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2

TRANSISTORSAND THEIR

LICATIOIN

TO/II/SION-RADIOELECTRONICS

Copyright --1953 byCOYNE ELECTRICAL SCHOOL

500 South Paulina St.Chicago, Illinois

o!,

All rights reserved. This book or any partsthereof may not be reproduced in any formwithout written permission of the publishers.

2nd Edition -1954

Printed in the United States of America

4

TRANSISTORS AND THEIR

APPLICATIONSIN

TELEVISION - RADIO

ELECTRONICSBy

Louis E. GARNER, JR.

Published byEDUCATIONAL BOOK PUBLISHING DIVISION

COYNE ELECTRICAL SCHOOLChicago, Ill.

ACKNOWLEDGEMENTS

The author and publishers wish to acknowledge the valu-able contributions made to this book by the following com-panies. In the case of photographs, illustrations and directquotations, further credit will be given in the body of the text.

BELL TELEPHONE LABORATORIES

CHICAGO STANDARD TRANSFORMER CO.

FANSTEEL METALLURGICAL CORPORATION

NATIONAL UNION RADIO CORPORATION

RADIO & TELEVISION NEWS

RADIO RECEPTOR CO., INC.

SYLVANIA ELECTRIC PRODUCTS, INC.

TRANSISTOR PRODUCTS, INC.

CBS-HYTRON

GENERAL ELECTRIC

THE HEATH COMPANY

P. R. MALLORY & CO., INC.

RADIO CORPORATION OF AMERICA

RAYTHEON MANUFACTURING CO.

TEXAS INSTRUMENTS, INC.

RADIO -ELECTRONICS MAGAZINE

6

FOREWORD

"TRANSISTORS AND THEIR APPLICATIONS" wasespecially written as a guide and reference volume to pro-vide a PRACTICAL explanation of these "wonder mites,"TRANSISTORS.

The author, Mr. Louis E. Garner Jr. is well qualified towrite this book. He has written many articles on the subjectof TRANSISTORS that have appeared in America's leadingtechnical magazines. He has been a Technical Consultantfor many years for several large Electronics Engineeringcompanies and has written a great deal of material for oneof the largest Radio -Television schools in the country.

His earlier background includes work as an Instructor ata Coast Guard Radio Material School, as a Consulting Elec-trical Engineer, and, during the second World War, work inthe Radar Section, Bureau of Ships, and with the Army -NavyElectronics Production Agency.

This practical field experience plus his training at GeorgeWashington University and the University of Maryland andhis study of the progress in Transistors since their inceptionqualifies him to explain the many applications of transistorsin the radio -television and electronics field.

This book is profusely illustrated with photographs andeasy -to -follow schematics. A "how -to -do -it" practical ap-proach was used throughout. Every effort has been madeto slant the data toward maximum value to the practicingserviceman, technician, or anyone interested in electronicdevelopment.

It is the authors opinion that Transistors will play a mostimportant part in the future of radio -television -electricityand all the phases of electronics. It is therefore paramountthat the present day serviceman acquaint himself with -WHAT TRANSISTORS ARE and WHAT THEY CANDO. This book will be very helpful in answering these ques-tions.

The COYNE ELECTRICAL SCHOOL wishes to expressits thanks to Mr. Garner and the many companies that havecontributed material to make the publishing of this refer-ence book possible.

B. W. COOKE, PresidentCoyne Electrical SchoolChicago 12, Ill.

7

f

TABLE OF CONTENTS

CHAPTER TITLE AND DESCRIPTION PAGE

1. INTRODUCTION-A discussion of transistorsand their effect on the Radio -TV and elec-tronics worker. Brief history of the transistor. 11 - 18

2. UNDERSTANDING TRANSISTOR ACTION-A non -mathematical explanation of how transistorswork. Basic types of transistors. 19 - 28

3. TRANSISTOR CHARACTERISTICS-Physical andelectrical properties of transistors. Definitionsof terms. 29 - 35

4. TRANSISTOR AMPLIFIER CIRCUITS-The threebasic types of transistor amplifiers, groundedbase, grounded emitter, and grounded col-lector. Gain in transistor circuits. Couplingtransistor amplifier stages. 36 - 50

5. TRANSISTOR OSCILLATOR CIRCUITS-Basictransistor oscillator circuits. "Tickler Feed-back." "Hartley." "Colpitts." Other types. 51 - 57

6. SPECIAL TRANSISTOR CIRCUITS-D.C. Ampli-fiers, R.F. Detectors, Clippers, Phase -

Inverters, Push -Pull Circuits, Multivibrators,etc. 58 - 68

7. TRANSISTOR COMPONENTS-Sub-miniaturetransformers, capacitors, other components.Transistor power supplies. 69 - 77

8. THE CARE AND SERVICING OF TRANSISTORS-How to avoid damaging transistors. Servicingtransistorized equipment. Testing transistors. 78 - 86

9. PRACTICAL TRANSISTOR CIRCUITS-Experi-mental circuits with typical parts values.Audio circuits. R.F. circuits. Experimentalcircuits. 87 - 98

10. APPENDIX-Reference data. Commerciallyavailable transistors and their characteristics. 99 - 103

9

t

:

,

AN INTRODUCTION TO TRANSISTORS

CHAPTER I

Figure 1. Three of the Bell Telephone Scientists responsible for the invention ofthe transistor. Seated is Dr. William Shockley, who initiates) .and directed theLaboratories' transistor research program. Standing are Dr. John Bardeen, left,and Dr. Walter H. Brattain, key scientists in bringing the invention to reality. Inthis photo, the scientists are shown working with apparatus used in making some ofthe first investigations leading to the discovery of the transistor.

CREDIT: Courtesy Bell Telephone Laboratories.

Although new inventions are made so rapidlyin the electronics field as to be considered al-most "every -day" occurrences, few are trulyrevolutionary in scope. The development of anew vacuum tube, transformer, or capacitor, im-portant as it may be to one branch of the field,generally has little over-all affect on the industry.

But revolutionary inventions are occasionallymade . . . . one such development was the inven-tion of the triode vacuum tube by DeForest. Itis reasonably safe to say that the present-day

11

12 TRANSISTORS AND THEIR APPLICATIONS

4,

Figure 2. An experimental personal -type AM receiver in which transistors areused to permit a better than three -fold reduction in the size and weight of batteriesneeded to maintain a 100 -hour operating life.

The young lady is placing an experimental transistor into its socket.

radio, television and electronics industry has re -sulted from the applications of this one inventionand its modifications.

And the invention of the transistor by Shockley,Bardeen and Brattain of the Bell Telephone Lab-oratories may be as revolutionary and have as

INTRODUCTION 13

far -reaching effects as the invention of the triodevacuum tube. For the transistor is capable notonly of doing most of the things that vacuum tubesdo but can also perform jobs which have hereto-fore been difficult, if not impossible, for vacuumtubes alone to handle.

The transistor may be used as an oscillator,as an a.c. amplifier, as a d.c. amplifier, as arelay or electronic switch, as a mixer or modu-lator, and as a detector. It may also be used asan attenuator, for impedance matching, and as anisolation or "buffer" amplifier.

But even with its amazing versatility, the tran-sistor is at one time much smaller, lighter,longer lived, more rugged, fantastically moreefficient and potentially less costly than the vac-uum tube. And the transistor offers the additionaladvantage of instant operation . . . . no "warm-up" time is required nor is "standy-by" powerneeded for its operation.

From this it is easy to see that virtually everyworker in the electronics field will eventuallycome into contact with the transistor, even ifonly indirectly.

The serviceman may expect a gradual intro-duction of the transistor in equipment he is calledon to repair and maintain. Starting with hearingaids, portable record players and radios, elec-tronic megaphones and similar equipment, hemay expect to see the transistor eventually usedin home broadcast receivers, auto radios, head-light "dimmers", television sets, recorders, andinter -communication equipment.

The ham will find many applications of thetransistor in portable and mobile equipment, insmall receivers, in transmitters, and in portabletest and measuring equipment.


Figure 3. An electronic megaphone in which a transistor operated amplifier isemployed. This piece of equipment illustrates one practical application of transis-tors to commercial equipment.

The electronics engineer will find that he isnot only required to design new equipment usingtransistors but that he will be frequently calledonto redesign existing equipment to make use ofthe advantages offered by transistor operation. . . . that is, to "transistorize" vacuum tubeequipment.

INTRODUCTION 15

The experimenter will find the transistor ideal-ly suited to the design of gadgets, electronic toys,and experimental circuits. The fantasticallysmall power requirements of transistor circuitsmake them ideal for portable equipment, whiletheir light weight and small size suit them foruse in remote control circuits.

The radio operator will find that a knowledgeof transistor circuitry will be useful in under-standing the functioning of transistorized equip-ment he is called on to operate and to adjust.

And the student will find that an understandingof transistor operation is as important as aknowledge of vacuum tube operation if a well-rounded and up -to -date education in the electron-ics field is to be acquired.

The military uses of the transistor are obvious,although no details have been officially releasedat this writing . . . the transistor is potentiallysuited for use in guided missiles, proximity fuses,portable radio and radar equipment, remote con-trolled devices, miniature teletype and commu-nication equipment, miniature television "spy"apparatus, radiation detection and measuring in-struments, "Secret Agent" radio transmittersand receivers, air-sea rescue radio equipment,compact artillery computers, detonating devices,etc. . . . in short, wherever light weight, smallsize and economy of operation are desirable inelectronic equipment.

Commercially, the transistor promises to rev-olutionize the design of many types of equipment.Giant electronic brains" or computers may bereduced to table -top size . . . in fact, miniatureall -electronic calculaters and adding machinesfor routine office work may sooa become a reality.Hundreds of thousands (if not millions) of transis-tors may eventually be used in telephone circuits


alone, performing dialing, switching, signaling,routing, relaying, amplifying, and control oper-tions.

But these potential applications of the tran-sistor are but a sample of what the future maybring. It must be remembered that the tran-sistor is a comparatively new invention . . . .

compared to the vacuum tube, the transistor isbut a mere babe -in -arms.

The following chronology, prepared by the BellTelephone Laboratories, gives a glance backwardover the history of transistor development, andillustrates, in concise form, the rapidity withwhich one transistor development has followedanother

JUNE, 1948 -- The point -contact transistormade its first public appearance at the WestStreet headquarters of Bell Laboratories in NewYork. It had been demonstrated privately to themilitary a short time before this announcementto the press.

MARCH, 1950 -- Invention of the photo -tran-sistor by the (Bell Telephone)Laboratories' Dr.J. N. Shive was announced. An entirely new typeof "electric eye" -- much smaller and sturdierthan present photo -electric cells and possiblycheaper -- it is a transistor controlled by lightrather than by electric current.

JULY, 1951 -- The junction transistor, a radi-callynew and in many ways more effective type,invented by Dr. Shockley, was announced. MorganSparks built the first of the new type transistors,using crystal processes developed by G.K. Tealand J.B. Little. Among others, R.L. Wallace, Jr.,and W.J. Pietenpol worked on their development.

The junction transistor was described as "ex-tremely small, occupying only about 1/400 of acubic inch" and even less power -consuming thanthe original point -contact type.

INTRODUCTION 17

The (Bell Telephone) Laboratories also an-nounced that development work on the point -con-tact type had resulted in understanding problemsinvolved in reliability and reproductibility, thusbringing regular production nearer.

SEPTEMBER, 1951 -- A transistor symposiumheld at Murray Hill. Nearly 300 guests attendedthe sessions; represented were industry, theArmy, Navy, Air Force, government agenciesand their contractors, and several universities.

OCTOBER, 1951 -- Production of the firsttransistors begun by the manufacturing depart-ment of Western Electric at its Allentown plant.

APRIL, 1952 -- Transistor technology sym-posium held at Murray Hill and Allentown, Pa.This was concerned with the development of tran-sistors, phototransistors and related semicon-ductor devices.

JUNE, 1952 -- Sixty-three faculty members,representing 33 universities, colleges, and insti-tutions of technology attended the first transistorschool, sponsored by the Laboratories (Bell Tele-phone) and held at Murray Hill, N.J. The schoolwas designed to facilitate the introduction oftransistor physics into university courses.

AUGUST, 1952 -- First announcement of autetroden transistor, invented at Bell Labora-tories by R.L. Wallace, Jr. By adding a fourthelectrode, Wallace was able to produce a trap-sistor usable at frequencies at least ten timshigher than would otherwise be possible.

OCTOBER, 1952 -- The transistor went to workfor the first time in the nation's telephone net-work. Oscillators employing transistor unitswere installed in dial switching equipment inEnglewood, N. J., as part of the customer longdistance dialing trial. The first commercial os-cillator model was assembled in the (Bell Tele-


phone)Laboratories by F . E. Blount, assisted byD. Houk.

END OF 1952 -- First over-the-counter con-sumer product to make use of transistors wasput on the market by manufacturers of hearingaids.

MARCH, 1953 -- A most important use of thetransistor was made public when the (Bell Tele-phone) Laboratories announced development ofthe "card translator," which will serve as anautomatic routing device for setting up long dis-tance calls across the country. The card trans-lator makes use of both the phototransistor andthe current -amplifying transistor.

* * * * * * * * * *

As for the future of the transistor. . .one en-gineer has said - "Within a few years. . .five toten at the most. . . .we may expect to see moretransistors than vacuum tubes used in new equip-ment !"

19

CHAPTER II

Understanding Transistor Action

At the left is a junction transistor, with next to it a point -contact transistor. Theminiature tube is a 6AL5 twin -diode type.

Transistors, important as they may be, rep-resent only a part of a large group of semi -con-ductor devices. Included with transistors in thislarge grouping are the selenium photo -cells usedin exposure and light meters, selenium rectifiersused in battery chargers, radio receivers andtelevision sets, the germanium diodes used asvideo 2nd detectors, as discriminators in FMreceivers, as clippers and as limiters, and thesilicon diodes used as high frequency mixers inUHF Television front -ends and converters, andin Radar equipment.

The old "crystal" radio receiver, employinga galena crystal and an adjustable "cat's whis-ker", represents one of the earliest applicationsof semi -conductor devices to radio.

20 TRANSISTORS AND THEIR APPLICA CIONS

Transistors, as well as the diodes and otherdevices mentioned, all depend,for their operation,on the electrical properties of a class of sub-stances known as semi -conductors. A semi -con-ductor is a substance or material which, undersome conditions, acts as a conductor (like cop-per or aluminum), but which, under other condi-tions, acts like an insulator (such as glass).

Selenium, silicon and germanium, when com-binedwith certain impurities, are the most pop-ular semi -conductors. There are other materialsexhibiting similar properties, but they are notused as extensively as the three mentioned.

Selenium is widely used in power rectifiersand in "self -generating" photocells.

Silicon is used primarily in high frequency"mixer" diodes.

Germanium is used in diodes, transistors, andphototransistors.

Almost all semi -conductors permit electricalcurrent flow to take place easier in one directionthan in the other and thus may be used as recti-fiers or diodes. In addition, other physical con-ditions may affect the flow of current -- suchconditions include light, heat, and the presenceof other electrical fields. It is the last propertythat makes the transistor possible.

Current flow through a semi -conductor maytake place by two different methods, although onegenerally predominates in a particular case.

Under some conditions, free electrons may bedetached from atoms of the material and maytravel through the substance . . . . much in thesame fashion that normal current flow takes placethrough ordinary conductors.

Those semi -conductors which permit currentflow to occur primarily by means of free elec-trons are called "negative" or 'u" type materials.A typical example of an N -type material is puregermanium to which a small amount of arsenic

UNDERSTANDING TRANSISTOR ACTION 21

has been added as an impurity.On the other hand, should there be few free

electrons in the material, the molecular structureof the substance may be such that valence bondsare left unsatisfied, and electrons are actuallyrequired by some of the molecules. Under suchconditions, the "absence" of an electron in a par -tidular molecule may be considered as a "hole".

O 0 0 0 00O 0 0 0 0 0O 0 POO00.000O 0

Terminal's

Figure 2-1. Schematic representationof an N -P junction of semi -conductor materialSuch a junction is made up of a single piece of material (a single crystal), but withthe ends having different electrical characteristics.

The molecule having the "hole" may rob anearby, electrically neutral, molecule of an elec-tron, leaving the second molecule with a hole.The holes may thus migrate through the material,acting, for practical purposes, like a current flowof positively charged particles.

If the current flow through the material is pri-marilyby means of "holes", then the semi -con-ductor is called a "positive" or "P" type material.A P -type semi -conductor may be formed by add-ing a small amount of indium to otherwise puregermanium.

Regardless of whether a particular substanceis an Nor a P -type material, it must be remem-bered that "holes" may exist in an N -type orfree electrons may exist in a P -type semi -con-ductor. Both holes and free electrons may alsobe formed by passing current through the ma-terial.

If a junction is made up of an N and a P -type

22 TRANSISTORS AND. THEIR APPLICATIONS

semi -conductor, as shown in Pig. 2-1, and a d.c.voltage is applied to it, the amount of currentflow that can take place will depend on the po-larity of the applied potential.

Should the negative terminal of the powersource be connected to the N -type material, freeelectrons will move away from the terminal andtoward the junction of the two substances. In asimilar fashion, the holes in the P -type materialwill migrate away from the positive terminaltowards the junction, where they can combinewith the free electrons of the N -type material.

The junction, under these conditions, permits afree exchange of positive and negative charges(holes and electrons ), and a comparatively largecurrent flow may take place. We can say that thejunction offers low resistance to current flow, orthat it acts like a good conductor.

Should the positive terminal of the voltagesource be connected to the N -type material, thefree electrons will move toward this terminaland away from the junction. Similarly, the holesin the P -type material will move away from thejunction and toward the negative terminal of thepower source. Thus, with the d.c. supply voltagereversed, few electrons and holes are present atthe junction.

Under these conditions, the previous exchangeof positive and negative charges is quite difficultand very little current flow past the junction cantake place. . . .the junction now offers a high re-sistance to current flow and acts somewhat likean insulator.

Since a junction of the type described offerslow resistance to current flow in one direction,and high resistance to current flow in the oppositedirection, it may be used as a rectifier or as adiode.

Let us now consider the action when two junc-tions of the type described above are provided.This maybe accomplished by using three layersof semi -conductor material, with the center layerof the opposite type from the two outer layers.


iA.C. Signal I

Inserted

R IN

ryfopo,Ar000s;,.

+11111

Power Supply Batteries)

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Figure 2-2. Schematic representation of an NPN junction tiyinsistor, showing powersupply connections and input and output load resistors.

If the outer layers are N -type material andthe inner layer is a P -type material, then theunit is called an NPN junction transistor. Sucha transistor is represented in Fig. 2-2.

On the other hand, should the outer layers beP -type material and the inner layer an N -typesemi -conductor, a PNP junction transistor re-sults.

Referring to Fig. 2-2, let us assume that avoltage source is connected to "bias" the left-hand junction in its low resistance direction. Agood current flow will result, limited primarilyby series resistor Rin, and producing an excessof electrons in the center material.

If the other junction is now biased in its "re-verse", or high resistance, direction, currentflow can only take place through this junctionbecause of the excess of electrons in the centermaterial.

Now, if the current flow through the left-handjunction is varied, the number of excess electronsin the center material will be changed, and sim-ilar variations will occur in the "reverse" cur-rent flow through the right-hand junction.

In practice, a change in the current flow in theleft-hand junction is produced by adding a smalla.c. signal in series with the d.c. "bias' current.


In many cases, the currents in the two circuitsmay be on the same order of magnitude. How-ever, an appreciable power gain may be realizedbetween the input and output circuits because ofthe difference in impedances.

With the circuit arrangement shown in Fig.2-2, the left-hand terminal of the transistor iscalled the "emitter". The center section is calledthe "base", and the right-hand terminal is the1'c ollector" .

Since the base -emitter circuit is biased in itsforward, or low resistance direction, it forms alow -impedance circuit. The collector (output)circuit, biased in its "reverse" or high -resist-ance direction, forms a high -impedance circuitIt is this difference between output and inputimpedances that permits gain to be obtained andthe transistor to be used as an amplifiereven though the emitter and collector currentsmay be nearly the same.

In the PNP junction transistor, power supplyvoltage polarities are reversed and the majorconduction is by holes instead of electrons .0ther -wise, the operation of the two transistors issimilar.

The part which is to be the emitter is treatedduring manufacture to perform this function best,while the part designated for the collector hasbeen prepared especially for such service.

The structure of the two types of junction tran-sistors is shown in principle in Fig. 2-3. Op-posite sides of a single piece of crystal are ofthe compositions which are to act as emitter andcollector. In between is a section, possibly one -thousandth of an inch thick, which will act as thebase.

The usual symbol for the junction transistor isshown at C. The base is represented by a straightline, with emitter and collector represented bylines at angles to the base. The slanting line forthe emitter is marked with a small arrowhead,which generally points towards the base in thecase of PNP transistors, and away from the base


O P N P

BaseEmitter Collector

O N P N

4/..- Leads-.BaseTerminal

Emitter Collector

Figure 2-3. Element arrangements for junction transistors, and the symbol forthis general type.

in the case of NPN junction transistors. However,there is some tendency to use the same symbolfor both types of transistors where thereis any doubt about the type of transistor indicatedon a schematic diagram, manufacturer's typenumbers should be checked.

There is a second method of transistor con-struction, resulting in a unit called a "point -con-tact" transistor. This method of construction isillustrated, in principle, in Fig. 2-4.

In the point -contact transistor, the internalconnections to emitter and collector are madethrough the ends of very small wires in contactwith the surface of the crystal at points very closetogether. As actually constructed the crystalof the point -contact transistor may be only about1/ 20 inch in diameter (or square), and about 1/ 50inch thick. Contacts of the two wires on thecrystal surface are separated by only a fewthousandths of an inch.

The design represented at A of Fig. 2-4 employsan N -type crystal; the main body of the crystalbeing of such material. During manufacturethere is a forming process which produces ateach contact position a small area of P -typematerial. Therefore, the contact points foremitter and collector actually connect to P -typematerial, between which is the N -type body of thecrystal. This makes the N -type point -contacttransistor equivalent in operation and in other


TerminalLe ads

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Emitter

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Ba seEmitter Collector

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Figure 2-4. Construction and element arrangement for point -contact transistors,and the usual symbol.

respects to the PNP junction type that is,emitter and collector leads go to P -type ma-terial, while the base lead goes to N -type ma-terial.

In diagram B (Fig. 2-4)the body of the crystalis P -type material. There are small areas ofN -type material formed at the emitter and col-lector contact points, giving a point - contacttransistor that is equivalent, in many respects,to the NPN junction transistor.

The construction of a point -contact transistoris clearly illustrated in the model shown in Fig.2-5. The small cube represents the piece ofsemi -conductor material.

Tetrode point -contact transistors are essen-tially similar to the triode types just discussed,except that an additional "point -contact" or cat's -whisker is provided.


.

Figure 2-5. Photographof a model showing the constructionof a point -contact tran-sistor. The two "cat -whiskers" and the small cube of semi -conductor materialare clearly visible. The "hole" in the front of the cartridge is provided so that thepoint -contacts may be adjusted during manufacture.

CREDIT: Courtesy Bell Telephone Laboratories.


COMPARISON OF TRANSISTORS AND VACUUM TUBES

Criteria TubeGain/Stage 0-40 dbNoise Figure at 1000 c p s,

BW = 1 c p sFreq. Limit as au AmplifierFreq. Limit as an OscillatorOutput Power (Po)Class A Efficiency

Class B EfficiencyClass C EfficiencyOscillator EfficiencyTotal Power Required

Physical N olumeTemperature LimitationsShock LimitationsLife

Transistors0-40 db

10-50 db0-30 mc

300 mc0-200 mwPoint Contact 35%Junction 35-49%>80%

99%>70%Point Contact 4-50Junction 1-1000.0005-0.02 in.3-60°c to 80°c20,000 to 30,000 G> 70,000 hours

0-30 db0-60 k mc0-60 k mc0-kilowatts

35%79%85%60-70%

mw50 mw to 2w0.125-1.0 in .3-60°c to 200°c750 G0.5,000 hours

t Laboratory experiments have indicated that transistors designed with 200 watt ratingsare possible.

29

CHAPTER DI

Transistor Characteristics

The electrical ratings of transistors are gen-erally given in terms of a few basic characteris-tics. Since these characteristics are, for themost part, different from those used to describevacuum tubes or other electronic components,it is important that the electronic technician,student, and serviceman become familiar withthem.

TRANSISTOR TYPES: As we have seen, tran-sistors can be divided into two general types,according to the method of construction employed. . . .point -contact and junction. In addition, bothbasic types can be further sub -divided.

Point -contact transistors can be classed ashaving either a P -Type or an N -Type base. Afurther sub -division into triodes and tetrodes isalso possible.

Junction transistors may be classified asPNP or NPN.

Still further classifications may be introducedas additional transistor types are produced. Forexample, a number of manufacturers are devel-oping multi -purpose transistors, high powertransistors, and other special types.

At present, there is considerable variation interminal lead connections between transistorssupplied by different manufacturers. However,there is a definite tendency toward standardiza-tion and, in the future, we may expect most tran-sistors to have similar lead connections. Thepresently accepted "standard" is illustrated inFig. 3-1.

However, even if the lead connections areknown, special care should be exercised to checkthe transistor types used whenever assemblingnew circuits or repairing transistorized equip-ment. The application of improper power supplyvoltage polarities, even if only momentarily, mayeasily ruin a transistor.


PHYSICAL CHARACTERISTICS: All com-mercially available transistors are physicallyquite small, with over-all dimensions, exclusive

Outline of Transistor may vary withAe,/Manufacturer

ww

m

NOT TO SCALE

0

04U

w

0

unequal Lead Spacing

.017 Leads to fit Standard SubminiatureTube Socket

Figure 3-1. The presently accepted "standard" for lead connections of triode tran-sistors. The pin size and spacing is chosen so as to permit the use of a standard5- or 7 -pin "in line" subminiature tube socket. Some manufacturers supply theirtransistors with long leads to permit the units to be wired directly into a circuit. . . these leads are cut short where a socket is employed.

of leads, seldom exceeding a fraction of an inch.Their weight, too, is generally minute, usuallybut a tiny part of an ounce.

The semi -conductor crystal itself may bemolded in plastic or glass, or both, for protec-tion and mechanical stability. There may be anouter case or shell of metal. Some manufac-turers also offer hermetically sealed units, inwhich the transistor crystal is sealed in a vac-uum. . . .such units offer superior stability undersome operating conditions.

Both ambient temperature and humidity con-ditions may affect transistor operation. At thiswriting, no manufacturer recommends operationof their units above 100°C, and many specify amaximum ambient temperature of 40 or 50°C.

TRANSISTOR CHARACTERISTICS31

As far as moisture is concerned, this problemis only important if the transistors used are nothermetically sealed, and if the humidity, underoperating conditions, is likely to be high. Insuch cases, special care must often be taken, whendesigning the transistorized equipment, to insurea reasonably well sealed case.

ELECTRICAL CHARACTERISTICS: Man u-facturers of transistors generally tabulate theelectrical specifications of their units in threeways, as follows: (1) MAXIMUM RATINGS, (2)TYPICAL OPERATING CHARACTERISTICS, al-so called AVERAGE CHARACTERISTICS, and as(3) CHARACTERISTIC CURVES (or CHARTS).

The Maximum Ratings are those that must notbe exceeded when operating the units. Thesevalues are especially important to the experi-menter, to the electronics engineer, and to anyworker designing or assembling new circuits inwhich transistors are employed.

In most cases, the Maximum Ratings will in-clude the Maximum Collector Voltage, MaximumCollector Current, Maximum Emitter Current,and Maximum Collector Dissipation. In the caseof point -contact transistors, the Maximum PeakInverse Emitter Voltage may also be included.Voltages are generally given with respect to thebase, with the values indicated in volts. Currentsare given in milliamperes or microamperes,and the power dissipation values are given inwatts or milliwatts, at a specific ambient tem-perature.

The Typical Operating Characteristics aregiven to serve as a guide to the engineer or de-signer building new equipment, or to the service-man or technician called on to service transistor-ized apparatus. These values may vary widely,depending on the "operating conditions" chosenby a particular manufacturer as typical.

From a practical viewpoint, the Typical Oper -ating Conditions need not be followed exactly indesigning transistor circuits, provided the Max-imum Ratings are not exceeded.


Under Typical Operating Characteristics willgenerally be listed such values as Collector Volt-age, Emitter Current, Current AmplificationFactor, Cut-off Current, Power Gain, Input Im-pedance (or Resistance), Output Impedance, Out-put Capacitance, Noise Factor, Frequency Cut-Off, Power Output, and Distortion. Several ad-ditional values may be given by some manufac-turers, including Base Resistance, Output Ad-mittance, and other characteristics, but thevalues listed above are the most common.

The Collector Voltage and Emitter Current aregiven in volts and milliamperes (or microam-peres), respectively. These values establish thebasic conditions under which the other character-istics have the indicated numeric al values.

In addition, it is generally customary to indicatethe circuit arrangement used to obtain the valuesgiven in the manufacturer's technical data sheet.Many of the values listed, especially those forPower Gain, Input and Output Impedance, PowerOutput and Output Capacitance, may have quitedifferent values with other circuit arrangements.

If not otherwise stated, the Typical OperatingCharacteristics given by a manufacturer general-ly apply where the Grounded Base (or CommonBase)circuit is employed (refer to the descrip-tion of basic transistor circuits in Chapter IV).

The Greek letter alpha ( a) is used to indicatethe Current Amplification Factor of a transistor.The alpha of junction transistors cannot be above1.0, but values appreciably above 1 may be ob-tained with point -contact transistors.

Alpha is related to the gain that can be obtainedfrom a transistor and is defined as "the ratio ofthe change in collector current for a specificchange in emitter current at a constant collectorpotential."

The Cut -Off Current is the collector currentdrawn under conditions of zero base current, witha d.c. voltage applied between emitter and col-lector. Its value is generally listed only for junc-tion transistors. The Cut -Off Current usuallyhas a value of from 5 to 25 microamperes, al-

TRANSISTOR CHARACTERISTICS 33

though it may be higher or lower with individualunits.

(NOTE: In some instances, a manufacturermay specify the Cut -Off Current as the collectorcurrent drawn under conditions of zero emittercurrent. This definition applies when the Ground-ed Base circuit is employed, and is most oftenused when referring to point -contact transistors.)

Both the Power Gain and the Noise Factor aregiven in decibels (db).

Internal noise, in transistors, is caused pri-marily by the molecular agitation resulting fromthe movement of holes and electrons through thematerial of the semi -conductor.

Although the Noise Factor for junction tran-sistors is generally lower than that for point -contact transistors, it is higher than that forvacuum tubes giving comparable gain. In thecase of junction transistors, the Noise Factormay vary appreciably with the collector voltageused (and hence the operating point), with a lowcollector voltage generally giving minimumnoise.

The Input and Output Impedances are particu-larly important when coupling two or morestages, when using the transistor to drive otherequipment, or when connecting a signal sourceto the input of a transistor. Both values willvary widely with the circuit arrangement em-ployed, but in most cases the input impedancewill have alow to medium value while the outputimpedance will be high. These values are spe-cified in ohms.

The Output Capacitance is given in micro -micro -farads (mmf). Typical values are gener-ally appreciably less than 100 mmf. This char-acteristic is of special importance when usingthe transistor in wide -band or high frequencycircuits.

The Frequency Cut -Off (or Cut -Off Frequency,as it is often called) is generally taken as thefrequency where the value of alpha is 3 db downfrom its low frequency value (usually 1000 cpsor less).


As operating frequency is increased, the gainof transistor circuits decreases because of twoactions in the transistor itself. The first is thetime required for electrons to move through thecrystal material and for holes to shift from placeto place. Both of these movements are muchslower than the movements of electrons througha vacuum tube, with the holes moving even slowerthan the electrons.

The other factor is the internal capacities be-tween elements of the transistor.

It is possible to reduce the transit time re-quiredby electrons and holes in both point -con-tact and junction transistors. In point -contacttransistors this is accomplished by reducing thespacing between contacts. Injunction transistorsthis is accomplished by using smaller dimensionsbetween end contacts and by using a thinner baselayer.

Unfortunate l y, both of these constructionchanges, although they reduce transit time, alsoincrease the internal capacities of the transistor,increasing the other limiting factor on frequencyresponse.

For practical purposes, junction transistorsare generally limited to the audio range of fre-quencies, although especially selected units mayoperate satisfactorily at R.F at least intothe AM Broadcast Band.

Point - contact transistors may generally beoperated at much higher frequencies, and someunits have been used as oscillators at hundredsof megacycles.

Regardless of the type of transistor, however,it is usually possible to operate units satisfac-torily at much higher frequencies as oscillatorsthan as amplifiers.

The Power Output obtained from a transistorwill depend on the circuit employed, the d.c.operating currents and voltages, the amount ofsignal drive (in the case of an amplifier), and onthe characteristics of the transistor itself. Thisvalue is generally given in milliwatts.

TRANSISTOR CHARACTERISTICS 35

The maximum Power Output that can be ob-tained from a specific transistor is dependent onthe Maximum Power Dissipation of the unit andon the efficiency of the circuit employed.

Distortion is expressed as a percentage and isimportant where the transistor is used in audiocircuits. The Distortion obtained in a particularcircuit increases as the operating voltages andcurrents approach the maximum rated values.

In addition to the tabulated numerical valuesfor the transistor characteristics, the majorityof transistor manufacturers also supply CHAR-ACTERISTIC CURVES (or CHARTS) as part ofthe technical data sheets on their units.

These charts are roughly analogous to theCharacteristic Curves furnished by vacuum tubemanufacturers and serve approximately the samepurpose. . . .to assist the electronics engineeror circuit designer in determining the properoperating conditions for a particular circuit.

As inthe case of vacuum tube curves, the datasupplied must be considered as approximate only,and as typical only of the average characteristicsof a particular type of transistor. Individualunits mayhave somewhat different characteris-tics due to production tolerances.

Typical transistor Characteristic Curves aregiven in Fig. 3-2.

36

CHAPTER IV

TRANSISTOR AMPLIFIER CIRCUITS

Similar circuit arrangements are used withboth vacuum tube and transistor amplifiers. Thissimilarity applies to the circuit arrangementonly, however, and it is incorrect to consider thetransistor as a direct substitute for the vacuumtube . . . the differences between the operationof transistors and vacuum tubes are far greaterthan their similarities.

The transistor is basically a current operateddevice. Thus, the transistor operates as a cur-rent amplifier and prefers a constant currentpower supply. The vacuum tube, on the otherhand, operates as a voltage amplifier and worksbest with a constant voltage power source.

In addition, most vacuum tube circuits featureboth high input and high output impedances.Transistor circuits generally have a low to mod-erate input and a moderate to high output imped-ance. In vacuum tube circuits, the input andoutput signals are generally well isolated, where-as this is often not the case with transistoramplifiers. And, as we discussed in the lastChapter, the transit time required for electrons(and holes) in transistors is considerably longerthan the transit time required by electrons in avacuum tube.

Base

Collector

P

N

P

Emitter

Plate

Cathode

Bose

Collector

N

N

Emitter

Figure 4-1, This sketch illustrates the three elements of a vacuum tube and theanalogous elements of both PNP and NPN transistors.

TRANSISTOR AMPLIFIER CIRCUITS 37

The similarities in the circuit arrangementsof transistor and vacuum tube amplifiers maybe most easily observed by considering the vari-ous elements of the transistor as analogous, butnot equivalent, to corresponding elements in thevacuum tube. These relationships are clearlyillustrated in Fig. 4-1.

Inthe transistor, the collector corresponds tothe plate, the base to the grid, and the emitter tothe cathode of the vacuum tube.

InputSig nal

OutputSignal

PlateBias --.-

Voltage

In putSignal

Bias

OutputSignal

CollectorVoltage

Figure 4-2. Grounded cathode circuit for a vacuum tube (left) and the groundedemitter circuit for a transistor (right). While the plate voltage of the vacuum tubewill always be positive with respect to its cathode, the voltage polarities in thetransistor circuit will depend on the type of transistor used.

There are three basic vacuum tube amplifiercircuits, each of which is identified by the ground-ed or common element. They are the groundedcathode the grounded grid, and the grounded plate(or cathode follower) amplifiers. Transistoramplifiers are identified in a similar fashion asgrounded emitter, grounded base and groundedcollector amplifiers. Let us discuss each ofthese three basic types.

THE GROUNDED EMITTER AMPLIFIER: Thegrounded cathode triode tube circuit is shown atthe 'left in Fig. 4-2. The input signal is appliedbetween the grid and cathode, while the outputsignal appears between plate and cathode. Aver-age grid potential is determined by a grid bias-ing voltage, and average plate potential withreference to the cathode is fixed by a d.c. platevoltage. The cathode is thus common to both theinput and output signals, and may be grounded.

The equivalent transistor amplifier is shownat the right (Fig. 4-2). Input is applied between


base and emitter. Average d.c. base potential,with reference to the emitter, is determined bythe bias voltage. However, it is the bias currentthat determines the mode of operation of the cir-cuit.

The output signal is taken from between thecollector and emitter, with average collectorpotential fixed by the d.c. collector voltage. Inmany cases, the actual collector voltage has verylittle effect in determining collector current, asthis is primarily a function of the bias current.

When a signal is applied to the input, the biascurrent varies about its average value, and cor-responding variations, though of much greateramplitude, take place in the collector circuit.This results in an amplified version of the inputsignal appearing across the load impedance.

Although this circuit is termed a groundedemitter amplifier, the emitter need not neces-sarily be grounded . . . it is simply common toboth the input and output circuits.

Note that the polarity of the d.c. power supplyvoltage is not indicated in Fig. 4-2. The properpolarity will depend on the type of transistorused . . . this is illustrated in Fig. 4-5 for thethree basic types of amplifier circuits and fordifferent types of transistors.

These polarities are indicated as positive ornegative when measured between the indicatedtransistor element (emitter, base, or collector)and the grounded element.

"Polarities of the output elements, whethercollector or emitter, always are as shown, withreference to the grounded or common element.However, polarities of the input elements maybe occasionally reversed in practice. This mayoccur when the biasing voltage, at the elementitself, is 0.2 volt or less. In a few instances,gain may be increased or distortion lessened byusing a 'reversed' bias polarity.

"Along the upper row of diagrams (Fig. 4-5)are shown the correct polarities for NPN junc-tion type transistors or for point -contact typeswith P -type base material. The collector -emitter


In put

Out put

G

PlateBi os Voltage

Input Output

_Bins CollectorVoltage

Figure 4-3. Grounded grid triode tube connections (left)° and grounded basecon-nections for a transistor (right). Note that these circuits are analogous, but notequivalent.

polarities are similar to those encountered invacuum tube circuits. Thus, just as the plate ofa vacuum tube is positive with respect to itscathode,so is the collector of an NPN transistorpositive with respect to its emitter.

"The lower row of diagrams shows relativepolarities for PNP junction transistors or forpoint -contact units with N -type material in thebase. Note that these polarities are exactly theopposite of those shown in the top row."

It is extremely important that the proper d.c.polarities always be ovserved when working withtransistor circuits. While the application of anegative voltage on the plate of a vacuum tube,for example, may not injure the tube,the applica-tion of improper voltages to a transistor willruin it almost instantly.

Electron current flow, in transistor circuits,is exactly like that in any electrical circuit. Themovement of electrons is from the point of high-est negative potential through the circuit ele-ments to those points of less negative (or morepositive) potential.

In the grounded emitter circuit, the collectoremitter circuit current flow may be onthe orderof a few milliamperes, while the base -emittercircuit current maybe from less than 10 to sev-eral hundred microamperes.

The grounded emitter circuit features a lowto moderate input impedance and a moderate out-put impedance. With typical junction transistors,the input impedance may be from 300 to 1500


ohms, and the output impedance may be on thesame order or somewhat higher.

Although not quite as stable as the groundedbase amplifier, the grounded emitter circuit of-fers the highest gain of the three basic transistoramplifier circuit arrangements.

Phase reversal occurs in a grounded emitterstage. Thus, if a positive -going signal pulse isapplied to the input, a negative -going signal pulsewill be obtained in the output circuit.

THE GROUNDED BASE AMPLIFIER: A popu-lar triode vacuum tube amplifier circuit is thegrounded grid amplifier, shown in simplifiedschematic form in the diagram to the left in Fig.4-3. The input signal is applied between thecathode and grid, by way of ground, with biasvoltage applied between cathode and ground . . .

this is equivalent, of course, to applying the biasbetween grid and cathode. Output is obtained be-tweenthe plate and grid, again by way of ground.Average plate potential is determined by the d.c.plate voltage.

The analogous transistor circuit, the groundedbase amplifier, is shown to the right in Fig. 4-3.The input signal is applied between the emitter,corresponding to the tube's cathode, and the base,comparable to the tube's grid. Average emitterpotential, with reference to the base, is deter-mined by a bias voltage inthe input circuit. Theoutput is taken between the collector and base,with the d.c. collector voltage obtained from aseparate power source.

As in the case of the grounded emitter circuit,the base of a grounded base amplifier need notbe connected to the circuit ground, as long as itis common to both input and output circuits.

The input impedance of a grounded base ampli-fier is very low, generally quite a bit less than100 ohms.* The output impedance, on the otherhand, is fairly high. But even with these differ-ences in input and output impedances, the d.c.in the emitter and collector circuits may be onthe same order of magnitude.*For junction transistors ... point contact unitshave an input impedance of between 100 and1000 ohms.


In operation, an a.c. signal applied to theemitter -base circuit results in current varia-tions in this circuit. These current changes,in turn, result in corresponding impedance vari-ations inthe collector circuit, so that an ampli-fied signal appears across the collector load.Thus, although signal amplification is possiblein a grounded base circuit, and a definite powergain maybe obtained, this gain is due almost en-tirely to the differences between the input andoutput impedances of the transistor. The actualcurrent amplification, in a grounded base circuit,is always less than 1.*

The d.c.polarities for the grounded base ampli-fier are shown in the middle pair of schematicsin Fig. 4-5. Again, different polarities are em-ployed for different types of transistors.

Phase reversal does not take place in a ground-ed base amplifier.

Quite stable, the grounded base circuit pro-vides a moderate power gain and is very popularfor some applications. It is more generally usedwith point -contact than with junction transistors,however.

In put

Bios

PlateVoltage

utput

4 Out put

Input

CollectorVoltage

Bios

Figure 4-4. Grounded plate(cathode follower) circuit for a triode vacuum tube (left)and the grounded collector circuit for a transistor (right). Both circuits are usedprimarily for impedance matching.

THE GROUNDED COLLECTOR AMPLIFIER:The grounded plate vacuum tube amplifier ismore popularly known as a cathode follower. Thebasic circuit is shown to the left in Fig. 4-4,while the equivalent (or, rather, analogous) tran-sistor circuit is shown to the right in the sameillustration.*For junction transistors.


In the vacuum tube version of the circuit, theinput signal is applied between grid and thegrounded plate circuit, with the output signalobtained across a load impedance placed betweenthe cathode and ground. Thus, it is the platethat is common to both input and output circuits.

Common Emitter

NPN Junction or P -Base Point -Contact

Cormon Base

PNP Junction or A -Base Point Contact

Common Collector

Figure 4-5. Transistor element d.c. polarities with reference to common orgrounded element. Note that different polarities are encountered with differenttypes of transistors.

In the grounded collector amplifier, the inputsignal is applied between base and the groundedside of the collector circuit, with the output ob-tained across a load impedance placed betweenemitter and ground. The collector thenbecomescommon to both input and output circuits.

As in the other transistor circuits, bias cur-rent and collector voltage are supplied by d.c.sources. D.C. voltage polarities inthe groundedcollector circuit ard- shown in the last pair ofschematics in Fig. 4-5.

The grounded collector circuit differs from theother basic transistor amplifiers in that theimpedance maybe relatively high . . sometimeseven approaching, in value, the input impedanceof a vacuum tube amplifier. The input imped-


ance is very dependent on load impedance, how-ever.

Since the output impedance of this circuit ismoderate to low, it may be used as an impedancematching device, if desired, and, in this respect,apes its vacuum tube cousin, the cathode follower.

A large current amplification and a definitepower gain may be obtained with the groundedcollector amplifier. The power gain obtained,unfortunately, is less than that obtained with thegrounded emitter and grounded base circuits.The voltage gain of the grounded collector cir-cuit cannot exceed 1 . . . thus, the circuit is oflittle value as a voltage amplifier.

Phase reversal does not take place in thegrounded collector stage. The input and outputsignals therefore have the same phase relation-ships.

The grounded collector amplifier has onefeature that is unique . . . it may serve as a bi-lateral or "two-way" amplifier under some con-ditions. The connections of the input and outputcircuits may be interchanged, permitting a sig-nal to be amplified in either the forward or theinverse direction."

GAIN IN TRANSISTOR CIRCUITS

Gains of transistor circuits usually are spec-ified in decibels of power, instead of as multi-plications of voltage, as is more common withvacuum tube circuits (except power outputstages).

Fig. 4-6 shows, in graphical form, the rela-tionship between power gain in decibels (db) andgain as an actual ratio between output and inputpowers. Power in milliwatts is indicated on thechart, although any other common unit of powermay be used. Milliwatts are indicated simplybecause power outputs and permissible powerdissipation in transistor circuits are usuallymeasured in these units.

Assuming that there is a reasonably good matchbetween the source and the transistor input im-

0

0IF0


100 000

50 000

10 000

5 000

1 100 0

500

a.

4-

100Cl.

50

7

10

5

1

0 10 20 30 4 0Gain - Decibels of Power

50

yf

Figure 4-6. Graph showing the relations between gains measured in decibels ofpower and in ratios of output to input milliwatts. This chart may also be used tocompare db gain for other power units.


pedance,and that the input signal current remainsconstant, the input power of a transistor stage isdirectly proportional to its input impedance.Comparatively little power will be needed at theinput when the input impedance is small.

Similarly, if the output signal current is con-stant, output power will be directly proportionalto output inpedance,with a high output impedancegiving a comparatively high output power for anygiven signal current. Thus, as in the case of thegrounded base amplifier, where input and outputcurrents may be on the same order of magnitude,the ratio of gain is equal to the ratio of outputto input impedances.

On the otherhand,with input and output signalcurrents which are not alike,the input and outputpowers are proportional to the squares of thecurrents, as well as being directly proportionalto the impedances.

Actual power gains depend not only on thecharacteristic impedances of the transistor, but,to a great extent, on how well these impedancesare matched by the source and the load, on thetype of circuit employed, and on the relative po-tentials at the high -side input and output elementswith respect to the common or grounded element.Gain which may be realized in practice is a mat-ter of correct circuit design.

Generally speaking, junction transistors arecapable of giving more power gain than point -contact types ,when both types are used in similarcircuits. However, gain does depend on the basiccircuit employed and, as we have seen, thegrounded collector circuit provides considerablyless gain then either the grounded base orgrounded emitter circuits.

There are so many variables that it is difficultto give specific values as representative of typi-cal power gains. However, a figure of 40 dbrepresents an average gain for a number of welldesigned circuits using junction transistors inthe grounded emitter arrangement, while about25 db might be obtained using typical point con-tact units. Grounded collector circuits give a


power gain on the order of 15 db, while groundedbase circuits give a gain somewhat less than,but approaching, the gain of the grounded emittercircuits.

COUPLING TRANSISTOR AMPLIFIERS

For maximum transfer of signal power froma source to a load the impedances of source andload must be equal. There is a rapid decreaseof power transfer as the load impedance is madeless thenthe source impedance, and a less rapiddecrease as the impedance of the load is madegreater than that of the source.

In a single amplifier stage there are twosources and two loads requiring matching of im-pedances. There is first the signal source con-nected to the transistor input, and for which theinput impedance of the transistor stage formsthe load. The transistor' itself is the secondsource, at its output terminals. For maximumoutput from the transistor, the impedance of theload connected across its output terminals mustapproach or equal its own internal impedance.

As we have seen, the input and output im-pedances of transistors depend on the type oftransistor employed on the operating condi-tions,and on the circuit arrangement used. How-ever, the input impedances of transistor stagesare likely to be moderate to small, while theoutpu't impedances are likely to be large ...thisbrings up special problems when coupling tran-sistor stages.

Probably the simplest means of coupling twotransistor stages is to employ an impedancematching transformer between the two stages.Such a circuit is illustrated in Fig. 4-7. In thiscircuit, two junction transistor amplifier stagesare transformer coupled and are used to drive abeam -power output tube. The grounded emittercircuit arrangement is employed.

In order to match the high output impedance ofthe first transistor stage to the plow input im-


pedance of the second transistor, a step-downturns ratio is used in the interstage couplingtransformer.

Figure 4-7. This simplified schematic illustrates the use of transformer couplingbetween transistor amplifier stages. The first coupling transformer has a stepdownturns ratio for matching the high output impedance of the first stage to the low inputimpedance of the second stage. Grounded emitter circuits are used. The two -stagetransistor amplifier is used to drive a beam -power output vacuum tube.

Resistance -capacity and impedance couplingmay be used betwpentransistor stages on occa-sion. Typical circuit arrangements are given inFig.4-8. With such circuits, it is assumed thatthe capacitance of blocking capacitor Cb is large(in audio circuits, on the order of several micro -farads) so that its reactance will be small at thesignal frequencies used. Thus, as far as thesignal is concerned, there is a direct connectionbetween the output of one stage and the input of

b cb

Equivalent Signal Circuit

Figure 4-8. Resistance -capacity and impedance coupling techniques, as applied totransistor amplifiers, are illustrated in this sketch. If Cb is large and offers alowreactance at the signal I requencies, the Equivalent Signal Circuit shown applies.Since no special provision is made for matching the input and output impedances ofthe coupled stages, the efficiency of such circuits is low.


the following stage, as shown by the EquivalentSignal Circuit.

This means that the output impedance of onestage and the input impedance of the next stageare essentially in parallel, giving a total im-pedance which is less than the smaller value.Because of the differences in the internal im-pedances of the transistors, a match is impos-sible, and maximum power transfer betweenstages cannot occur.

This means that the gain of a resistance -capacity coupled multi -stage transistor ampli-fier will be considerably less than that obtainedwith transformer coupling. In some instances, athree or four stage R -C coupled amplifier willbe required to give the same gain as obtainedwith a two stage transformer coupled circuit.

Because there is less difference between theinput and out put impedances, the groundedemitter arrangement is most often used whereR -C or impedance coupling is necessary.

Figure 4-9. Impedances may be matched fairly well by using a grounded collectorcircuit between two transistor cirucits of other types. The grounded collectorcircuit has a high input and moderate to low output impedance.

There is a method of obtaining a reasonablygood impedance match between transistor stages,while retaining the advantages of R -C coupling. . . a grounded collector stage may be used asan impedance matching device between other


amplifier stages. This circuit arrangement isillustrated in Fig. 4-9. In the upper diagram agrounded collector circuit is between two ground-ed emitter circuits. In the lower diagram thereis shown a grounded collector circuit betweentwo grounded base amplifiers.

In either case, the grounded collector circuitserves to match the high output impedance of onestage to the low input impedance of the followingstage.

GENERAL CONSIDERATIONS

Transistor amplifier circuits may be usedquite successfully at audio frequencies. At R.F.,however, special consideration must be given tothe high internal capacities and comparativelylong transit time of transistors. Careful circuitdesign is necessary, and considerably less gainis generally obtained than at audio frequencies.

Where the desired power output is greater thanthat which can be obtained from a single tran-sistor, push-pull and parallel push-pull circuitarrangements may be employed . . . we will dis-cuss such circuits in a later chapter. If evenhigher power outputs are required, the transistoramplifier may be used to drive a vacuum tubestage . . , see Fig. 4-7.

Where distortion and noise must be kept to lowvalues, it is often desirable to adjust each am-plifier stage individually for optimum operation. . . or, where economically feasible, to provideadditional stages and to operate each transistorat well within its maximum ratings.

Generally speaking, the grounded emitter andgrounded base circuits are used where signalamplification is desirable, the grounded emittercircuit being the more popular where junctiontransistors are used, and the grounded base themore widely used where point -contact tran-sistors are employed. The grounded collectorcircuit i s used primarily for impedancematching.


Regardless of the type of circuit employed,the terminology refers only to a circuit type . . .

not necessarily to the element connected to cir-cuit ground. Thus, with the grounded emittercircuit, the emitter is simply common to boththe input and output signal circuits. It need notnecessarily be connected to ground.

51

CHAPTER V

Transistor Oscillator Circuits

An oscillator, basically, is but an amplifier towhich a circuit has been added to provide posi-tive (in -phase) feedback between the input andoutput circuits. It is only natural, therefore, tofind that the characteristics of transistor am-plifier circuits must be considered when design-ing transistor oscillators.

In a transistor oscillator, it is necessary thatthe feed -back circuit chosen serve a multiplefunction -- (a) it must provide an in -phase feed-back signal; (b) it must feed back sufficient signalto overcome circuit losses and thus to start andsustain oscillation; (c) it must serve to matchthe high output to the low input impedance of thetransistor; and (d) in some cases it must selectthe type of feed -back signal and thus determinethe frequency of operation.

Where the oscillator frequency is important,inductance -capacity or resistance -capacity tunedcircuits may be included in the feed -back circuit.Piezoelectric (quartz) crystals may be usedinstead of conventional tuned circuits if unusuallystable operation is required.

There are a large number of possible tran-sistor oscillator circuits, depending on the typeof transistor used,the basic circuit configurationemployed (whether grounded base or groundedemitter), and the means provided to select andfeed back energy between the output and inputcircuits. We will describe a number of the morepopular circuit arrangements in this Chapter.

Remember, however, that the circuits to beshown and described do not represent all thepossible oscillators, but only a representativegrouping.

"Tickler Feedback" Oscillators: A transform-er, T, provides the necessary coupling betweenthe transistor's input and output circuits to start


T

Bias Collector.Voltage Voltage

A

PowerSource

C

Figure 5-1. Three basic "tickler feedback" transistor oscillator circuits. Thegrounded base circuit is shown at A; grounded emitter circuits are shown at B andC. Power supply voltage polarities are not shown, as these will depend on the typeof transistor employed.

and maintain oscillation in the circuits shown inFig. 5-1. Ideally, the transformer should havea turns ratio to match the high output impedanceto the low input impedance.

A grounded base circuit arrangement is shownat A and a grounded emitter circuit at B. Thecircuit shown C is a modification of circuit Band is often used where "blocking oscillator"action is desired. In the grounded base circuit,R1 limits emitter current while R2 limits col-lector current. In the grounded emitter circuit,R1 limits both base and collector current.

Bias and collector voltage polarities are notshown, as these will vary with the type of tran-sistor used . . . refer to the preceding Chapter.

The frequency of operation as well as the signalwaveform obtained will vary with the type oftransformer used, although, in some cases, it ispossible to tune the circuit by connecting a ca-

TRANSISTOR OSCILLATOR CIRCUITS 53

pacitor across the primary or secondary (orboth) windings of the transformer.

When the circuit shown at C is employed, andthe transformer chosen to supply sufficient feed-back energy to cause blocking, the frequency ofoperation is determined primarily by the timeconstant of the R1 -C1 combination.

"Hartley" and "Colpitts" Oscillators: A vacu-um tube Hartley oscillator utilizes a tapped in-ductance coil to provide the necessary feed -backenergy to sustain oscillation. A Colpitts oscil-lator operates in a similar fashion, except thata "tapped" condenser (really two condensers inseries) is employed in place of the tapped in-ductance.

Analogous transistor circuits are shown inFig. 5-2. A tapped coil or "Hartley" oscillatoris shown at A, while "tapped condenser" or"Colpitts" oscillator circuits are shown at B andC. The circuits at A and B use the groundedemitter arrangement, while the grounded basecircuit is shown at C.

ai

A B COLLECTORVOLTAGE

Figure 5-2. Three additional transistor oscillator circuits. The circuit shown atA is somewhat analogous to a vacuum tube "Hartley" oscillator, while those shownat B and C are similar to the vacuum tube "Colpitts" oscillator. Grounded emittercircuits are used at A and B, and a grounded base circuit at C.

The operation of the "Hartley" transistoroscillator is similar to that of the transformercoupled oscillator circuits previously described,except that a tapped coil rather than separate


windings is used to provide the necessary feed-back energy. The value of Ri determines bothbase and collector currents.

The ideal coil to use in this circuit is one withthe tap adjusted to provide the desired high -to -low impedance match between collector and basecircuits. However, oscillation is possible evenif this match is not obtained, although the signalobtained may not be a sine -wave. It is sometimeseven possible to use a center -tapped coil for L.

The "Colpitts" circuits shown at B and C de-pend on the ratio of two capacitive reactancesto establish the necessary impedance match be-tween input and output circuits. In both cases,a "tapped condenser" consisting of C1 and C2 inseries is employed. In practice C2 has approxi-mately ten times the capacity of C I (or, C2 =10C1). The frequency of operation is determinedprimarily by the tuned circuit made up of L,C1 and C2.

In the circuit shown at B, collector currentdepends on the electrical sizes of both Ri andR2. In most instances Ri will be larger thanR2, but the exact ratio will depend on the circuitoperation desired.

Sine -wave operation is possible if Ri and R2are carefully chosen. In addition, these resistorsserve to give a form of d.c. stabilization and thuspermit quite stable operation.

In the circuit shown at C, emitter current de-pends on the value of Ri and collector currenton the value of R2. A tapped power source maybe substituted for individual power supplies inthis circuit, if desired.

A Crystal -Controlled Oscillator: Many tran-sistor oscillator circuits may be adapted tocrystal control if necessary to obtain exceptionalfrequency stability. One such circuit, especiallysuited to junction transistors, is shown in Fig.5-3. Another citcuit, suitable for use with sometypes of point -contact transistors, is shown atC in Fig. 5-4.

Special Common Base Circuits: The circuits


XTAL

POWERSOURCE

Figure 5-3. One of several possiblecrysts3-controlled oscillator circuits for junc-tion transistors. Note that only a single power source is required for this circuit.

Bias Collector Bias CollectorVoltage = Voltage Voltage Voltage

A

C

B

Figure 5-4. Three common base oscillator circuits which are suitable for use withpoint -contact transistors only. The circuit shown at C is crystal -controlled.


shown in Fig. 5-4 can be used where the alpha(current amplification factor) of the transistoremployed is greater than 1.0, and hence are suit-able for point -contact transistors.

The feed -back necessary to start and sustainoscillation is obtained by means of the high im-pedance in the common base circuit. Signalvoltages developed across this impedance by col-lector -base signal currents are thus common tothe emitter -base circuit. R1 and R2 are currentlimiting resistors, with R1 being effective in theemitter circuit, and R2 controlling collectorcurrent.

With a common base circuit, oscillation canbe initiated and maintained in three ways: (a)By using a high impedance in the common basecircuit; (b) By using a low impedance in theemitter circuit; (c) By using a low impedancein the collector circuit.

In operation, the high impedance in the basecircuit of circuit A consists of the tuned circuitmade up of L and C. In circuit B of Fig. 5-4,osoillation depends on the low impedance of theseries circuit (L -C) between the emitter andground. Resistor R3 is included to supplementthe internal base resistance of the transistorand thus to help maintain oscillation. If thetransistor has a high internal base resistance,oscillation may be possible without R3.

Circuit C is quite similar to circuit A in op-eration, except that the frequency is controlledprimarily by the piezoelectric crystal (XTAL).

Blocking Oscillator Circuits: Either of thecircuits shown at C in Fig. 5-1 or at A in Fig.5-2 may be used as "blocking" oscillators by aproper choice of R1, Cl, and the transformer(or coil). As blocking oscillators, these circuitscan supply fairly sharp pulsed signals over widerepetition rates.

When the circuits are adjusted as blockingoscillators, the frequency of operation is de-termined primarily by the time constant of theR1 -C1 combination. Pulse width is determined


by the characteristics of the transformer (orcoil) used.

Blocking oscillator action is generally obtainedwhen the coupling between base and collectorcircuits is such as to provide a large amount offeedback energy. The condenser,C 1, then chargesrapidly through the transistor and dischargesslowly through Ri. During the discharge of thecondenser,little or no collector current can flow.

Average collector current thus depends on thesize of Ri, so that it is often necessary to takeover-all circuit operation as well as frequencyinto account when choosing a particular R1 -C1combination. Several different R1 -C1 combina-tions may result in the same frequency of opera-tion, but only a few of these values may give thedesired circuit stability and economy of operationneeded for a specific application.

In many instances it is quite difficult to calcu-late the values of Ri and C1 in advance and it ismore practicable to determine their sizes ex-perimentally.

58

CHAPTER VI

Special Transistor Circuits

Transistor circuit applications are by nomeans limited to their use as simple oscillatorsand amplifiers. These semi -conductor devicesmay serve equally well as detectors, as clippers,as multivibrators or as phase inverters. With-in their frequency and power limitations, theymay, in fact, perform almost any of the basicjobs normally handled by vacuum tubes.

The electronics engineer, serviceman, student,experimenter or ham may often find it necessaryto use or to work with these more specializedcircuits, either alone or in combination with more

D.C.Input

Rt

Input

Ci

R2

Power

Source

A

C

Output

C2

Power

Source

B

Output

Figure 6-1. Three useful transistor circuits. A d.c. amplifier is shown at A; anR.F. detector or Sine -wave Clipper at B; and a two -stage direct -coupled amplifierat C.

SPECIAL TRANSISTOR CIRCUITS 59

conventional amplifier and oscillator arrange-ments. A brief review of these circuits should,therefore, prove helpful to almost any worker inthe electronics field.

A D.C. Amplifier: A junction transistor, usedin the grounded emitter circuit, may serve as adirect current amplifier. The basic circuit ar-rangement is shown at A in Fig. 6-1.

In operation, a small direct current applied tothe base -emitter circuit can control a muchlarger current in the collector -emitter circuit.In practice, current amplifications of ten ortwelve to one are comparatively easy to achievewith standard commercial transistors. A base -emitter current of 100 microamperes may re-sult in a current through the load of 1 milliampereor more.

The load need not necessarily be a resistor,choke or similar impedance but may well be arelay, meter, small motor or any similar elec-trical device. It is therefore possible to use atransistor in this circuit to increase the effectivesensitivity of a relay or meter by a factor of 10:1or better.

Applied d.c. control voltage and power sourcepolarities will depend on the type of transistorused and on the action desired.

An R.F. Detector: The input circuit of a tran-sistor amplifier, when the transistor is operatedwithout external bias current, acts like a simplecrystal diode. Thus, it will serve to rectify anapplied a.c. voltage or to "detect" an amplitude -modulated R.F. signal. A suitable circuit isshown at B in Fig. 6-1.

In operation, the detection action is like thatobtained with any diode, whether a vacuum tubeor a semi -conductor device. Since the diodepermits current to flow easier in one directionthan in the other, either the positive or the neg-ative going "half" of the applied signal will beremovedby simple rectification, and a pulsatingdirect current flow will occur through the diode. . . .in this case, the base -emitter circuit of


the transistor. The peak value of this pulsatingdirect current will vary in accordance with themodulation of the original R.F. signal.

The rectified R.F. signal thus contains bothan a.c. component (the audio modulation) and ad.c. component (its average value). The d.c.component serves to "bias the transistor andpermits the a.c. component to be amplified, ap-pearing as an audio signal voltage across thecollector load impedance. Although there maybe R.F. pulsations present in the original de-tected signal, these are generally "lost" due tothe high internal capacities and poor high fre-quency response of the transistor. If this doesnot occur, however, and R.F. pulsations are stillpresent in the collector circuit, they may be re-moved by adding a small R.F. by-pass capacitorbetween the collector terminal and ground.

This basic circuit thus performs two functionssimultaneously. . .it detects an applied R.F. sig-nal and then amplifies the resulting audio signal.

A Sine -Wave Clipper: The R.F. Detector cir-cuit just described (B in Fig. 6-1) may also beused as a sine -wave clipper.

In operation, when a pure sine -wave signal ofsufficient amlitude is applied to its input, the"diode action' of the base -emitter circuit re-moves either the positive or the negative peaks

whether the positive or the negative peaksare removed, in a particular case, will depend onthe type of transistor used.

The opposite peaks are limited by the voltagedrop across the collector load resistor. . . .thatis, when collector current reaches the pointwhere the entire supply voltage is dropped acrossthe collector load, no further increase is possible,even though the applied signal may continue torise in amplitude.

Thus, the net result is that both "halves" of theapplied signal are clipped. With proper design,this circuit is capable of supplying good qualityrectangular waves when driven with a sine -waveof sufficient amplitude. If the applied sine -wave

'4


is not of sufficient amplitude to limit collectorcurrent, then only one-half of the signal will beclipped and a rounded, rather than a rectangular,output signal will be obtained.

Since the transistor also serves to amplify theapplied signal, this clipper circuit is unique inthat the output signal, though "clipped", may beof greater amplitude than the applied sine -wave.

A Direct -Coupled Amplifier: If the amplitudesand polarities of all d.c. voltages and currentsare taken into account, and proper componentvalues are selected, it is entirely practicable todirect -couple transistor amplifier circuits. Onepossible arrangement is shown at C in Fig. 6-1.

In this circuit a common collector stage isdirect -coupled to a grounded emitter amplifier.Note, too, that the circuit arrangement permitsthe use of a single power source.

In operation, the voltage divider made up of Riand R2 provides a source of "bias" current forthe common emitter stage. R3 serves as the loadimpedance for this stage.

Resistor R4 serves as the load resistor forthe grounded emitter stage and the output signalis developed across this component. Bias cur-rent, in the second stage, is obtained by direct -coupling its base to the emitter of the precedingstage, and providing R5, by-passed by C2, tolimit this current to the proper value.

Phase -Inverter Circuits: For the most part,transistor phase -inverter circuits are analogousto their vacuum tube counter -parts. Two popu-lar types are illustrated in Fig. 6-2.

A transformer is used in the circuit shown atA. In order to operate the circuit with a singlepower source, "bias" current is provided by avoltage divider consisting of Riand R2.

The circuit shown at B in Fig. 6-2 is an adap-tation of the well known "split -load" vacuum tubephase - inverter circuit. Load impedances areplaced in both the collector (R3) and emitter (R4)circuits simultaneously. Bias current is suppliedby the R1 -R2 voltage divider.


T

PowerSupply

A

PowerSupply

Gnd.

2

Figure 6-2. Transistor phase -inverter circuits. Note that only a single powersource is required for either circuit.

Although the two load impedances, R3 and R4,are generally the same size, this circuit does notsupply a perfectly balanced output. . . .that is,the amplitude of the signal at "1" is not exactlythe same as the amplitude at "2". However, thebalance is close enough for most practical work. . .and especially if the circuit is used to drivea fairly linear amplifier.

Push -Pull Amplifier Circuits: Whenever acircuit designer requires a signal amplitude orpower output greater than can be obtained froma single transistor, he generally uses two ormore transistors in a push-pull arrangement.Three suitable circuits are shown in Fig. 6-3.

F = 0. C

A

4- 0_ +- 0

B

4- 7 O.

4- 7 0

Figu

re 6

-3.

Push

-pul

l tra

nsis

tor

circ

uits

.T

he c

ircu

its s

how

n at

B a

nd C

req

uire

but a

sin

gle

-end

ed in

put s

igna

l.T

he c

ircu

it sh

own

at C

can

dri

ve a

low

impe

danc

elo

ad d

irec

tly.

C

a) 03


A "standard" push-pull circuit arrangement isshown at A. With this circuit, both transistorsmay be of the same type and a balanced inputsignal is required. A single power source isneeded. The output load will generally be acenter -tapped transformer coupled to some otherdevice. . .such as a loud -speaker voice coil.

The push-pull circuit arrangements shown atB and C offer the advantage of requiring a single -ended input signal. . .it is not necessary to usea phase -inverter ahead of these circuits as isthe case with the circuit shown at A. However,for these two circuits to operate properly, it isnecessary that different type transistors be em-ployed, as shown in the diagrams. Appropriatepower supply polarity connections must also bemade, of course, so that either two power sup-plies or a tapped power source is needed.

The circuit shown at C offers the additionaladvantage of being capable of driving a low im-pedance load directly.

Both of these circuits operate by virtue of theopposite polarity of the transfer characteristicsof different type transistors. Thus, the outputsignals obtained from the two transistors are180° out -of -phase even though the same inputsignal is supplied to both. This characteristicis known as the complementary -symmetry prin-

Pulsein

Sowtooth R4Output

PowerA B

Figure 6-4. The transistor circuit shown at A can supply linear saw -tooth signals.A transistorized "flip-flop" circuit is shown at B.


ciple.A Saw- Tooth Oscillator: The linear saw -tooth

type of signal used as a sweep in oscilloscopesand television receivers may be obtained in var-ious ways from transistor circuits. One simplearrangement is shown at A in Fig. 6-4.

In operation, sufficient feed -back signal is sup-plied by tapped coil, L, to operate the circuit asa blocking oscillator. During the period when thetransistor is blocked, condenser C2 chargesslowly through R2. The condenser is then dis-charged rapidly by the pulse of collector currentduring the short "unblocked" period.

In order to insure a linear output signal, thetime constant of the R2 -C2 circuit is made longcompared to the repetition time of the oscillator.

A "Flip -Flop" Circuit: The transistor circuitshown at B in Fig. 6-4 has two stable conditions.It may be transferred from one condition to theother by the application of a pulse signal of theproper polarity to the appropriate base. A sin-gle power source is required.

In operation, one or the other of the two tran-sistors will start conducting first. Let us say,for purposes of illustration, that transistor IIstarts first. If this is the case, the collector -emitter impedance of the transistor is very low,and the potential at the juncture of R2 and R3 isvirtually the same as at the top of R7.

This means that little or no bias current canflow in the base -emitter circuit of transistor Iso that its collector -emitter circuit remains inanon -conducting (or high -impedance) condition.With the collector -emitter circuit of transistorI offering a high -impedance, there is virtuallyno voltage drop across RI, and the potential atthe juncture of RI. and R5 is sufficient to main-tain a bias current flow in transistor II. . . .andthis transistor stays in its low -impedance con-ducting condition.

Suppose, now, that a signal pulse of the correctpolarity and of sufficient amplitude to start tran-sistor I conducting is applied to its base. Tran-sistor I will then change from acting as a high-


impedance to acting as a low -impedance, andthere will be an appreciable voltage drop acrossR1. This means that there is no longer sufficientpotential applied through R5 to keep the base -emitter bias current of transistor II flowing, andthis transistor changes rapidly from a conductingto a nonconducting condition.

A B

Figure 6-5. Transistor multivibrator circuits. The one shown at A may be usedwith point -contact transistors; the one. shown at B is suitable for use with junctiontransistors.

Once this action has taken place, the circuitremains stable (but now transistor I is conduct-ing) until a pulse of the opposite polarity is appliedto the base of transistor I. . . . in such a casethe circuit returns to its original condition.

Since this circuit has two stable conditions, itis useful in switching and in counting operations.It is seldom used alone, however, but, more often,in conjunction with other circuits.

Multivibrator Circuits: Two transistorizedmultivibrator circuits are illustrated in Fig. 6 -5.The circuit shown at A uses a single transistorand requires a minimum of components. How-ever, since this circuit depends, for its operation,on the coupling obtained through the commonbase impedance, Ri, the transistor used musthave an alpha greater than 1.0. . . .therefore,this circuit is primarily of value when used withpoint -contact transistors.

The multivibrator circuit shown at B is analo-gous to a conventional vacuum tube plate -coupled


Input I

Input 2C2

MEWIID

BIASVOLTAGE COLLECTOR

VOLTA GE

utput

Figure 6-6. A mixer or modulator circuit suitable for use with a point -contacttetrode transistor. The tetrode transistor has two emitter terminals.

CREDITI Circuit Courtesy SYLVANIA.

circuit. It consists, essentially, of two groundedemitter amplifier stages with resistance -capa-city coupling between the input and output cir-cuits. A single power source is required. Junc-tion transistors will work well in this circuit.

A Tetrode Mixer Circuit: The circuit shownin Fig. 6-6 will serve well as a modulator ormixer and is designed to utilize a tetrode point -contact transistor. Note that the common basecircuit is employed.

In operation, signals are fed simultaneouslyto the two emitter contacts, with the combinedsignal being obtained across the collector loadimpedance.

Other Circuits: The specialized transistorcircuits described in this Chapter do not, by anymeans, represent all possibilities. Rather, theyserve as illustrations of what can be done withtransistors and how transistor circuits may bedevised to accomplish jobs performed, in thepast, by vacuum tube circuits alone.

As new transistor types are produced and asnew applications are found for these components,additional specialized circuits will undoubtedlybe developed, as well as new combinations of thetried and proven circuit arrangements.


RCA2N33

t

RCA2N35

1CA33

RCA2N32

RCA2N34

Figure 7-1. Commericaily available transistors are so small that several may beeasily contained within a standard sized thimble.

CREDIT: Courtesy RCA.

69

CHAPTER VII

Transistor Components

Most commercially available transistors areso small physically that several may be easilyfitted into a standard sized thimble. See Fig.7-1. Their small size ideally suits them to theconstruction of sub -miniature radio and elec-tronic equipment. However, since the transistorsthemselves represent only a part of any practicalelectrical circuit, it is not surprising to find thata number of manufacturers are producing com-paratively sized transformers, capacitors andother small components for use in transistorcircuits.

Not all sub -miniature components are designedexclusively for transistor use, of course. Forexample, sub -miniature "in -line" tube socketsare made by the majority of socket manufac-turers. Although designed originally for vacuumtubes, they are suitable for use with all tran-sistors in which the standard lead connectionsare employed . . . see Chapter III. As can beseen in Fig. 7 -2,these sockets are not appreciablylarger then the transistor itself and, even withthe transistor inserted, may be easily "lost" inthe palm of the hand.

Sub -miniature carbon potentiometers, used asvolume and tone controls, have also been madefor vacuum tube circuits, and especially for usein hearing aids. These controls, in the properresistance values, are quite suitable for use intransistor circuits. The small size of these com-mercially available potentiometers is apparentfrom the over-all dimensions of a typical con-trol - exclusive of terminal leads, but includingthe knob, one unit measures less than 3/4" indiameter by approximately 5/16" thick, a ndweighs less than 1/4 ounce!

TRANSFORMERS: The amount of iron neededin the core of a transformer is dependent uponseveral design factors. However, two of the most


Figure 7-2. A standard "in -line" sub-minature tube socket may be used for alltransistors employing the standard lead connections. This photo shows the smallsize of the socket and transistor the transistor shown is a point -contact unit.

CREDIT: Courtesy General Electric.

important are the amount of direct current flow-ing in the transformer's winding (in its intendeduse) and the power to be handled by the unit. Aswe have seen, both factors are quite small intransistor circuits. It is possible, therefore, tobuild extremely tiny iron -core "transistor"transformers.

One such unit is shown in Fig. 7-3, where it iscompared in size with a standard miniature

TRANSISTOR COMPONENTS 71

transformer. Over-all dimensions of the tran-sistor transformer are approximately 3/8" x3/8" x 3/8", while the weight is approximately1/10 of an ounce. The power handling capacityof such units is on the order of 1 milliwatt or less.

Figure 7-3. A transistor transformer is compared to a conventional "miniature"transformer in this photo. As may be readily observed, the transistor transformeris much smaller than the conventional unit.

CAPACITORS: Because of the low -impedancesencountered in transistor circuits, large valueby-pass and coupling capacitors are virtually anecessity. Fortunately the low d. c. voltagesnormally employed in transistorized equipmentmake it quite feasible to use low working voltageunits, and several manufacturers have designedsmall electrolytic condensers having fairly largecapacities.


One commercial unit measures only a littlemore than 1/8" x 3/4" over-all, exclusive ofleads, yet is rated at 2 mfd at 6 v. d.c.

Another type of small capacitor that is becom-ing increasingly popular are the extremely smalltantalum "electrolytics". These capacitors con-sist of a porous tantalum anode permanentlysealed into a small container, with a silver cath-ode and the electrolyte. In some commercialunits, the silver cathode also serves as a caseand as a container for the electrolyte.

STANDARD ELECTROLYTIC

Tantalum Capacitor

SIDE VIEW END VIEW(Scale Drawing )

Figure 7-4. A miniature tantalum capacitor is compared to a conventional standardsized electrolytic condenser in this scale drawing. Tantalum capacitors are widelyused in subminiature equipment employing transistors.

The small size of these units is illustratedgraphically in Fig. 7-4, where a typical com-mercial tantalum capacitor is compared with astandard sized electrolytic condenser. Over-alldimensions,exclusive of leads,are approximately5/16" x 31/64" . . . the capacitor occupies lessthan 1/10 of a cubic inch!

Electrically, most tantalum capacitors aresuperior to conventional electrolytics. For ex-ample, the manufacturer of the capacitor de-scribed offers these units over the range from1/5 mfd. to 30 mfd., with d.c. working voltagesfrom 125 to 6 v.d.c. The maximum d.c. leakageof the higher voltage capacitors, even in hightolerance grades, does not exceed 6.0 micro-amperes, while the low voltage units have amaximum leakage of 2.0 microamperes, or less.The normal operating temperature range of tan-talum capacitors is exceptionally good . . . from-55° C to 85° C at rated working voltages, with


operation as high as 1000C possible if the work-ing voltage is derated 15%.

Small paper, ceramic, and mica condensersmay also be encountered in transistorized equip-ment. Except for their physical size and workingvoltages, they are generally similar to the morefamiliar units employed in conventional elec-tronic equipment.

TRANSISTOR POWER SUPPLIES

The truly minute power requirements of tran-sistor c ircuit s make it economically practicableto operate transistorized equipment with bat-teries, even where the equipment may be subjectto almost continuous use. Either conventionalzinc -carbon or the newer mercury batteries maybe found in use in such equipment.

Zinc -carbon batteries are quite familiar tothe electronics technician, being used in portableradio receivers, in flashlights, in "breadboard"experimental circuits, in ohmmeters, in hearingaids, and in many other types of equipment.

In transistor circuits, the lower voltage zinc -carbon batteries, such as pen -light cells, flash-light cells, and bias batteries, may be used bothas bias and collector voltage power sources. Themedium voltage "B" batteries, supplying from15 to 30 volts, may be used as collector voltagesources. Small Hearing Aid Type "B" batterieswill more often be used in sub -miniature equip-ment, or where intermittent operation -is antici-pated. The higher voltage "B" batteries may beused most often in circuits employing point -contact or special power transistors.

The mercury cell, though more costly thansimilar zinc -carbon batteries, offers severaladvantages over them in many applications.Mercury cells are generally more stable andmore rugged, both mechanically and electrically,than their zinc -carbon counterparts. They havea much longer shelf life and a greater capacitythan similar sized zinc -carbon batteries and thus


TR-133 R$-12 3/1-502

r650

321-4

190

AM -3

.91110

T3-132 11111-401 TR-120

MO -6211

Figure 7-5. A number of commercially available mercury batteries. Several oithese are designed specificallyfor use as transistor power- supplies. The dimensionsshown are in inches.

CREDIT: Courtesy P.R. Mallory & Co.

are exceptionally well -suited to the design ofsub -miniature equipment. Because of this, mer-cury cells will undoubtedly find wide applicationin transistorized equipment.

Unfortunately, the average electronics techni-cian, unless he has worked with modern hearingaids, sub -miniature equipment, or a few special-ized types of military equipment, may never haveencountered mercury cells in his work and,therefore, may be unfamiliar with them. A num-ber of typical mercury cells and batteries areshown in Fig. 7-5. Several of these have beendesigned specifically to serve a s transistorpower supplies.

Mercury cells produce electrical energy byan electrochemical reaction between zinc andmercuric oxide, with a potassium hydroxide so-lution used as the electrolyte. The anode maybe a pressed pellet of amalgamated zinc powderor a winding of thin corrugated zinc strip. Thecathode consists of mecuric oxide to which asmall amount of micronized graphite has beenadded. The case of mercury cells is generally


of steel. Due to the high stability of the chemi-cals used in mercury cells and the inactive casematerial, there is virtually no internal cell re-action until electrical energy is drawn from them.

JO

Figure 7-6. A 5U4 -G power rectifier tube (left) is compared to a complete A.C. -operated d.c. power supply for transistors.

A.C. POWER SUPPLIES: A.C.-operated d.c.power sources designed to supply transistorcircuits are generally as much smaller thantheir vacuum tube counterparts as transistorsare when compared to tubes. This fact is visiblydemonstrated in Fig. 7-6, where a vacuum tuberectifier,of the type used in public address am-plifiers and television receivers, is comparedto a complete d.c. power supply designed for ex-perimental transistor amplifier circuits.

At the to -poi the transistor power supply, nearthe output terminals, are two by -pass condensersfor the input and output circuits of the amplifierwith which the power supply is used. Immedi-


ately below are two potentiometers used for ad-justing bias and collector voltage . . . or thevoltage to the "high -side" output elements of theamplifier. Near the center of the unit are thefilter capacitors and filter resistor and at thelower left is the power transformer. At the right,opposite one of the filter capacitors, is the "powerrectifier," which is a high -back -voltage type ofgermanium crystal diode . . . it is interestingto compare the size of this "power rectifier" withthe 5U4 -G vacuum tube shown, for both unitsoccupy comparable positions in their respectivecircuits.

Output

Bias Ground Element

Filter

CrystalRectifier

(3aFigure 7-7. The schematic. diagram of the transistor d.c. power supply shown inFigure 7-6.

The circuit for this power supply is shown inFig. 7-7. The rectifier is connected to provide,at the bias terminal, a positive potential withrespect to circuit ground, and, at the output ter-minal, a negative voltage with respect to ground.The electrolytic by-pass and filter condensersused must be connected to suit circuit polarities. . . thus, were the d.c. output voltage polaritiesto be reversed, it would be necessary to reversethe connections to all capacitors (and to therectifier). Such a change might be made in thecircuit if it were to be used with different typetransistors.

Power supplies designed to operate vacuum


tube circuits are generally built to supply a con-stant output voltage so that variations in signalcurrents in theconnected circuits will not causeexcessive changes of plate, screen and grid biasvoltages. Similar supplies are sometimes usedwith transistor circuits, where provision is madefor current limiting resistors in the individualstages.

However, with many circuit applications oftransistors (especially point -contact units) theapplied potentials are of such polarity as mightcause excessive current through the low imped-ance side, were these supply potentials to remainconstant regardless of current. That is, thetransistor acts like a rectifier to which voltage isapplied in the forward or low -resistance direc-tion, and possible destruction may occur due toexcessive current . . . unless the supply voltagedrops with increasing current drain.

This condition requires that the d.c. powersupply used be designed to furnish a nearly con-stant, or, at least, a limited, current. This isaccomplished, in practical circuits, by makingd.c. resistances in the power supply and otherseries resistances much greater than elementresistances in the transistor itself. Thus, shouldthe current drain tend to become excessive, suf-ficient voltage drop will occur across the variousseries resistances to effectively limit the voltageapplied to the transistor, and hence the currentthrough it.

78

CHAPTER VIII

The Care and Servicing of Transistors

Transistors, although generally more ruggedmechanically than vacuum tubes, are still com-paratively easy to damage by improper treatmentor electrical overload. Many of the earliertransistor experimenters have learned this thehard way, at the cost of one or more expensivetransistors.

While the technician who has acquired con-siderable experience working with transistorsmay take many liberties in using them, knowing,almost subconsciously, how to avoid damage,the beginner, be he engineer or student, techni-cian, ham or serviceman, will find it worthwhileto keep certain general precautions in mind atall times. A number of these are listed below:

a. Do not overheat transistors or their leads. . . excessive heat may easily cause permanentdamage. Where practicable use sockets whenbuilding transistorized equipment and do not in-stall the transistors until all wiring is completedand checked. If it is necessary to solder tran-sistors directly into the circuit, however, com-plete the soldering as quickly as possible. Usea clean, hot, well -tinned iron and good qualitysolder. Don't cut the transistor leads any shorterthan necessary. For maximum protectionhold the lead being soldered with a pair of longnose pliers. The pliers should be between thepoint where heat is applied and the body of thetransistor. Used in this fashion, the pliers forma heat "sink", conducting heat away from thetransistor itself.

b. Do not exceed the "absolute maximum elec -trical ratings . . . transistors are generally notunder -rated, and the absolute maximum ratingsspecified by a manufacturer mean exactly that.

THE CARE AND SERVICING OF TRANSISTORS 79

While these ratings may sometimes be momen-tarily exceeded in experimental work withoutapparent damage to the transistor involved,another unit might be hopelessly damaged by thesame overload. There is sufficient variation intransistors that a given overload may not injureone unit, may change the characteristics slightlyof another unit, and may completely ruin a third.

If the transistors are to be used in equipmentexposed to higher than normal temperatures, itmay be necessary to operate them at a fractionof the maximum values . . . such a step will notonly permit increased life but also better circuitoperation and improved stability.

c. Observe proper power supply polarities ...this point has been mentioned earlier, but isworth repeating. Reversing the plate voltagepolarity of a triode vacuum tube will keep thestage from operating but will generally not in-jure the tube . . . but reversing the collectorvoltage polarity of a transistor will often ruin it,instantly and permanently.

Since different types of transistors requiredifferent power supply connections,the technicianworking with these components must always be"on his toes". Whenever there is any questionconcerning the connections to a particular unit,the manufacturer's specification sheet shouldbe carefully checked ... before the transistor isinstalled in the circuit, and before power is ap-plied!

d. Guard against high transient current orvoltages . . . this precaution must be observedboth When working with new circuits and whentesting or servicing old equipment. A damagingtransient pulse may be caused in a number ofways . . . by the application of a signal having asharp leading or trailing edge, by inductive kick-back, by circuit switching, and even by the mereprocess of turning the equipment "on" or "off."

When working with new equipment or circuits,


carefully study the schematic for transient pos-sibilities before making final connections. Ifpossible, use a variable voltage power supplyand increase the operating voltages slowly fromzero to the desired operating values.

When working with built-up and tested equip-ment, do not insert into or remove transistorsfrom their sockets with the power "on". If thecircuit application requires an interchange oftransistors with the power on, provision shouldbe made for making the base connection first.And avoid all test techniques or methods thatmight result in sharp voltage transients in theequipment.

TESTING TRANSISTORS

The serviceman will have to rely on some ofthe following techniques for checking tran-sistors.

In some cases, it is possible to tell that atransistor is defective from the circuit operationobtained. For example, if the collector currentruns very high in a particular circuit using thegrounded emitter arrangement, and changes inbase current seem to have little or no effect onit, there is a good possibility that the transistoris defective.

Another technique some technicians use is toconsider the transistor as equivalent, as far asd. c. resistances are concerned, to two diodesconnected "back-to-back". An ohmmeter is thenused to check "forward" and inverse resistancebetween base and emitter, and between base andcollector. In each case there should be an ap-preciable difference between the "forward" andthe inverse resistance measurements . . . justas is the case with conventional crystal diodes.

If the two resistance measurements are simi-lar in either (or both) case, there is a good chancethat the transistor is defective.

Still another technique, perhaps the most ef-fective of all, is the old "stand-by" of substitution...that is, trying another transistor of the same


type in the circuit. However, this techniqueshould only be used after the serviceman hasassured himself that there are no other circuitdefects . . . especially those that might damagea transistor.

The experimenter will find a variation of thislast technique useful, especially where he usesa limited number of transistor types. A simpleamplifier circuit may be permanently wired, withterminal connections for checking d.c. currentsand voltages, and the input and output a.c. sig-nals. A socket is provided for the transistor.

To check a transistor, it is inserted in thesocket, instrument connections are made, andpower is applied. Operating currents are checkedand compared to average" values as specifiedby the transistor manufacturer. Finally, an a.c.signal is applied to the input and stage gainchecked. If the d.c. values are not too far off,and if reasonable signal gain is obtained, thetransistor may be considered "good" ... at leastfor most practical work.

Where the circuit in which the transistor is tobe used is especially critical, the only real testis to actually try the transistor and to checkoperation.

SERVICING TRANSISTORIZED EQUIPMENT

The serviceman need not anticipate specialdifficulties when he is eventually called on torepair transistor operated equipment. In general,he will find that the techniques he has used inthe past may be applied, with but little modifica-tion, to the servicing of such equipment.

Transistorized equipment should normally re-quire less servicing than equipment in whichvacuum tubes are used . . . assuming that gooddesign practices have been followed by the manu-facturer and that good quality parts are used,and, of course, that the equipment has not beendeliberately mistreated. There a r e severalreasons for this.

First, transistors themselves should have a


STEP 1Check OperationConfirm ComplaintCheck on MistreotmentCheck on Power Supply

STEP IIAnalyze Operation

STEP IIITest to IsolateDefective Section

STEP IVTest to IsolateDefective Stage

STEP VTest to IsolateDefective Part(s)

STEP VIReplace DefectivePart Carefully

STEP VIIFinal Test andAdjustment(If Neccessary)

Figure 8-1. A block diagram illustrating the steps to be followed when servicingtransistorized equipment. The same steps are followed, generally, when servicingall types of electronic equipment.


much longer life than vacuum tubes. In sometypes of equipment, in fact, the transistors usedshould last as long as the normal service life ofthe equipment itself.

Secondly, very little heat is generated in com-pletely transistorized equipment . . . this meanslower operating temperatures with correspond-ingly longer life for electrolytic condensers,insulation, resistors, batteries, and similarelectrical components.

And, finally, transistorized equipment gener-ally operates with much lower d. c. potentialsthan are found in vacuum tube circuits. Thismeans that less strain will be placed on insulationand components with reduced chances of voltagebreakdown.

The basic servicing procedure applicable toall types of electronic equipment, whether tran-sistorized or not, is shown in block diagram formin Fig. 8-1. In following this general procedure,the serviceman, at all times, should keep in mindthe general precautions outlined in the first sec-tion of this Chapter, and should exercise specialcare in every step where physical work with theequipment is involved. Let us review, briefly,the various steps in the servicing procedure.

STEP I: When the equipment is first broughtto the serviceman for repair, he should checkits operation and confirm, for himself, the com-plaint. He should also check on the possibilityof mistreatment - for example, if a transistorizedhearing aid were left in a closed car on a hotsummer day for any period of time, the temper-atures encountered could very easily change thecharacteristics of the transistors . . . perhapseven ruining them completely.

In addition, since batteries are used in manytypes of transistorized equipment, he shouldcheck the power supply at this point ... particu-larly if the complaint is "dead" or "fails to op-erate".


STEP II. From the nature of the complaintand from his knowledge of transistor circuitoperation, the serviceman should next apply rea-soning to the problem. He should briefly review,mentally, the defects that might cause the com-plaint encountered and where in the circuit thedefect is likely to be located. This same rea-soning procedure should be reapplied at eachstep in the servicing procedure as new informa-tion is acquired.

Where the serviceman has worked with similarequipment in the past, he may be able to deter-mine the defect at this point, perhaps by makingone or two simple confirming tests. He can thengo directly to the "heart" of the trouble, skippingall intermediate steps.

STEPS III, IV, V: The serviceman should nextapply conventional servicing techniques,choosingthose techniques best suited to the equipment be-ing serviced and to the nature of the complaint.Signal Tracing, Signal Injection, Parts Substitu-tions, Resistance Tests, Voltage Measurements,and similar test methods may all be employed.The defective section is generally isolated first,then the stage, and finally the defective part it-self.

When carrying out tests, care must be takenthat no test method is employed which may causedamaging voltage or current transients. For thisreason, Circuit Disturbance tests should gener-ally be avoided, and Parts Substitutions shouldnot be made with the power "on".

STEP VI: When replacing the defective part(or parts) the serviceman must be especiallycareful not to overheat transistors or their leads.He should also make sure that proper connec-tions are made for all "polarized" components. . . he may sometimes find, for example, thatthe positive lead of electrolytic condensers isgrounded.


STEP VII: Once the defect is repaired, theserviceman should carefully recheck the opera-tion of the equipment and make any final adjust-ments (or alignment) that may be necessary.

86 PRACTICAL TRANSISTOR CIRCUITS

Figure 9-1. A transistor code practice oscillator. Parts values are as follows:

TR1 - RAYTHEON type CK722 PNP junction transistor.Ti - Audio interstage transformer, 3:1 turns ratio.RI - 15K rheostat ... adjust for optimum operation.Swi - SPST "Off -On" Switch.131 - 1.5 volt bias battery.152 - 3 to 15 volt collector voltage battery.PHONES - 2000 ohm headset.

CREDIT: Circuit Courtesty RADIO & TELEVISION NEWS.Adapted from "A Translator Code Practice Oscillator".

87

CHAPTER IX

Practical Transistor Circuits

The experimenter, student, ham, and practicalengineer will often find that considerable timecan be saved when designing and wiring experi-mental circuits if typical component parts valuescan be obtained. In order to help these workers,a number of characteristic circuits illustratingpractical applications of transistors will beshown and described in this chapter.

These circuits have been obtained from anumber of sources and may not, in every case,represent optimum final design. The readershould, therefore, use the suggested circuits asa guide only, making any experimental modifi-cations as may be necessary to suit special con-ditions or his own individual requirements.However, if the reader has not previously workedwith transistors, he should exercise special carewhen wiring, testing, or operating any of the cir-cuits shown. A review of the suggestions givenin the preceding Chapter may be found helpful.

For convenience in listing, the various circuitshave been divided into three groupings:- AUDIOCIRCUITS, R.F. CIRCUITS, and GENERAL EX-PERIMENTAL CIRCUITS.

AUDIO CIRCUITS

A Code Practice Oscillator: A grounded basearrangement is used in the transistor audio os-cillator shown in Fig. 9-1. This oscillator maybe easily assembled by a skilled technician in afew hours time. The circuit is non -critical andmay be adapted not only for use as a code prac-tice oscillator, but as a general-purpose audiosignal source.

To adjust the completed oscillator for opera-tion, first set R1 for maximum resistance. Next,close the "OFF -ON" switch and hold the key


down, listening to the earphones. Gradually re-duce the resistance of Ri until the desired op-eration is obtained. The size of this resistor willeffect both tone and power output. However, donot use values below 2000 ohms for R1normal operating values should be in the rangefrom 2500 to 12,000 ohms.

If oscillation does not occur, try reversingthe connections to either the primary or thesecondary windings of Ti. Since this transformerserves to feed back the electrical energy neededto start and maintain oscillation, it is extremelyimportant that the lead connections be such thatan in -phase feed -back signal is obtained.

The size (voltage) of B2 is not too important,and satisfactory operation of this,pircuit has beenobtained with from 3 to 15 volts collector voltage.However, the value of R1 is less critical, andbetter stability and greater power output are ob-tained with the higher voltages.

In some cases, this oscillator will supply suf-ficient power to operate a small loudspeaker.Simply connect the primary winding of a standardaudio output transformer in place of the magneticheadset. The secondary winding is connected tothe voice coil of a PM loudspeaker . . . any sizespeaker may be used, but best results are ob-tained with a 6 or 8 inch speaker, or larger.

A Phonograph Amplifier: The circuit shownin Fig. 9-2 gives a moderately good signal-to-noise ratio and is suitable for use in a smallrecord player. A 10" or 12" PM loudspeakershould be used.

As can be easily seen by reference to theschematic diagram, this circuit uses a singlepre -amplifier stage, transformer coupled to apush-pull output stage. Grounded emitter circuitarrangements are used throughout. TransformerT 1 is used primarily for phase inversion andhence its characteristics are not too critical.

Once the circuit is completed and tested, biascontrol R3 should be adjusted for optimum op-eration . . . that is, for maximum power output

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A Transistor Hearing Aid: The transistorizedhearing aid circuit given in Fig. 9-3 may be as-sembled as quite a small unit if the recommendedcomponents are used and if care is taken in layoutand wiring. As with any multi -stage audio am-plifier, the layout chosen should be as "clean"and open as practicable, with ample separationbetween the input and output leads of individualstages as well as of the amplifier as a whole.

Small rheostats should be used in place of R1,R2 and R3 during the preliminary assembly ofthe circuit. These rheostats should then be ad-justed for optimum operation with the batteryvoltage used. Thot is, to give the best compro-mise between low background noise, maximumgain, and minimum distortion. Once the valuesof R1, R2 and R3 are determined in this fashion,fixed resistors mby be installed.

Either tantalum capacitors or sub -miniatureelectrolytics should be used for the interstagecoupling condensers, C1, C2 and C3. For longbattery life, a mercury cell is the best choicefor B1, although conventional zinc -carbon bat-teries will give satisfactory results.

If desired, a high impedance (2000 ohm) mag-netic earset may be substituted for the PHONEspecified, and T4 omitted. Simply connect thehigh impedance phones in place of the primarywinding of T4.

R. F. CIRCUITS

A Crystal -Controlled TransistorTransmitter:The oscillator circuit shown in Fig. 9-4 uses apoint -contact transistor in a grounded base ar-rangement. 'Phone operation is obtained bymodulating the collector current, using a carbonmicrophone. This circuit is suitable for com-munication only over very short distances.

Inductance coil L1 is made up by winding ap-proximately 16 turns of # 24 wire on a 3/8"diameter form. The antenna is inductively cou-

PRACTICAL TRANSISTOR CIRCUITS 91

2C

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pled to this coil through a few turns wound on thesame form. The amount of coupling to employas well as the exact number of turns to use forthe antenna winding may be best determined ex-perimentally after the circuit is wired.

RFC

R1

Figure 9-4. A crystal -controlled transistor transmitter. Parts values are as follows:

XTAL - 7 Mc crystal.TR]. - CBS-HYTRON type PT -2A or PT -2S point -contact transistors.RFC - Small radio frequency choke.RI - 1K carbon resistor.Ci - 220 mfd.C2 - .001 mfd.L1 - Approx. 16 turns # 24 wire on 3/e D. form.M - Carbon microphone.Sw1 - SPST "Off -On' switch.131 - 12 volt battery.

CREDIT: Circuit Courtesy C BS -HYTRON

A 50 -Mc Oscillator: A specially designed andselected point -contact transistor is used for theoscillator circuit given in Fig. 9-5. The poweroutput obtained from this circuit, though quitesmall (approximately 1 milliwatt), is ample formany types of experimental work.

A Broadcast -Band Radio Receiver: The radioreceiver circuit shown in Fig. 9-6 requires agood antenna and ground for best performance.Two transistors are used, with the groundedemitter circuit arrangement being employed in


R

Figure 9-5. A 50-mc transistor oscillator circuit. Parts values are given below:

TR]. - RCA type 2N33 point -contact transistor.- 0.46 microhenry inductance.

L2 - 1 millihenry R.F. choke.C1 - 1 mmf ceramic condenser.C2 - 4-30 mmf adjustable ceramic condenser.C3 - 270 mmf mica condenser.C4 - 470 mmf mica condenser.RI - 5100 ohm, 1/2 watt, carbon resistor.R2 - 1K, 1/2 watt, carbon resistor.BI - Power supply battery .. not over 8 volts.

CREDIT: Circuit courtesy RCA.

both cases. TR1 serves as a detector -amplifier,with its output audio signal, appearing acrossVOLUME control Ri, coupled through C5 to theinput of TR2 which, in turn, operates as a con-ventional amplifier stage. In order to obtainreasonable selectivity in the broadcast band (550to 1500 Kc), two tuned circuits are employed.

When assembling the circuit, a standard broad-cast band antenna transformer may be used forT1. The detector transformer, T2, is made upby using a second antenna transformer and re-moving the primary (antenna) winding. The wirefrom this winding is then used to rewind about60 turns closely coupled to the tuning coil. Thisnew winding then becomes the "secondary", whichis connected to TR1. The parts layout chosenshould permit mutual coupling between T1 and

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T2,and this coupling should be adjusted for max-imum gain consistent with good selectivity.

High impedance magnetic headphones may beconnected directly into the collector circuit ofTR2, as snown. If low impedance phones or aloudspeaker are to be used, however, a smallimpedance matching transformer must be em-ployed. If crystal headphones are to be used,a 2K to 5K resistor should be connected in thecollector circuit of TR2, and a .05 mfd couplingcondenser used between the headphones and thecollector terminal.

Controlled Circuit

Figure 9-7. A sensitive Relay Circuit. Parts values are as follows:

TR - RAYTHEON type CK722 PHP junction transistor.RLY - High resistance relay, such as ADVANCE type 850 or 1200; Should not re-

quire more than 5 ma for coil operation; choose coil resistance to suit bat-tery used.B1 - Batter7 ... from 8.3 to 15 volts.SW1 - SPST 'Off -On" Switch.R1 - Base current limiting resistor; determine value by Ohm's law for battery

voltage used.R2 - 250 K rheostat (SENSITIVITY adjustment).

GENERAL EXPERIMENTAL CIRCUITS

An Ultra -Sensitive Relay Circuit: A direct -coupled common emitter transistor amplifierstage may be used to increase the sensitivity of


a relay by a factor of 10 or 12 to one, using thecircuit arrangement shown in Fig. 9-7. Thus, ifthe relay used normally requires 2 milliamperescoil current to operate, a current flow of only200 microamperes in the control (base) circuitwill permit operation.

The relay chosen should not require a coilcurrent of more than 5 milliamperes, so as notto exceed the maximum collector current ratingof the transistor. Battery B1 should supply suf-ficient voltage to easily drive the necessary cur-rent through the resistance of the relay's coil . . .

however, under no conditions should more than20 volts be used.

Resistor R1 is a current limiting resistor usedto prevent excessive currents through the tran-sistor elements. Its size will vary with thebattery voltage used. In general, the resistancevalue chosen should not permit a current flow ofgreater than 5 ma. through any of the transistor'selements, even with R2 turned to minimum re-sistance (0).

To operate the circuit, close Sw1. With R2set to its maximum resistance, short the "controlcircuit" terminals together. Now, gradually re-duce the value of R2 until positive relay closureis obtained. This setting is satisfactory wherethe "control circuit" has little or no resistance. . if appreciable resistance is likely to bepresent in the control circuit, a lower valuesetting of R2 may be employed.

The final sensitivity of the completed instru-ment will vary considerably with the basic relayemployed and, to a lesser extent, with the in-dividual transistor used and the battery voltage.

A Photo -Cell Relay Circuit: The Photo -Cellrelay circuit shown in Fig. 9-8 represents anadaptation of the Sensitive Relay Circuit givenin Fig. 9-7. The operation of the two circuits isalmost identical, except that a "self -generating"selenium photo -cell has been substituted for ad.c. voltage source in the base -emitter circuitof the transistor.


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Figure 9-8. A transistorized photo -cell relay. Circuit values as follows'

TR - RAYTHEON type CK722 PNP junction transistor.RLY - Sensitive Relay - ADVANCE type 1200 - choose coil resistance to suit bat-

tery voltage used.Bi - Battery - 6.3 to 15 volts.Swi - SPST "Off -On" Switch.P -C - Barrier type selenium photo -cell.

CREDIT: Basic Circuit Courtesy Radio & Television News.Adapted from "Transistor -Operated Photocell Relay".

The selenium photo -cell (P -C) used is the sametype unit that is employed in photographer's ex-posure meters. A suitable unit may often besalvaged from a second hand exposure or lightmeter . . . . or purchased from one of the largemail order radio supply houses.

In operation, light striking the photo -cellgenerates a small d.c. voltage, permitting currentflow through the base -emitter circuit of the tran-sistor. This current flow, in turn, permits acorrespondingly greater collector current flow,operating the relay. With most photo -cell units,a reasonably strong light source is needed.

As in the preceding circuit, the relay chosenshould not require more than a few milliamperesfor operation...best results are obtained whereless than one milliampere is required. Batteryvoltage should be suited to relay coil resistancebut, again, should not exceed 20 volts.


A Sine -Wave Clipper: When driven with sine -wave signals of sufficient amplitude, the circuitshown in Fig. 9-9 is capable of delivering goodquality rectangular waves over the audio band.The output signals obtained are satisfactory for

TRi

INPUTSine -Wave SignalFrom 3-6 Volts

Amplitude B1 Sw.1

111-0'

OUTPUTRectangular Pulses

Figure 9-9. A Sine -Wave Clipper using a single transistor. Parts values are asfollows.

TRi - RAYTHEON type CK722 PNP Junction transistor.C1 - .5 mfd, 200 volt paper tubular condenser.R1 - 18K, 1/2 w. carbon resistor.R9 - 10K pa., OUTPUT LEVEL control.Svn - SPST "Off -On" Switch, may be on OUTPUT control.

- 15 volt battery.

CREDIT: Circuit Courtesy RADIO & TELEVISION NEWS.Adapted from "A Transistor Sine -Wave Clipper".

square -wave testing of pre -amps, wide -bandamplifiers, filters, and similar audio circuits.

Battery voltage is not critical, but the peakamplitude of the output signal obtained will varydirectly with the battery voltage used, assumingthat ample signal drive is supplied in every case.If an adjustable output level is not needed, R2may be replaced by a fixed resistor, and theoutput obtained directly from the collector ter-minal.

In operation,the output signal should be checkedwith an oscilloscope as the applied sine -wavesignal is gradually increased in amplitude. Ifinsufficient drive is supplied, a rounded, ratherthan a "flat", output pulse will be obtained. Iftoo much drive is used, the output signal will bedistorted, and the transistor itself may be over-loaded.

TRANSISTORIZED WRIST- RADIO

99

The transistor wrist -watch receiver is a regenerative, capacitance -tuned set.

TECHNICAL details on the transistorizedwrist radio have been released by the SignalCorps. The radio was built to demonstrate thefeasibility of constructing a small radio receiverwith transistors. The reduced power require-ments of transistors as compared to vacuumtubes made it possible to use a very small bat-tery which is included in the wrist case.

The receiver (see photo) uses three trans-sistors; one as a regenerative r . f stage andtwo as audio amplifiers . A point - contacttransistor (type 1729) is used in the regenerativestage. Regeneration is controlled by varying thecoupling between the two coils. A miniaturecapacitor is used for tuning. The audio is am-plified by two p -n -p junction transistors (typeTA -153). A bead diode (type 1764) is used as adetector and another one is used as a d.c. return.(Information on the exact commercial equivalentsof the transistors and diodes was not availablebut we understand that the 1729 is similar to the2N25, the TA -153 is similar to the 2N34, and thediodes (1764's) are roughly equivalent to 1N84' s.)

The power supply is a 6.5 -volt battery (1/2 x5/8 inch) consisting of five 1.3 -volt RM-412 mer-cury cells. Battery drain is about 20 milliwattsand battery life about 10 hours. Although in strong


-signal areas no antenna is needed, usually a 1-foot wire should be used. The 2,000 -ohm ear-phone is a small Telex hearing -aid type. Thetransistors can be replaced without making anycircuit adjustments.

The receiver tunes from 1000 to 1600 kc; it hassharp selectivity, and a sensitivity of 50 micro-volts. A number of New York stations (35 milesfrom Fort Monmouth) can be heard quite satis-factorily. When the receiver is in the vicinity ofradiators,such as telephones,the reception,is im-proved to the extent that signals from the setcan be heard 30 feet from the earphone.

With the addition of a signal audio stage and anoutput transformer, a loudspeaker could be used.Two stages of audio were needed to compensatefor the elimination of the antenna. In metropol-itan areas the coils alone will pick up sufficientsignal. In the wrist version, the antenna can bebuilt into the strap.

When the receiver is held near the body boththe tuning and regeneration are affected by bodycapacitance. The regeneration should be checkedfor each tuning adjustment. Regeneration can bemore easily controlled electronically than bymoving a coil. One method is to insert a small2,000 -ohm potentiometer in series with the col-lector coil. Another is to useatrimmer capac-itor for feedback from collector to emitter. Inthis case, the collector coil can be replaced witha resistor.

The selection of a power source was primarilydetermined by the point -contact transistor whichoperated best with 6 volts. Junction transistors,however, can be satisfactorily operated from a1 -1/2 -volt source. With minor modifications,the power requirements can be reduced by a fac-tor of 2.5; and by replacing the point -contacttransistor in the regenerative stage with a_junc-tion type the requirements can be reduced con-siderably more.

The publishers thank Radio -Electronics Mag-azine for information on this transistorizedwrist -radio.

101

CHAPTER X

Appendix

A. Transistor Amplifier Characteristics:GROUNDED BASE AMPLIFIER:

Input Impedance: Very low.Output Impedance: High.Power Gain: Moderate to high.Current Amplification: Less than 1.0.*Frequency Response: Poor.Output Signal Phase: In phase withinput.

GROUNDED COLLECTOR AMPLIFIER:Input Impedance: High, but dependson load.Output Impedance: Moderate to low.Power Gain: Low.Current Amplification: Large.Frequency Response: Good.Output Signal Phase: In phase withinput.

GROUNDED EMITTER AMPLIFIER:Input Impedance: Moderate.Output Impedance: Moderate to high.Power Gain: High.Current Amplification: Large.Frequency Response: Moderate.Output Signal Phase: Phase reversaloccurs.

B. New Transistor Types: - Transistor manu-facturers are constantly carrying out develop-ment work on new type units. As new types aredeveloped and announced, the electronics tech-nician should obtain technical data and informa-tion on them, thus keeping abreast of the field.Some of the types now in the developmental stageare listed below:

PHOTOTRANSISTORS: Although a few photo -transistors are currently on the commercial*For junction transistors


market, new types are being developed. Thebasic phototransistor may be either a junctionor a point -contact unit. They operate somewhatsimilarly to conventional transistors except thatthey are basically diode units, with the presenceof light energy taking the place of the emitter. . . .

thus, light striking the sensitive surface in-creases the number of electrons and holes, de-creasing the resistance of the unit.

ANALOG TRANSISTOR: A theoretical typeutilizing a special construction. If productionproves practical, this unit should have charac-teristics similar to those of a vacuum tube.

TETRODE JUNCTION TRANSISTOR: A spe-cial junction transistor with improved high fre-quency response. This is accomplished by re-ducing the thickness of the center layer of ma-terial and adding a fourth electrode lead. Theadded lead is on the base, directly opposite thenormal base connection. A 'comparatively highbias is applied to this connection, with respectto the regular base lead. The net result is toreduce the conducting area of the base, reducingthe effective inter -terminal capacities.

POWER TRANSISTORS: Several manufactur-ers are developing diffused junction power tran-sistors with increased power handling capacities.Some experimental types are able to handle morethan a watt of audio power.

P -N 'HOOK" TRANSISTOR: A special typejunction transistor employing three junctions(such as a PNPN transistor). Theoretically,these transistors may have an alpha greater than1.0, and may thus provide exceptionally high cur-rent gains.

MULTI -PURPOSE TRANSISTORS: Multi -pur-pose transistors, analogous to multi -purposetubes, may eventually be introduced. One possi-ble type would combine a PNP and a NPN junc-tion transistor into a single unit, for applicationin a push-pull stages using the complementarysymmetry principle.

APPENDIX 103

C. Commercially available Transistors: - -Someof the transistors commercially available at thiswriting are listed below. Part of the data listedis tentative, however, and revised informationis released from time to time. In addition, tran-sistor types, like tube types, may become obseleteas new and improved transistors are introduced.Therefore, the engineer or technician workingwith transistor circuits should always obtain thelatest available technical data directly from themanufacturer.

In the listing below, the following symbols willbe used to identify the transistor types: J - junc -tion transistor, with the type given in brackets(PNP), PC - point -contact transistor, PT - photo -transistor, TPC - tetrode point -contact tran-sistor.

MANUFACTURER: Transistor Products

TYPE NUMBERS: 2A 2B 2C 2D 2E 2F 2G

TRANSISTOR TYPES: PC PC PC PC PC PC PC

MAXIMUM RATINGS:

Power Dissipation: 120 120 100 100 100 120 120 MW.

Collector Voltage: -50 -50 -50 -50 -50 -100 -100 VoltsCollector Current: -8 -8 -8 -8 -8 -40 -40 Ma.

TYPE NUMBERS: X-4 X-22 X -23 X -25

TRANSISTOR TYPES: PT J (NPN) J (NPN) PTMAXIMUM RATINGS:

Power Dissipation: 50 50 MW.

Collector Voltage: 40 40 VoltsCollector Current: 5 5 Ma.

Note: Type X-4 in a PN junction phototransistor designed for use asa light detector; Type X-25 is an NPN junction phototransistor witha power output of approximately 60 MW.


MANUFACTURER: General Electric

TYPE NUMBERS: G11 G1 1A 2N43 2N44 2N45

TRANSISTOR TYPES: PC PC J (PNP) J (PNP) J (PNP)

MAXIMUM RATINGS:

Power Dissipation: 100 100 150 150 150 MW.

Collector Voltage: -30 -30 -45 -45 -45 Volts

Collector Current: -7 -7 -10 -10 -10 Ma.

MANUFACTURER: National Union

TYPE NUMBERS: T18A T18B 2N39 2N40 2N42

TRANSISTOR TYPES: PC PC J (PNP) J (PNP) J (PNP)

MAXIMUM RATINGS:

Power Dissipation: 120 120 50 50 50 MW

Collector Voltage: -50 -50 -30 -30 -30 Volts


MANUFACTURER: CBS-HYTRON

TYPE NUMBERS: PT -2A PT -2S 2N36 2N37 2N38

TRANSISTOR TYPES: PC PC J (P NP) J (PNP) J (PNP)

MAXIMUM RATINGS:

Power Dissipation: 100 100 50 50 50 MW.

Collector Voltage: -40 -40 -20 -20 -20 Volts.


MANUFACTURER: Raytheon

TYPE NUMBERS: CK716 CK718* CK721 CK722

TRANSISTOR TYPES: PC J (PNP)J (PNP) J (PNP)

MAXIMUM RATINGS:

Power Dissipation: 100 30 30 MW.

Collector Voltage: -40 -20 -20 Volts

Collector Current: -4 -5 -5 Ma.

*Type CK718 is similar to type CK721, except that it has passed a

special noise test. Generally available direct to manufacturers only.

APPENDIX 105

MANUFACTURER: Sylvania Electric Products

TYPE NUMBERS: 2N32 2N34 3N21

TRANSISTOR TYPES: PC J (PNP) TPC

MAXIMUM RATINGS:



Collector Current: -8 -8 --- Ma.

MANUFACTURER: Radio Corporation of America

TYPE NUMBERS: 2N32 2N33 2N34 2N35

TRANSISTOR TYPES: PC PC J (PNP) J (NPN)

MAXIMUM RATINGS:

Power Dissipation: 50 30 50 50 MW.Collector Voltage: -40 -8.5 -25 25 Volts

Collector Current: -8 -7 -8 8 Ma.

MANUFACTURER: Radio Receptor Co.

TYPE NUMBERS: RR14 RR20 RR21

TRANSISTOR TYPES: J (PNP) J (PNP) J (PNP)MAXIMUM RATINGS:



Collector Current: -5 -5 -5 Ma.

MANUFACTURER: Texas Instruments

TYPE NUMBERS: 100 (100A) 101 (101A) 200 201

TRANSISTOR TYPES: PC PC J (NPN) J (NPN)MAXIMUM RATINGS:

Power Dissipation: 120 120 50 50 MW.

Collector Voltage: -100 -30 30 30 VoltsCollector Current: -15 -25 5 5 Ma.

'''1

AAlpha, definitionAction, ofAdvantages, of junction type

transistorstransistors

Amplifiers,circuitsgrounded emitter typegrounded base typegrounded collectorcoupling transistormulti -stage transistordirect coupled type

diagramPush -Pull circuit

diagramPhono

diagram

B

Base,Battery vs. Transistor

experimental type receiverusing transistor

Batteries, mercuryphoto of

Bias

C

Capacitors,diagramtantalum

Capacitance, outputCapacity, resistanceCell, mercuryCharges, positive & negativeCharts, characteristic curves

comparison transistors &vacuum tubes

Characteristics,analog transistorscommercially availablegrounded base amplifiergrounded collector amplifiergrounded emitter amplifiermulti -purpose transistorphoto transistorsP -N hook transistorspower

Chronology,Circuits,

amplifieranalogous transistoraudioblocking oscillator

diagram ford -c amplifier

diagram forelectron flow inflip-flop

diagram for

te

INDEXContinued

92 Circuits,19 multivibrator

diagram for26 oscillator13 photo cell relay

sine wave clipper96-51 push-pull amplifier

37 diagram for40 phase inverter41 diagram for46 sensitive relay48 special common base61 diagram for58 special transistor62 tetrode mixer83 diagram for88 Clipper, sine wave89 Coil, Hartley

Comparison, elements of vacuumtubes and transistors

Components27 Connections, terminal leads12 Construction, point contact types

methods of12 Collector,74 Coupling, means of74 use of transistors for26 Current flow, methods

Cut off current,frequency

Curves, characteristicsCurrent flow, electron in

transistor circuits7172723347732535

2829-35

100101

999999

10099

100100

16-1838-51

38408756525858396564

D

Detector, R -F typediagram for

E -F -G

Element, polaritiesEmitter, explanation of termEmitter

Frequency, cut offFree exchange, explanation

Graph, relation between gainsGain, transistors circuitsGrid, average potentialGain, power

H

Hearing aid, batteries fordiagram for

"Hole", explanation of

6666

51-5796-97

9862636162955455

58-8967676054

9669-77

2924

23, 9023, 27

484720

32-333335

39

5958

422723

3322

44433733

73, 909121

Impedance, 33matching 48

ContinuedInventors, 12 transistorized equipment 81Invention, of transistors 18-18 Signals, input & output 45Inverters,

phase circuit8162

linear outputrectified RF signal

8560

diagram of 82 Sine Waves, clipper 60, 98diagram of 98

Layers, semi conductive materials 26 Socket, placing transistor in 12sub -miniature 70

M -N -O -P Specifications, comparison ofMegaphone, transistor type 14 transistors & vacuum tubes 28

cut off frequency 33N -type, 20 cut off current limit 33Noise, internal 33 factors to be considered 32

factor 33 methods of rating 30noise factor 33

Oscillator, blocking oscillatorcircuit 56

Symbols,for junction transistors

2922

code practice schematic 86Colpitts, principle of 53 Temperature, effect on 30crystal controlled 54-55 Terminal lead, connections 29

diagram of 55 Terms, Alpha, definition 32definition of 51 Testing, transistors 80-8150 MC type

diagram offrequency of

929351

Transformers,Transistors,

amplifier circuits

69

36-51Hartleys, principle of 53 amplifier couplings, means of 48

diagram of 53 analogous circuits 40materials of 51 care & servicing 78-88Saw Tooth,tickler feedback

6551

characteristics ofcomponents for

2989-77

diagram of 52 effect of temperature on 30Oscillator, transistor circuit 51-57 transistor vs battery inOutput, high side elements 38 AM receiver 12

inventors of 12P -types, 21 junction types 22PNP types 28 oscillator circuits 51-57Phase, reversal 43 point contact types 25Photos, experimental type AM power supplies for 73

receiver using transistors 12 rectified RF signal 80point contact transistor 25 RF detector 59electronic megaphone 14 diagram for 58comparative size of transistors 19 special circuits 58-89in -line sub -miniature tube size 89-77socket 70 testing 80

Photo cell, relay circuit 96 transformer 69diagram of 97 diagram for 71

Polarity, d -c power supply 38 uses of 13

high side output elements 38 wrist radio 99Potentiometers, sub -miniature 69 Transit time, 34

Power, a -c supplies 75 Transmitter, crystal controlled 90

diagram of 75 diagram for 92

d -c supply polarity 38 Tube, 5Ua compared with transistor 75

maximum dissapationsupplies

3573

Types,P type

2921

Power Gain, 33 hermetically sealed 30

Power Output, 34 junction types 22PNP type 28

R -S -T Tetrode point contact 24

Radio, wrist 99Receiver, AM receiver 12 U -V -W -X -Y -Z

broadcast band,diagram of

Research, pioneers in

929411

Uses, in military,commercially

1515

Servicing 78-86 Vacuum tube, amplifier 37

block diagram for servicing 88 Wrist radio 99

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