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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
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DC Equivalent CircuitDC Equivalent Circuit
12
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AC Equivalent CircuitAC Equivalent Circuit
Without AC SourceWithout AC Source
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AC Equivalent CircuitAC Equivalent Circuit
With AC SourceWith AC Source
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AC Equivalent Base CircuitAC Equivalent Base Circuit
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Input Resistance at the BaseInput Resistance at the Base
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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))
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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)
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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
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Overall Current Gain= Ai = Ic/Is
Common Collector AmplifierCommon Collector Amplifier
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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
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NumericalNumerical
3636
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Darlington PairDarlington Pair
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Application ofApplication of Darlington PairDarlington Pair
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2
Application of Darlington PairApplication of Darlington Pair (Continued ….)(Continued ….)
Equation 1
And,
,
3939
Common Base AmplifierCommon Base Amplifier
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2
DC FormulasDC Formulas
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AC FormulasAC Formulas
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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.