6- BJT Amplifiers

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BJT AmplifiersBJT Amplifiers

CHAPTER #6CHAPTER #6

1

AC QuantitiesAC Quantities

DC Quantities: R C, IC, IE,VCE

2

AC Quantities: Ie,Vce

Instantaneous Quantities:

ic ie vce



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Linear AmplifierLinear Amplifier

3

Graphical AnalysisGraphical Analysis

4Example 6-1 Do Yourself



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AC Equivalent CircuitsAC Equivalent CircuitsAn Equivalent circuit uses various internal transistor’s

parameters to represent the transistor’s operation e.g.

r parameters an parameters.

5

AC Equivalent Circuits (continued…)AC Equivalent Circuits (continued…)

Short (Low)

Open

(High)

6



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FindingFinding rree´́

´́ == oo EE

r ree´́ is the most important ris the most important r--parameter and isparameter and is

temperature dependent.temperature dependent.

7Example 6-2 Do Yourself

Comparison ofComparison of ββacac andand ββDCDC

••At different points onAt different points on

the nonthe non--linear curve, thelinear curve, the

8

ratio will be different.ratio will be different.

••ββacac andand ββDCDC can differcan differ

slightly.slightly.



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h h ParametersParameters

9

Relationship betweenRelationship between h h && r r ParametersParametersDirect Relation:Direct Relation:

hh--parameters are provided byparameters are provided by

,,

while rest of the parameterswhile rest of the parameters

are calculated.are calculated.

10



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CommonCommon--Emitter AmplifierEmitter Amplifier

11

DC Equivalent CircuitDC Equivalent Circuit

12



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AC Equivalent CircuitAC Equivalent Circuit

Without AC SourceWithout AC Source

13

AC Equivalent CircuitAC Equivalent Circuit

With AC SourceWith AC Source

14



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AC Equivalent Base CircuitAC Equivalent Base Circuit

15

Input Resistance at the BaseInput Resistance at the Base

16



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1717

Voltage GainVoltage GainRatio of ac Output voltage (VRatio of ac Output voltage (Vcc) to ac Input voltage (V) to ac Input voltage (Vbb))

18



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Base CircuitBase Circuit

Attenuation and Overall GainAttenuation and Overall Gain

19Overall Gain = Attenuation (Source to Base) x Gain (Base to Collector)

Effect of Emitter Bypass Capacitor (C2)Effect of Emitter Bypass Capacitor (C2)on Voltage Gainon Voltage Gain

It rovides an effective short to the ac si nalIt rovides an effective short to the ac si nal

around R around R EE keeping the emitter at ac groundkeeping the emitter at ac ground

Voltage gain is maximum and equal to Rc/re’Voltage gain is maximum and equal to Rc/re’

CC should be large enough so that Xcshould be large enough so that Xc zerozero

RuleRule--of of--ThumbThumb

20

Example 6-4 Do Yourself



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Some other Effects on Voltage GainSome other Effects on Voltage Gain Voltage gain without bypass capacitorVoltage gain without bypass capacitor

AAvv = R = R CC / (/ (rree ́ +́+R R EE)) (Gain decrease)(Gain decrease)

Effect of load RL on voltage gainEffect of load RL on voltage gain

R R cc = R = R CC R R LL / (R / (R CC++R R LL))

AAvv = R = R cc // rree ́ ́ (If(If R R cc < R < R CC then reduced. If R then reduced. If R LL>>Rc then>>Rc then

R R = R = R and ne li ible effect on ainand ne li ible effect on ain

Stability of voltage gainStability of voltage gain rree ́ ́ depends on Idepends on IEE and Temperature.and Temperature.

If RIf REE >>>> rree ́ ́ then Athen Avv is independent of ris independent of ree ́ ́ andand

AAvv = R= RCC / R / REE 2121

SwampingSwamping(Method of minimizing effect of r(Method of minimizing effect of ree ́ without ́ without

reducing Areducing Avv to its minimum value)to its minimum value)

== ́ ́++

22

vv ee

If R If R EE11 >> r>> ree ́ ́ ((1010x) thenx) then

AAvv = R = R CC / R / R EE11



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23

DC AnalysisDC Analysis(Determination of DC Bias Values)(Determination of DC Bias Values)

24



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AC AnalysisAC Analysis

(Determination of AC Bias Values)(Determination of AC Bias Values)

25

Collector Voltage (Peak Values)Collector Voltage (Peak Values)

2626



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VVrmsrms Riding on VRiding on VDCDC

2727

Current GainCurrent Gain

1. Current Gain: βac = Ic/I b

2. Overall Gain= Ai = Ic/Is

= +

28

.(tot)

s

in(tot)

4. Is = Vs/R (tot)

OR

Is = Vs/(R s + R in(tot))



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Power GainPower Gain

O. Power Gain = O. Volta e Gain x O. Current Gain

Ap = Av' x Ai

Where

Overall Voltage Gain= Av' = Vc/Vs

29

Overall Current Gain= Ai = Ic/Is

Common Collector AmplifierCommon Collector Amplifier

30



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Voltage gain (AVoltage gain (Avv))

3131

Why EmitterWhy Emitter--Follower?Follower?

3232



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Input ResistanceInput Resistance

3333

Output ResistanceOutput Resistance

3434

(For Derivation, See Appendix B)



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Current GainCurrent Gain

Power GainPower Gain

35

NumericalNumerical

3636



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Darlington PairDarlington Pair

37

Application ofApplication of Darlington PairDarlington Pair

38



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2

Application of Darlington PairApplication of Darlington Pair (Continued ….)(Continued ….)

Equation 1

And,

,

3939

Common Base AmplifierCommon Base Amplifier

40



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2

DC FormulasDC Formulas

41

AC FormulasAC Formulas

42



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2

MultiMulti--StageStage AAmplifiersmplifiers

43

Example 6-12 Do Yourself

MultiMulti--Stage Amplifier AnalysisStage Amplifier Analysis

44



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MultiMulti--Stage Amplifiers (continued…)Stage Amplifiers (continued…)

45

Direct Coupled (Direct Coupled (MultiMulti--StageStage

AmplifiersAmplifiers))

1. In capacitively coupled amplifiers,

increased reactance of capacitors at

lower frequencies produces gain

reduction.

2. Direct-coupled amplifiers, on the other

hand, can be used to amplify low

frequencies all the way down to dc

(0Hz) without loss of voltage gain.

46

3. However, small changes in DC voltagesfrom temperature effects or power-

supply variation are amplified by the

succeeding stages, which can result in

a significant drift in the dc levels

throughout the circuit.

6- BJT Amplifiers

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