bjt operations

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BIPOLAR JUNCTION

TRANSISTORS (BJTs)

Chapter 4



INTRODUCTION

What is transistor? A three-terminal device whose output current,

voltage and/or power are controlled by its input.

Commonly used in audio application as anamplifier, in switching application as a switch andin power supply voltage and current regulatorcircuit.

2 basic transistor types: BJT and FET

These two transistor differ in their operatingcharacteristic and their internal construction.



Describe the basic structure of the bipolar junctiontransistor (BJT)

Explain and analyze basic transistor bias andoperation

Discuss the parameters and characteristics of atransistor and how they apply to transistor circuits

OBJECTIVES



1. BJT STRUCTURE



1. BJT STRUCTURE

The BJT is constructed with three doped semiconductor regionsseparated by two pn junctions.

The three region are called emitter (E),base (B) and collector (C)

The BJT have 2 types:

1. Two n region separate by a p region – called npn

2. Two p region separated by a n region – called pnp

The pn junction joining the base region and the emitter region iscalled the base-emiter junction

The pn junction joining the base region and the collector region is

call base-collector junction

The base region is lightly doped and very thin compared to theheavily doped emitter and the moderately doped collector region



1. BJT STRUCTURE



1. BJT STRUCTURE

BJT schematic symbol The arrow on schematic symbol is important

because: Identify the component terminal

The arrow is always drawn on the emitter terminal.The terminal opposite emitter is collector and thecenter terminal is base.

The arrow always points toward n-type material

If the arrow point toward base, transistor is pnp type.If it points toward emitter, transistor is npn type.



1. BJT STRUCTURE

Transistor terminal current



1. BJT STRUCTURE

Transistor Currents:

The directions of the currents in npn transistor and pnp transistor are

shown in the figure. The emitter current (IE) is the sum of the collector current (IC) and the

base current (IB)

IB << IE or IC

The capital letter – dc value

Transistor is a current-controlled device - the value of collector andemitter currents are determined by the value of base current.

An increase or decrease in value of IB causes similar change in valuesof IC and IE.

C B E I I I

B DC C I I

Current gain (β) factor

by which current increasesfrom base of transistor toits collector.



1. BJT STRUCTURE

Transistor Voltages:

VCC

– collector supply voltage. This is a power supply voltageapplied directly to collector of transistor.

VBB – base supply voltage. this is dc voltage used to bias baseof transistor.

VEE – emitter supply voltage. dc biasing voltage and in many

cases, VEE is simply a ground connection.



1. BJT STRUCTURE


VC – dc voltage measured from collector terminal ofcomponent to ground

VB – dc voltage measured from base terminal to ground.

VE – dc voltage measured from emitter terminal to ground.



1. BJT STRUCTURE


VCE – dc voltage measured from collector to emitter terminal

of transistor. VBE – dc voltage measured from base to emitter terminal of

transistor.

VCB – dc voltage measured from collector to base terminal of

transistor.



2. BJT OPERATION



2. BJT OPERATION

To operate the transistor properly, the two pn junction must be correctly biased with external dc

voltages.

The figure shown the proper bias arrangement forboth npn and pnp transistor for active operation asan amplifier.



2. BJT OPERATION

Transistor is made of 3 separate semiconductor

materials that joined together to form two pn junction.

Point at which emitter and base are joined forms asingle pn junction base-emitter junction

Collector-base junction point where base and

collector meet.



2. BJT OPERATION

Cutoff region

Both transistor junctions are reversebiased.

With large depletionregion between C-Band E-B, very smallamount of reversecurrent, I

CEO passes

from emitter tocollector and can beneglected.

So, V CE = V CC



2. BJT OPERATION

Saturation region

Both transistor junctions areforward-biased.

IC reaches its maximum valueas determined by VCC and total

resistance in C-E circuit. IC is independently from

relationship of β and IB.

VBE is approximately 0.7V and VCE < VBE.

E C

CC

C R R

V I



2. BJT OPERATION

Active region

BE junction is forward biasedand the BC junction is reversebiased.

All terminal currents have

some measurable value. The magnitude of I C depends

on the values of β and I B .

VCE is approximately near to0.7V and V

CE falls in ranges

VBE<VCE<VCC.



3. BJT CHARACTERISTICS &PARAMETERS



3. BJT CHARACTERISTICS & PARAMETERS

DC Beta ( ) and DC Alpha ( ):

The ratio of the dc collector current (IC) to the dc base current (IB) is thedc beta

( ) = dc current gain of transistor

Range value : 20< <200

Usually designed as an equivalent hybrid (h ) parameter, ontransistor data sheet –

The ratio of the dc collector current (IC) to the dc emitter current (IE) isthe dc alpha ( ) – less used parameter in transistor circuits

Range value-> 0.95< <0.99 or greater , but << 1 (Ic< IE )

DC

DC

DC

DC

FE h DC FE h

DC

DC

B

C DC

I

I

E

C DC

I

I



3. BJT CHARACTERISTICS & PARAMETERSCurrent and Voltage Analysis:

The current and voltage can be identified as follow:

Current: Voltage:

dc base current, dc voltage at base with respect to emitter,

dc emitter current, dc voltage at collector with respect to base,

dc collector current, dc voltage at collector with respect to emitter,

B I

E I

C I CE V

CBV

BE V

Transistor current & voltage

reverse-biased the

base-collector junction

forward-biased the

base-emitter junction



3. BJT CHARACTERISTICS & PARAMETERSCurrent and Voltage Analysis:

The voltage at the collector with respect to the grounded emitteris:

Since the drop across is:

The dc voltage at the collector with respect to the emitter is:

where

The dc voltage at the collector with respect to the base is:

C RCC CE V V V

C C CC CE R I V V

BE CE CB V V V

C R C C RC

R I V

B DC C I I



Solution Example 1

V mAV R I V V C C CC CE

55.3)100)(5.64(10

When BE junction is FB, act as normal diode. So, VBE=0.7V.

The base current,

Collector current,

Emitter current,

Solve for VCE and VCB.

V V V V BE CE CB

85.27.055.3

Ak R

V V I

B

BE BB

B 430

10

7.05

mAAI I B DC C

5.64)430(150

mAAmAI I I B C E

9.644305.64



3. BJT CHARACTERISTICS & PARAMETERSCollector Characteristic Curve:

Using a circuit as shown in below, we can generate a set ofcollector characteristic curve that show how the collector current,Ic varies with the V CE voltage for specified values of base current,I B .

variable voltage



Collector characteristic curve:





Assume that VBB is set to produce a certain value of IB and VCC is zero.

At this condition, BE junction and BC junction are forward biasedbecause the base is approximately 0.7V while the emitter and thecollector are zero.

IB is through the BE junction because of the low impedance path to

ground, therefore IC is zero. When both junctions are forward biased – transistor operate in

saturation region .

As VCC increase, VCE is increase gradually, IC increase – indicated bypoint A to B.

IC increase as VCC is increased because VCE remains less than 0.7V dueto the forward biased BC junction.

When VCE exceeds 0.7V, the BC becomes reverse biased and thetransistor goes into the active or linear region of its operation.




Once BC junction is RB, IC levels off and remains constant for given

value of IB and VCE continues to increase.

Actually IC increases slightly as VCE increase due to widening of the BC depletion region

This result in fewer holes for recombination in the base region which

effectively caused a slight increase in indicated in pointB and C.

When VCE reached a sufficiently high voltage, the reverse biased BC junction goes into breakdown.

The collector current increase rapidly – as indicated at the right point C

The transistor cannot operate in the breakdown region.

When IB=0, the transistor is in the cutoff region although there is a verysmall collector leakage current as indicated – exaggerated on the graphfor purpose of illustration.

B DC C I I



DC Load Line:

Cutoff and saturation can be illustrated in relation to

the collector characteristic curves by the use of a load line.

DC load line drawn on the connecting cutoff and saturation point.

The bottom of load line is ideal

cutoff where IC=0 & VCE=VCC.

The top of load line is saturation

where IC=IC(sat) & VCE =VCE(sat)

In between cutoff and saturation

is the active region of transistor’s

operation.




Example 2

Determine whether or not the transistor in figure

below is in saturation. Assume VCE(sat) = 0.2V



Solution Example 2

First, determine IC(sat),

Now, see if IB is large enough to produce IC(sat),

With specific βDC, this base current is capable of

producing IC greater than IC(sat). Thus, transistor issaturated and IC = 11.5mA is never reached. Iffurther increase IB, IC remains at its saturation value.

mAk R

V V

I C

sat CE CC

sat C 8.90.1

2.010)(

)(

mAk R

V V I

B

BE BB

B 23.010

7.03

mAI I B DC C

5.11)23.0(50



Derating :

-Specified at 25°C, for higher temp, PD(max) is less.

-Data sheet often give derating factor for determining at > 25°C

-Example: derating factor of 2mW/°C indicates that the max. power

dissipation is reduced 2mW for each degree increase in temperature.

(max) D P

(max) D P




ON Characteristics

DC current gain ( I C = 0.1 mA dc, V CE = 1.0 V dc)

( I C = 1.0 mA dc, V CE = 1.0 V dc)

( I C = 10 mA dc, V CE = 1.0 V dc)

( I C = 50 mA dc, V CE = 1.0 V dc)

( I C = 100 mA dc, V CE = 1.0 V dc)

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

hFE 20

40

3570

50100

3060

1530

–

–

–

–

150300

–

–

–

–

–

Characteristic Symbol Max UnitMin

Data Sheets

Data sheets give manufacturer’s specifications for maximum operatingconditions, thermal, and electrical characteristics. For example, an

electrical characteristic is β DC , which is given as h FE. The 2N3904shows a range of β’s on the data sheet from 100 to 300 for I C = 10mA.



4. BJT AS AN AMPLIFIER

4 BJT AS AN AMPLIFIER




B DC C I I

• Transistor amplify currentbecause

• IB is very small, so IC ≈ IE.

• Amplification of a small acvoltage by placing the acsignal source in the base

circuit.• Vin is superimposed on the

DC bias voltage VBB byconnecting them in serieswith base resistor R B.

• Small changes in the basecurrent circuit causes largechanges in collector currentcircuit.

4 BJT AS AN AMPLIFIER



Voltage gain:resistanceemitter acinternal ' r e

C ec

R I V

b

c

V

V

V A

C c c

R I V

e e

C e

b

c

V

r I

R I

V

V A

'

e

C

V

r

R A

'

ec I I

• Ac emitter current is Ie ≈ Ic = Vb / r’e.

• Ac collector voltage, Vc equals ac voltage drop across Rc.

•Since , ac collector voltage is

• Vb is considered as ac input voltage where Vb=Vin - IbR B. Vc as the transistorac output voltage. The ratio of Vc to Vb is ac voltage gain, Av of the circuit.

•Substituting IeR C for Vc and Ier’ e for Vb, yields:




5. BJT AS A SWITCH

5 BJT AS A SWITCH



5. BJT AS A SWITCH

A transistor when used as a switch is simply being biased sothat it is in:

1. cutoff (switched off)

2. saturation (switched on)

5 BJT AS A SWITCH



Conditions in Cutoff

CC cutoff CE V V )(

C

sat CE CC sat C

R

V V I

)()(

DC

sat C B

I I

)((min)

Conditions in Saturation

Since VCE(sat) is very small compared to VCC, it can be neglected.

Neglect leakage current and all currentsare zero. BE junction is reverse biased.

5. BJT AS A SWITCH

E l 3



Example 3

a) For the transistor circuit in below figure, what is V

CE when V

IN=0v?

b) What minimum value of IB is required to saturatethis transistor if βDC is 200?

c) Calculate the maximum value of R B when VIN=5V.

Solution Example 3



Solution Example 3

a) When VIN=0V, the transistor is in cutoff (act asopen switch), so VCE(cutoff)=VCC = 10V.

b) Since VCE(sat) is neglected (assumed 0V),

This is the value of IB necessary to drive transistor topoint of saturation.

c) When transistor is ON, VBE=0.7V. The voltage acrossRB is

VRB=VIN – VBE = 5 – 0.7 = 4.3VBy Ohm’s Law, the maximum value of RB is:

AmAI

I

mAk

V

R

V I

DC

sat C

B

C

CC

sat C

50

200

10

100.1

10

)(

(min)

)(

k I

V R

B

RB

B 86

50

3.4

(min)

(max)



6. TROUBLESHOOTING

6 T bl h ti



6. Troubleshooting

Troubleshooting a live transistor circuit requires us to befamiliar with known good voltages, but some general rulesdo apply. Certainly a solid fundamental understanding ofOhm’s law and Kirchhoff’s voltage and current laws isimperative. With live circuits it is most practical to

troubleshoot with voltage measurements.

6 Troubleshooting



6. Troubleshooting

Voltage measurements that are typicallylow are caused by a point that not “electrically connected to ground”. Thiscalled a floating point. This is typically

indicative of an open.

More in-depth discussion of typicalfailures are discussed within thetextbook.

Possible faults are open bias resistors, open or resistive connections,shorted connections and open or short internal to the transistor itself.

Correct voltage measurement

6 Troubleshooting



6. Troubleshooting

Testing a transistor can be viewed more simply if you view itas testing two diode junctions. Forward bias having lowresistance and reverse bias having high resistance.

6 Troubleshooting



6. Troubleshooting

The diode test function of a multimeter is more reliable thanusing an ohmmeter. Make sure to note whether it is an npn or

pnp and polarize the test leads accordingly.



Summary



Summary

The bipolar junction transistor (BJT) is constructed of

three regions: base, collector, and emitter. The BJT has two p-n junctions, the base-emitter junction and the base-collector junction.

The two types of transistors are pnp and npn.

For the BJT to operate as an amplifier, the base-emitter junction is forward biased and the collector-base junction isreverse biased (transistor in active region).

Of the three currents IB is very small in comparison to IE

and IC. Beta is the current gain of a transistor. This the ratio ofIC /IB.

Summary



Summary

A transistor can be operated as an electronics switch.

When the transistor is off it is in cutoff condition (nocurrent).

When the transistor is on, it is in saturation condition(maximum current).

Beta can vary with temperature and also varies fromtransistor to transistor.

bjt operations

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