Electronic Circuits ELCT604
Lecture 11: Output Stages (Power Amplifiers)
Dr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: eman.azab@guc.edu.eg
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Introduction
 Cascaded voltage amplifiers are used extensively in Circuit design
 The output stage of the voltage amplifier needs to have the
following specs:
1.
2.
3.
4.
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It must deliver a specified amount of signal power to a load (Power
amplification)
It should have low levels of signal distortion (Linear)
It has low output impedance
Low DC Standby power (Quiescent)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Introduction
Example of Cascaded Voltage Amplifiers:
OP-AMP
 It consists of three voltage amplifiers
stages:

1.
2.
3.
3
Differential Amplifier: to provide a
differential input signal and high Input
impedance
A
Gain
Stage:
A
Common
Source/Emitter with high gain. However,
these configurations has high output
impendence
Output Stage: deliver large signal with no
distortion to the load and has low output
impedance
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Output Stages Classification
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Output Stages classifications
 Output
stages are Classified
according to the basic amplifier
transistor’s Large signal Current
(iC or iD) versus the input voltage


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If the transistor conducts through
the complete cycle of the input
voltage with a DC average, then it
is a “Class A” Power Amplifier
This classification is based on the
DC Biasing of the Transistor
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Example of Class A
Copyright © 2010 by Oxford University Press, Inc.
Output Stages classifications
If the transistor conducts through the half cycle of the input voltage,
then it is a “Class B” Power Amplifier
 If the transistor conducts through more than half cycle of the input
voltage, then it is a “Class AB” Power Amplifier


This classification is based on the DC Biasing of the Transistor
Example
of Class B
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Example of
Class AB
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class A: Power Amplifier
Emitter/Source Follower
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class A: Emitter Follower
 Class A output stage is designed using Emitter Follower (Common
Collector)
OneTransistor is conducting for all values of ‘vin’
 The circuit small signal voltage gain is approximately unity
 Low Output impedance
 The circuit should be able to deliver high output current to the load

 In the following analysis we will deal with the large signal analysis
of the amplifiers

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Since the input voltage signal is already amplified
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class A: Emitter Follower
 Circuit Description:
Q1 is the basic amplifier
(Active mode)
 Q2, Q3 and R are used to set
the DC Current for Q1
(Current Source)

v O  v I v BE 1
vo
iE1  I 
RL
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class A: Emitter Follower
 Circuit Analysis:

If β is large, then the emitter
and collector currents are
almost equal
vO

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vO 

I R 
L 
 v I V T ln 
 Is 




This is a non-linear Equation,
thus we will consider
different values for RL
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class A: Emitter Follower
 Circuit Transfer CKHs:
For Large RL, the term
vO/RL can be neglected
 For positive vI, Q1 is
driven from active to
saturation
v O ,max V CC V CE 1,sat

v O  v I V T
 I 
ln 

I
 s 
For negative vI, Q2 is
driven from active to
saturation, Q1 is off
v O ,min  V CC V CE 2,sat

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class A: Emitter Follower
 Circuit Transfer CKHs:
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
For Small RL, the term
vO/RL can be neglected
for positive vI, Q1 is
driven from active to
saturation
v O ,max V CC V CE 1,sat

For negative vI, Q1 is off
when v
O ,min  IR L
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
v O  v I V T
vO

I


RL
ln 
Is









Copyright © 2010 by Oxford University Press, Inc.
Class A: Emitter Follower
 Notes:
For Class A to have a maximum output swing the following
condition must be satisfied:
V CC V CE 2,sat
IR L  V CC V CE 2,sat
R L ,min 
I
 Power Conversion Efficiency: ηc
 It is the ratio of the average power delivered to the load to the
average power drawn from the supply

C
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
PL

PDC
Class A: Emitter Follower
 Notes:

“Class A” Power Conversion Efficiency: ηc
 PL is the load average power, (root mean square of half the peakto-peak voltage and current at the load)
1 ˆ ˆ
1 VˆO2
PL  V O I O 
2
2 RL
PDC is the power drawn from the supply
 Maximum ηc is at the minimum Load

R L ,min 
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V CC V CE 2,sat
I
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
C ,max
PDC  2V CC I
1 V CC V CE ,sat

2
2V CC IR L

2
 0.25
Class B: Power Amplifier
Push-Pull Emitter/Source Follower
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class B: Push-Pull Emitter Followers
 Class B output stage is designed using two Emitter Follower
(Common Collector)
TwoTransistors are conducting, each for half a cycle of ‘vin’
 The circuit small signal voltage gain is approximately unity
 Low Output impedance
 The circuit should be able to deliver high output current to the load

 In the following analysis we will deal with the large signal analysis
of the amplifiers

16
Since the input voltage signal is already amplified
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class B: Push-Pull Emitter Follower
 Circuit Description:
QN and QP are the basic
amplifier (Active mode)
 For
-0.7<vI<0.7
both
transistors are off and vO is
zero
 For vI>0.7, QN will conduct
and QP will be off

v O  v I v BE ,N

For vI<-0.7, QP will conduct
and QN will be off
v O  v I v EB ,P
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class B: Push-Pull Emitter Follower
 Circuit Transfer CKHs: :
Each transistor conducts only for a half cycle and thus PDC decreases
in Class B compared to Class A
 Crossover distortion exists in Class B (zero output while the input is
crossing the zero value)

v O ,max V CC V CEN ,sat
v O ,min  V CC V ECP ,sat
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class B: Push-Pull Emitter Follower
 Notes:

“Class B” Power Conversion Efficiency: ηc
 PL is the load average power, (root mean square of half the peakto-peak voltage and current at the load)
1 ˆ ˆ
1 VˆO2
PL  V O I O 
2
2 RL

PDC is the power drawn from the supply
PDC  2V CC I avg

2VˆOV CC

 RL
Maximum ηc is at maximum output voltage C ,max    0.785
4
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class B: Push-Pull Emitter Follower
 Notes:
At Zero input voltage, no power is drawn from the supplies, thus
the Quiescent power is Zero
 To eliminate the crossover distortion, many techniques are used to
allow a small current to flow in the two transistors at zero input
 This new modification is called class AB Output stages

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class AB: Power Amplifier
Push-Pull Emitter/Source Follower
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Class AB: Push-Pull Emitter Follower
 Circuit Description:
QN and QP are the basic
amplifier (Active mode)
 For vI>0, QN will conduct
and QP will be off

v O  v I v BE ,N 

V BB
v I
2
For vI<0, QP will conduct
and QN will be off
v O  v I v EB , p 
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V BB
v I
2
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Class AB: Push-Pull Emitter Follower
 Circuit Transfer CKHs: :
Each transistor conducts only
for a half cycle and thus PDC
decreases in Class B
compared to Class A
 Crossover
distortion
is
eliminated

v O ,max V CC V CEN ,sat
v O ,min  V CC V ECP ,sat
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Sedra/Smith
Copyright © 2010 by Oxford University Press, Inc.
Power Amplifiers
 Notes:
There are different techniques to remove the crossover distortion in
Class B other than the ones mentioned in the slides
 Linearity of power amplifiers is very important, it can be calculated
using the System and Control theory of linear systems


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The output voltage is analyzed and the any components function in higher
orders of the input voltage are calculated and considered nonlinear terms.
(Harmonics)

Please refer to the text book for this part (Sedra or Gray)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo