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

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

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

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. A B C D The major classes of amplifier operation IC IC t t IC IC ISAT

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. RB = 1.2 kW RL = 12 W VCC = 18 V C b = 60 B CC E VCC 18 V IB = =
A large-signal amplifier can also be called a power amplifier.
This class A amplifier has a large quiescent collector current.
IB =
VCC RB
18 V
1.2 kW =
= 15 mA
IC = b x IB = 60 x 15 mA = 0.9 A
RB = 1.2 kW
RL = 12 W
VCC = 18 V C
b = 60 B CC E

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

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

10. 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
Class B
The collector signal
is too distorted for
linear applications.

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

12. Class B
NPN
PNP

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

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

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

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

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. The class of an amplifier is determined by the bias
1.4
1.2
1.0 AB B
0.8
0.6
0.4
0.2 C 2 4 6 8 10 12 14 16 18
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 %*
Class B = 180o %
Class AB @ 200o (between A & B)
Class C @ 90o %
*Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

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

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

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

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

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

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. The low-pass filter rejects the switching frequency.
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