This document discusses different classes of power amplifiers: - Class A amplifiers operate near the center of the load line and have a conduction angle of 360 degrees but maximum efficiency of only 25%. - Class B amplifiers have their operating point at cutoff, a conduction angle of 180 degrees, and a maximum theoretical efficiency of 78.5%. - Class AB amplifiers have conduction angles between Class A and B to reduce crossover distortion. - Class C amplifiers use tank circuits and have a small conduction angle below 90 degrees but maximum theoretical efficiency of 100%. - Class D or switch-mode amplifiers use pulse-width modulation to rapidly switch transistors between cutoff and saturation for very high efficiency.

1. Electronics
Principles & Applications
Sixth Edition
Chapter 8
Large-Signal Amplifiers
©2003 Glencoe/McGraw-Hill
Charles A. Schuler

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

3. Power
Amplifier
PIN
Efficiency =
Input signal
POUT
POUT
PIN
Output signal
HEAT = PIN - POUT
High efficiency means less heat.

4. Efficiency
• The dc power supplied to an amplifier is
PIN = VCC x IDC
• Efficiency = POUT/PIN 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
IC
t
IC
t
IC
t
IC
ISAT
A B
C D
The major classes of amplifier operation

6. Class and Efficiency Quiz
If POUT = 100 W and PIN = 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
VCC = 18 V
RL = 12 W
RB = 1.2 kW
CC
b = 60
IB =
VCC
RB
18 V
1.2 kW
= = 15 mA
IC = b x IB = 60 x 15 mA = 0.9 A

8. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
ISAT =
VCC
RL
18 V
12 W
= = 1.5 A
Q
This is a Class A amplifier.
PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

9. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
Q
This is a Class B amplifier.
PC = VCE x IC = 18 V x 0 A = 0 W
Its quiescent power dissipation is zero.

10. 0 2 4 6 8 10 12 14 16
0.2
0.4
0.6
0.8
1.0
1.2
1.4
5 mA
0 mA
25 mA
20 mA
15 mA
10 mA
The collector signal
is too distorted for
linear applications.

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

12. NPN
PNP
Class B

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

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

15. 0 2 4 6 8 10 12 14 16 18
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCE in Volts
IC in A
Q
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.

16. Cap. required
+VCC
RL
RL
+VCC
Single-ended amplifier
A bridge-tied load provides four times the output
power for a given supply voltage and load resistance.
+VCC
2
Max.
Max. = VCC
Bridge amplifier
Max. = 2 x VCC
Max.

17. 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
_________. 180o
To reduce distortion, two class B transistors
are arranged in _____________. push-pull
Class AB is a solution for __________
distortion.
crossover

18. 0 2 4 6 8 10 12 14 16 18
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.

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

20. C
B
E
VCC
RB
CC
VBB
Class C Amplifier
Tank circuit
The transistor is
off for most of
the input cycle
and the conduction
angle is small.
VBB reverse biases the base-emitter junction.

21. VBB
0.7 V
0 A
VBE waveform
IC waveform
VCE waveform
Class C amplifier
waveforms
(with tank circuit)
Low VCE when IC is flowing

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

23. IB >> 0
Three transistor operating modes:
IB = 0 IB > 0
Cutoff Linear Saturation
PC = 0 in both of these modes

24. 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
RB
They are also
called Class D
amplifiers.

25. 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

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

27. 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

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