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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition
Chapter 8Large-Signal Amplifiers
Charles A. Schuler
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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• Amplifier Class• Class A• Class B• Class AB• Class C• Class D
INTRODUCTION
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PowerAmplifier
PIN
Efficiency =
Input signal
POUT
POUT
PIN
Output signal
HEAT = PIN - POUTHigh efficiency means less heat.
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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.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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t
IC
t
IC
t
IC
t
IC
ISAT
A B
C D
The major classes of amplifier operation
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Class and efficiency quiz
If POUT = 100 W and PIN = 200 W, theefficiency is _________. 50%
The efficiency of an ideal amplifier is__________. 100%
When efficiency is poor, too much of the inputis converted to ________. heat
An amplifier that conducts for the entire cycleis operating Class _______. A
An amplifier that conducts for half the cycleis operating Class _______. B
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A large-signal amplifier can also be called a power amplifier.This class A amplifier has a large quiescent collector current.
C
BE
VCC = 18 VRL = 12 RB = 1.2 k
CC
= 60
=VCC
RB
18 V
1.2 k= = 15 mA
IC = x IB = 60 x 15 mA = 0.9 A
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VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
mA
10 mA
ISAT =VCC
RL
18 V12 = = 1.5 A
Q
This is a Class A amplifier.
PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W
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VCE in Volts
IC in A
5 mA
0 mA
25 mA
20 mA
mA
10 mAQ
This is a Class B amplifier.
PC = VCE x IC = 18 V x 0 A = 0 W
Its quiescent power dissipation is zero.
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5 mA
0 mA
25 mA
20 mA
mA
10 mA
Cla
ss B
The collector signal is too distorted forlinear applications.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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C
B
C
B E
E
+VCC
The complementary-symmetry Class B push-pull amplifier has acceptable
linearity for some applications.
NPN
PNP
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C
B
C
B E
E
+VCC
Since the base-emitter junction potentialis 0.7 V, there is some crossover distortion.
NPN
PNP
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C
B
C
B E
E
+VCC
Crossover distortion is eliminatedby applying some forward bias
to the transistors (class AB).
NPN
PNP
1.4 V
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VCE in Volts
IC in AQ
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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Cap. required
+VCC
RL
RL
+VCC
Single-ended amplifier
A bridge-tied load provides four times the outputpower for a given supply voltage and load resistance.
+VCC
2
Max.
Max. = VCC
Bridge amplifier
Max. = 2 x VCC
Max.
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Class A, B, and AB quiz
Class A amplifiers are biased to operate nearthe ________ 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 transistorsare arranged in _____________. push-pull
Class AB is a solution for __________distortion. crossover
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A
B
C
AB
The class of an amplifieris determined by the bias
which establishes the Q-point.
Class C is established by reversebiasing the base-emitter junction.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
8-19
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.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
8-20
C
BE
VCC
RB
CC
VBB
Class C amplifier
VBB reverse biases the base-emitter junction.
Tank circuit
The transistor isoff for most ofthe input cycle
and the conductionangle is small.
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VBB
0.7 V
0 A
VBE waveform
IC waveform
VCE waveform
Class C amplifierwaveforms
(with tank circuit)
Low VCE when IC is flowing
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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C
BE
VCC
RB
CC
Class C amplifier with signal bias
The base-emitterjunction rectifiesthe input signaland charges CC.
Signal bias increases when the input signal increases in amplitude.
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IB >> 0
Three transistor operating modes:
IB = 0 IB > 0
Cutoff Linear Saturation
Ideally, PC = 0 in both of these modes
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A switch-mode amplifier uses a rectangular input signal to drive power transistors rapidly between cutoff and
saturation. The efficiency is high, allowing large power outputs from relatively small boards and heat sinks.
C
BE
RB
They are alsocalled Class D
amplifiers.
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
8-25Two, 250-W outputs into 4-loads, or one 500-W bridged
output into an 8-load
classdaudio.com
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Power Out
Note that Class D offers high efficiency when the amplifier output is modest or low!
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If the switching frequency is a good deal higherthan the signal frequency, a Class D amplifier is
capable of linear amplification. Pulse-widthmodulation and a low-pass filter are often used.
PWM Signal
Input Signal
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Low-pass filterComparator Class D amplifier
Triangular wave
Input signal Output signal
Max.pulsewidth
Min.pulsewidth
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PWM LPF
The low-pass filter rejects the switching frequency.
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Class C and D quiz
Class C amplifiers use _______ circuits torestore sinusoidal signals. tank
The base-emitter junction in a class Camplifier 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