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UNIT I - TRANSISTOR & ITS BIASING

• Transistor Symbols – Transistor as an Amplifier–

Connections– CB, CE,&CC–

• Characteristics– Comparison of Transistor

Connection.

• Transistor biasing:

• Methods of transistor Biasing– Base resistor method–

Biasing with feedback

• resistor– Voltage divider bias method.

TRANSISTOR AND ITS BIASING

• The transistor was invented by a team

of three scientists at Bell Laboratories,

USA in 1947.a

• A transistor is a semiconductor device

used to amplify and switch electronic

signals and electric power.

• It composed of semiconductor material

with at least three terminals for an

external connection.

• There are two basic types of transistors

(1) The bipolar junction transistor (BJT)

(2) Field effect transistor (FET)

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FIELD-EFFECT TRANSISTOR

• The field-effect transistor, sometimes called

a unipolar transistor, uses either electrons (in n-

channel FET) or holes (in p-channel FET) for

conduction.

• The four terminals of the FET are

named source, gate, drain, and body

• On most FETs, the body is connected to the source

inside the package,.

BIPOLAR JUNCTION TRANSISTOR

• The bipolar junction transistor consists of two back-to back P-

N junctions manufactured in a single piece of a semiconductor

crystal.

• The bipolar junction transistor is used in two broad areas of

electronics : as a linear amplifier to boost an electrical signal

and as an electronic switch

• These two junctions give rise to three regions called

• Emitter,

• Base and

• Collector

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REGIONS OF TRANSISTOR

1. Emitter

It is more heavily doped than any of the other regions because its main function is to supply majority charge carries

to the base.

2. Base

It forms the middle section of the transistor. It is very thin (10-6 m) as compared to either the emitter or collector and is very lightly-doped.

3. Collector

Its main function is to collect majority charge carriers coming from the emitter and passing through the base. Collector region is made physically larger than the emitter region because it has to dissipate much greater power.

JUNCTION TRANSISTORS

Junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type.

• There are two types;

1. PNP transistors

A layer of N-type material sandwiched between two layers of P-type material. It is described as a PNP transistor.

2. NPN transistors

NPN – transistor consisting of a layer of P-type material sandwiched between two layers of N-type material

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Contd..,

TRANSISTOR CIRCUIT

CONFIGURATIONS

• There are three types of circuit connections for operating a

transistor.

1. common-base (CB),

2. common-emitter (CE),

3. common-collector (CC)

• The term ‘common’ is used to denote the electrode that is common

to the input and output circuits.

• Because the common electrode is generally grounded, these modes

of operation are frequently referred to as grounded-base, grounded-

emitter and grounded-collector configurations

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CB CONFIGURATION

• Emitter current IE is the input current and collector current IC

is the output current. The input signal is applied between the

emitter and base whereas output is taken out from the collector

and base

• The ratio of the collector current to the emitter current is called

dc alpha (αdc) of a transistor.

• The negative sign is due to the fact that current IE flows into

the transistor whereas IC flows out of it. Hence, IE is taken as

positive and IC as negative.

Contd..

• The α of transistor is a measure of the quality of a transistor ;

higher the value of α, better the transistor in the sense that

collector current more closely equals the emitter current. Its

value ranges from 0.95 to 0.999.

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CE CONFIGURATION

• Input signal is applied between the base

and emitter and output signal is taken

out from the collector and emitter

circuit. As seen from Fig, IB is the input

current and IC is the output current.

• The ratio of the dc collector current to

dc base current is called dc beta (βdc)

or just β of the transistor

• .

• It is also called common-emitter dc

forward transfer ratio and is written as

hFE.

• It is possible for β to have as high a

value as 500

∴ β = –IC /–IB = IC /IB

CC CONFIGURATION

• In this case, input signal

is applied between base

and collector and output

signal is taken out from

emitter-collector circuit.

• Conventionally speaking,

here IB is the input

current and IE is the

output current

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TRANSISTOR STATIC CHARACTERISTICS

• There are the curves which represents relationship

between different d.c. currents and voltages of a

transistor. These are helpful in studying the operation of

a transistor when connected in a circuit.

• The three important characteristics of a transistor are :

• 1. Input characteristic,

• 2. Output characteristic,

• 3. Constant-current transfer characteristic.

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TRANSISTOR INPUT CHARACTERISTICS

• The static characteristics of an NPN transistor connected in

common-base configuration can be determined by the use of

test circuit shown in Figure.

• Milli-ammeters are included in series with the emitter and

collector circuits to measure IE and IC.

• Similarly, voltmeters are connected across E and B to measure

voltage VBE and across C and B to measure VCB.

• The two potentiometer resistors R1 and R2 supply variable

voltages from the collector and emitter dc supplies

respectively

CONTD..,

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COMMON BASE STATIC

CHARACTERISTICS

• Input Characteristic:

• It shows how IE varies with VBE when

voltage VCB is held constant.

• First, voltage VCB is adjusted to a

suitable value with the help of R1.

• Next, voltage VBE is increased in a

number of discrete steps and

corresponding values of IE are noted

from the milliammeter connected for

the purpose.

• When plotted, we get the input

characteristic shown in Figure. one for

Ge and the other for Si.

• This characteristic may be used to find

the input resistance of the transistor.

• Its value is given by the reciprocal of its

slope.

•

VCB constant.

Rin= ΔVBE / ΔIE

COMMON BASE STATIC

CHARACTERISTICS

• Output Characteristic:

• It shows the way IC varies with

VCB when IE is held constant. The

method of obtaining this

characteristic is as follows:

• First, movable contact, on R2 is

changed to get a suitable value of

VBE and hence that of IE.

• While keeping IE constant at this

value, VCB is increased from zero

in a number of steps and the

corresponding collector current IC

that flows is noted.

• Next, VCB is reduced back to zero,

IE is increased to a value a little

higher than before and the whole

procedure is repeated. In this way,

whole family of curves is obtained,

a typical family being shown in Fig

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• The reciprocal of the near horizontal part of the characteristic gives the output resistance R out of the transistor which it would offer to an input signal. Since the characteristic is linear over most of its length (meaning that IC is virtually independent of VCB). Rout is very high, a typical value being 500 kΩ.

Contd..,

• It is seen that IC flows even when VCB = 0.It is due to the fact that electrons are being injected into the base under the action of forward-biased E/B junction and are being collected by the collector due to the action of the internal junction voltage at the C/B junction. For reducing IC to zero, it is essential to neutralize this potential barrier by applying a small forward bias ac-ross C/B junction.

• Another important feature of the characteristic is that a small amount of collector current flows even when emitter current IE = 0.

• This characteristic may be used to find αac of the transistor

• Another point worth noting is that although IC is practically independent of VCB over the working range of the transistor, yet if VCB is permitted to increase beyond a certain value, IC eventually increases rapidly due to avalanche breakdown

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Contd..,

• Current Transfer

Characteristics:

• It shows how IC varies with

changes in IE when VCB is

held constant.

• For drawing this

characteristic, first VCB is set

to a convenient value and

then IE is increased in steps

and corresponding values of

IC noted.

COMMON EMITTER TEST CIRCUIT

• A milli-ammeter is connected in series with the base to measure IB.

• Similarly, a milli-ammeter is included in the collector circuit to measure IC.

• A voltmeter with a typical range of0 –1 V is connected across base and

emitter terminals for measuring VBE.

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COMMON EMITTER STATIC

CHARACTERISTICS

• Input Characteristic:

• It shows how IB varies with

changes in VBE when VCE is held

constant at a particular value.

• To begin with, voltage VCE is

maintained constant at a convenient

value and then VBE is increased in

steps.

• Corresponding values of IB are

noted at each step. The procedure is

then repeated for a different but

constant value of VCE.

COMMON EMITTER STATIC

CHARACTERISTICS

• Output or Collector

Characteristic:

• It indicates the way in which IC

varies with changes in VCE when

IB is held constant.

• For obtaining this characteristic,

first IB is set to a convenient value

and maintained constant and then

VCE is increased from zero in

steps, IC being noted at each step.

• Next, VCE is reduced to zero and

IB increased to another convenient

value and the whole procedure

repeated

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COMMON EMITTER STATIC

CHARACTERISTICS

• Current Transfer

Characteristic:

• It indicates how IC varies

with changes in IB when

VCE is held constant at a

given value.

• It is seen that a small

collector current flows even

when IB = 0. It is the

common- emitter leakage

current ICEO = (1 + β) ICO.

Like ICO,

• It is also due to the flow of

minority carriers across the

reverse-biased C/B junction.

COMMON COLLECTOR

STATIC CHARACTERISTICS

• In this case, collector terminal is common carrier to both the input (CB) and output

(CE) carriers circuits

• The output characteristic is IE versus VCE for several fixed values of IB.

• The CC input characteristic is a plot of VCB versus IB for different values of VCE

• Similarly, its current gain characteristic IC versus IB for different values of VCE is

similar to that of a CE circuit because IC = IE.

• It is quite different from those for CB or CE circuit. This difference is due to the

fact that input voltage V CB is largely determined by the value of CE voltage

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OUTPUT AND CURRENT GAIN

CHARECTERISTICS

TRANSISTOR BIASING

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TRANSISTOR BIASING

• Biasing in electronics is the method of establishing pre-

determined voltages or currents at various points of an

electronic circuit for the purpose of establishing proper

operating conditions in electronic components.

• This bias point is called the quiescent or Q-point as it gives

the values of the voltages when no input signal is applied. To

determine the Q-point we need to look at the range of values

for which the transistor is in the active region.

• It is also known as the dc operating point or working point.

The best position for this point is midway between cut-off and

saturation points where VCE= 1/2 VCC

SATURATION POINT AND CUTOFF REGION

•

(i) when IC = 0, VCE= VCC

— cut-off point A

(ii) when VCE = 0, IC = VCC/RL

— saturation point B

Q- Point VCE= 1/2 VCC

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STABILITY FACTOR

• The operating point can be made stable by keeping IC and VCE constant.

• There are two techniques to make Q point stable. – stabilization techniques

– compensation techniques

• In first, resistor biasing circuits are used which allow IB to vary so as to keep IC relatively constant with variations in bdc, ICO and VBE.

• In second, temperature sensitive devices such as diodes, transistors are used which provide compensating voltages and currents to maintain the operating point constant.

• To compare different biasing circuits, stability factor S is defined as the rate of change of collector current with respect to the ICO, keeping bdc and VCE constant

• S = ¶ IC / ¶ ICO

• If S is large, then circuit is thermally instable. S cannot be less than unity

NEED FOR BIASING A TRANSISTOR

• For normal operation of a transistor amplifier circuit, it is essential that there should be a

(a) forward bias on the emitter-base junction and

(b) reverse bias on the collector-base junction.

• In addition, amount of bias required is important for establishing the Q-point which is dictated by the mode of operation desired.

• If the transistor is not biased correctly, it would

1. work inefficiently and

2. produce distortion in the output signal.

3. It is desirable that once selected, the Q-point should remain stable i.e. should not shift its position due to temperature rise etc. Unfortunately, this does not happen in practice unless special efforts are made for the purpose.

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METHODS OF TRANSISTOR BIASING

• Base bias or fixed current bias

• Base bias with emitter feedback

• Base bias with Collector feedback

• Base bias with collector and emitter feedback

• Voltage divider bias

BASE BIAS WITH EMITTER FEEDBACK

• The circuit is obtained by

simply adding an emitter

resistor to the base bias

circuit

• At saturation, VCE is

essentially zero, hence

VCC is distributed over

RL and RE.

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CONTD..,

• IC can be found as follows :

• Consider the supply, base, emitter and ground route. Applying Kirchhoff’s

Voltage Law ie for a given junction or node in a circuit, the sum of the

currents entering equals the sum of the currents leaving.

• Substituting these values in the above equation, we have

PROBLEM

• For the circuit shown in Fig. 58.9, find

• (i) IC(sat) , (ii) IC , (iii) VC ,

• (iv) VE, (v) VCE

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BASE BIAS WITH COLLECTOR FEEDBACK

• The circuit is like the base bias

circuit except that base resistor is

returned to collector rather than to

the VCC supply.

• It derives its name from the fact that

since voltage for RB is derived from

collector, there exists a negative

feed back effect which tends to

stabilise IC against changes in β.

• To understand this action, suppose

that somehow β increases. It will

increase IC as well as IC RL but

decrease VC which is applied across

RB

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BASE BIAS WITH COLLECTOR AND

EMITTER FEEDBACK

• Both collector and emitter are use in this circuit.

• This is used to make the circuit less sensitive to changes in β.

• Applying kirchoff’s voltage law

-(IC+IB)RL-VBE-IC(RE+RB/β)VCC=0

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BASE RESISTOR METHOD

• In this method, a high resistance RB

is connected between the base and

+ve end of supply for npn transistor

and between base and negative end

of supply for pnp transistor.

• Here, the required zero signal base

current is provided by VCC and it

flows through RB.

• It is because now base is positive

w.r.t. emitter i.e. base-emitter

junction is forward biased.

• It is required to find the value of

RB so

• that required collector current flows

in the zero signal conditions.

BASE RESISTOR METHOD

• Let IC be the required zero signal collector current.

• Considering the closed circuit ABENA and applying Kirchhoff 's voltage

law, we get,

• Since VBE is generally quite small as compared to VCC, the former can be

neglected.

IB = IC / β

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BASE RESISTOR METHOD

ADVANTAGES :

(i) This biasing circuit is very simple as only one resistance RB is required.

(ii) Biasing conditions can easily be set and the calculations are simple.

(iii) There is no loading of the source by the biasing circuit since no resistor is

employed across base-emitter junction.

DISADVANTAGES :

(i) This method provides poor stabilisation. It is because there is no means

to stop a self increase in collector current due to temperature rise and

individual variations.

(ii) The stability factor is very high. Therefore, there are strong chances of

thermal runaway. Due to these disadvantages, this method of biasing is

rarely employed

VOLTAGE DIVIDER BIAS

• This is the most widely used

method of providing biasing and

stabilization to a transistor.

• In this method, two resistances

R1 and R2 are connected across

the supply voltage VCC and

provide biasing.

• The emitter resistance RE

provides stabilization.

• The name ‘‘voltage divider’’

comes from the voltage divider

formed by R1 and R2.

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THANK YOU