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

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

3. Dear Student:
This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you
to view that segment again, if you want to.

4. Concept Preview Efficiency is most important in power amplifiers.
Poor efficiency means that much of the input power is converted to heat.
A class A amplifier conducts for the entire signal cycle and has the lowest efficiency.
A class B amplifier conducts for only half of the signal cycle.
A class C amplifier conducts for less than half of the signal cycle.
A class D amplifier switches between cutoff and saturation.

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

6. 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.

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

8. 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

9. Concept Review Efficiency is most important in power amplifiers.
Poor efficiency means that much of the input power is converted to heat.
A class A amplifier conducts for the entire signal cycle and has the lowest efficiency.
A class B amplifier conducts for only half of the signal cycle.
A class C amplifier conducts for less than half of the signal cycle.
A class D amplifier switches between cutoff and saturation.
Repeat Segment

10. Concept Preview
Class A amplifiers operate at the center of the load line and have a large quiescent current flow.
Class B amplifiers operate at cutoff and have no quiescent current flow.
Class B amplifiers are usually operated in push-pull configurations.
Class B amplifiers have crossover distortion.
Class AB reduces crossover distortion.
Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

11. 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

12. 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

13. 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

14. 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.

15. 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

16. Class B
NPN
PNP

17. 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

18. 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

19. 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

20. 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

21. 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

22. Concept Review
Class A amplifiers operate at the center of the load line and have a large quiescent current flow.
Class B amplifiers operate at cutoff and have no quiescent current flow.
Class B amplifiers are usually operated in push-pull configurations.
Class B amplifiers have crossover distortion.
Class AB reduces crossover distortion.
Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.
Repeat Segment

23. Concept Preview
Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency.
Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.
Class C amplifiers cannot be used in wideband applications like audio.
Class D amplifiers switch between cutoff and saturation for very high efficiency.
Class D amplifiers operate at a relatively high switching frequency and often use PWM.
Class D can be used in audio applications.

24. 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.

25. 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.

26. 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.

27. 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

28. 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.

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

30. 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

31. 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

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

33. 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

34. Concept Review
Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency.
Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.
Class C amplifiers cannot be used in wideband applications like audio.
Class D amplifiers switch between cutoff and saturation for very high efficiency.
Class D amplifiers operate at a relatively high switching frequency and often use PWM.
Class D can be used in audio applications.
Repeat Segment

35. REVIEW
Amplifier Class
Class A
Class B
Class AB
Class C
Class D