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Biasing the BJT

To operate as an amplifier a transistor must be biased in

the active region

The transistor, when properly biased can amplify a small

ac signal faithfully

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Transistor Circuit

under D. C. Condition

Transistor Circuit withA. C. Signal at input

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iB= IB+ IbiE = IE+ ieiC = IC+ ic

IB, IC,IE - D.C. currentsIb, ic, ie A.C. currents

iB, iC, iE D.C + A.C. currents

Similarly,

VBE, VCE D.C. Voltages

vbe, vce A.C.VoltagesvBE, vCE D.C+ A.C Voltages

Transistor Circuit with A. C. Signal

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vCE = VCC- iCRC= VCC - (IC+ ic)RC= VCC- ICRC - icRC= VCE icRC

VCE = D.C. Bias voltage at

the collector

Considering only the A. C. Voltage at the collector,

vo= vce = - icRC

Voltage gain = vo/vi

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VBE =VBB IBRB

Transistor Circuit under D. C. Condition

VBB/RB

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VCE =VCC- ICRCWhen IC = 0, VCE =VCCWhen VCE = 0, IC =VCC/RC

VCC/RC

Q is the operating point

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Operating Point or Quiscent Point

The intersection of the load line with the transistoroutput characteristic for a particular IB gives the

operating point Q(VCEQ, ICEQ)

VCC/RC

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Effect of Q-point location on allowable signal swing

Load- line A results in an

operatingpoint too close to the

cut-off region and thus limits

the positive swing ofvCE

Load-line B results in an

operatingpoint too close to the

saturation region and thus limits

the negative swing of vCE

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Selection ofOperating Point Q

A good selection of the operatingpoint Q is essential inorder that an amplifier circuit amplifies an A.C signal

without any distortion

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BiasingMethods

1. Fixed Bias

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Fixed Bias Circuit

D.C. EquivalentC1 and C2 open circuit under D.C.

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Using KVL in the base-emitter loop,

VCC IBRBVBE = 0

or, IB = (VCC-VBE)/RB

IC = IB=(VCC-VBE)/RB

Using KVL in the collector-emitter

loop,

VCC ICRCVCE = 0

or, VCE = VCC - ICRCQ(VCE,IC) is set

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2. Emitter and Collector Feedback Bias

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Emitter and Collector Feedback Bias

D.C. EquivalentApplying KVL

or, VCC- (IC+IB)RC- IBRB-VBE-(+1)IBRE= 0

or, VCC- (IB+IB)RC- IBRB-VBE- (+1)IBRE = 0or, VCC {RB+(+1) (RC+RE)}IB-VBE = 0

or, )R(R1RVV

IECB

BECC

B

! VCE = VCC- (IC+IB)(RC+RE)

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BiasS

tabilization

)(1)(

I

If RB

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3. Voltage Divider Bias

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D.C. EquivalentVoltage Divider Bias

IR1

IR2 IB

IC

IE

Ignoring IB(as very small), IR1 = IR2We can use Thevenins Theorem

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Thevenins Equivalent Circuit

for the base-emitter loop

)( 21

2

Th

!

)( 21

21

21Th

!!

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Base-Emitter Loop

ETh

BEThB

EBBEThBTh

R1R

VVIor,

0RI1VRIV

!

!

Collector- Emitter Loop

ETh

BEThBC

R1R

)V(VII

!!

EBCCCCCEECCCCCE )RI(IRIVRIRIVV !!

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Bias Stabilization

ETh

ETh

1)(I

!

If RTh

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Ex.1

Fig.1

For the circuit shown in Fig.1,

find the change in IC when

changes from 100 to 300.