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1. Solid State Devices and Solid State Devices and
Circuits (ECN-13104)Circuits (ECN-13104)
Unit-6
Power Amplifiers

2. •Amplifier Class
•Class A
•Class B
•Class AB
•Class C
•Class D
INTRODUCTION

3. Power
Amplifier
P
IN
Efficiency =
Input signal
P
OUT
P
OUT
P
IN
Output signal
HEAT = P
IN
- P
OUT
High efficiency means less heat.

4. Efficiency
•The dc power supplied to an amplifier is
P
IN = V
CC x
I
DC
•Efficiency = P
OUT/P
IN x 100%
•The maximum efficiency for Class A
amplifiers with a dc collector resistance
and a separate load resistance is 25%.
•Class A is usually not acceptable when
watts of power are required.

5. t
I
C
t
I
C
t
I
C
t
I
C
I
SAT
A B
C D
The major classes of amplifier operation

6. Class and Efficiency Quiz
If P
OUT
= 100 W and P
IN
= 200 W, the
efficiency is _________. 50%
The efficiency of an ideal amplifier is
__________. 100%
When efficiency is poor, too much of
the input is converted to ________. heat
An amplifier that conducts for the entire
cycle is operating Class _______. A
An amplifier that conducts for half
the cycle is operating Class _______.B

7. A large-signal amplifier can also be called a power amplifier.
This class A amplifier has a large quiescent collector current.
C
B
E
V
CC = 18 V
R
L = 12 R
B = 1.2 k
C
C
= 60


=
V
CC
R
B
18 V
1.2 k
= = 15 mA
I
C
=  x I
B
= 60 x 15 mA = 0.9 A

8. 024681012141618
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V
CE
in Volts
I
C in A
5 mA
0 mA
25 mA
20 mA
mA
10 mA
I
SAT
=
V
CC
R
L
18 V
12 
= = 1.5 A
Q
This is a Class A amplifier.
P
C
= V
CE
x I
C
= 7.2 V x 0.9 A = 6.48 W

9. C
B
C
B
E
E
+V
CC
The complementary-symmetry Class B
push-pull amplifier has acceptable
linearity for some applications.
NPN
PNP

10. 024681012141618
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V
CE in Volts
I
C in A
5 mA
0 mA
25 mA
20 mA
mA
10 mA
Q
This is a Class B amplifier.
P
C
= V
CE
x I
C
= 18 V x 0 A = 0 W
Its quiescent power dissipation is zero.

11. 0246810121416
0.2
0.4
0.6
0.8
1.0
1.2
1.4
5 mA
0 mA
25 mA
20 mA
mA
10 mA
C
la
ss B
The collector signal
is too distorted for
linear applications.

12. C
B
C
B
E
E
+V
CC
Since the base-emitter junction potential
is 0.7 V, there is some crossover distortion.
NPN
PNP

13. C
B
C
B
E
E
+V
CC
Crossover distortion is eliminated
by applying some forward bias
to the transistors (class AB).
NPN
PNP
1.4 V

14. 024681012141618
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V
CE in Volts
I
C
in A
Q
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.

15. Class A, B, and AB Quiz
Class A amplifiers are biased to operate
near the ________ of the load line.center
Class B amplifiers have their Q-points
at ____________. cutoff
The conduction angle for class B is
_________. 180
o
To reduce distortion, two class B transistors
are arranged in _____________. push-pull
Class AB is a solution for __________
distortion.
crossover

16. 024681012141618
0.2
0.4
0.6
0.8
1.0
1.2
1.4
A
B
C
AB
The class of an amplifier
is determined by the bias
which establishes the Q-point.
Class C is established by reverse
biasing the base-emitter junction.

17. Conduction Angles
& theoretical max. efficiencies:
•Class A = 360
o
50 %*
•Class B = 180
o
78.5 %
•Class AB  200
o
(between A & B)
•Class C  90
o
100 %
*Class A amplifiers are seldom driven to
maximum output and typically provide much
less efficiency.

18. C
B
E
V
CC
R
B
C
C
V
BB
Class C Amplifier
Tank circuit
The transistor is
off for most of
the input cycle
and the conduction
angle is small.
V
BB reverse biases the base-emitter junction.

19. V
BB
0.7 V
0 A
V
BE waveform
I
C waveform
V
CE
waveform
Class C amplifier
waveforms
(with tank circuit)
Low V
CE when I
C is flowing

20. C
B
E
V
CC
R
B
C
C
Class C Amplifier with Signal Bias
The base-emitter
junction rectifies
the input signal
and charges C
C
.
Signal bias increases when the input
signal increases in amplitude.

21. I
B >> 0
Three transistor operating modes:
I
B
= 0 I
B > 0
Cutoff Linear Saturation
P
C = 0 in both of these modes

22. A switch-mode amplifier uses a
rectangular input signal to drive the
transistor rapidly between cutoff and
saturation. The efficiency is very high.
C
B
E
R
B
They are also
called Class D
amplifiers.

23. If the switching frequency is a good deal higher
than the signal frequency, a Class D amplifier is
capable of linear amplification. Pulse-width
modulation and a low-pass filter are often used.
PWM Signal
Input Signal

24. PWM LPF
The low-pass filter rejects
the switching frequency.

25. Class C and D Quiz
Class C amplifiers use _______ circuits
to restore sinusoidal signals.
tank
The base-emitter junction in a class C
amplifier is ________ biased. reverse
The theoretical maximum efficiency for
class C is ___________. 100%
Class D amplifiers are also known as
__________ amplifiers. switch-mode
Class D amplifiers employ a varying
duty-cycle known as _________. PWM

26. REVIEW
•Amplifier Class
•Class A
•Class B
•Class AB
•Class C
•Class D