power amplifiers 4
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ElectronicsElectronics Principles & ApplicationsPrinciples & Applications
Seventh EditionSeventh Edition
Chapter 8Large-Signal Amplifiers
(student version)
Charles A. Schuler
McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.
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• Amplifier Class• Class A• Class B• Class AB• Class C• Class D
INTRODUCTION
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Dear Student:
This presentation is arranged in segments. Each segmentis preceded by a Concept Preview slide and is followed by aConcept Review slide. When you reach a Concept Reviewslide, you can return to the beginning of that segment byclicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.
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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.
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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.
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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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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
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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.
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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 V1.2 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 mA20 mAmA
10 mA
ISAT =VCC
RL
18 V12 = = 1.5 A
QThis 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 mA20 mAmA
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 mA20 mAmA
10 mA
Class B
The collector signal is too distorted forlinear applications.
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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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NPN
PNP
Class B
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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 A Q
The quiescent power dissipation is moderate for class AB.
The efficiency is much better than class A.
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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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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
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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.
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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.
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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.
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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
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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(PC > 0)
Saturation
PC = 0 in both of these modes
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A switch-mode amplifier uses a rectangular input signal to drive the transistor rapidly between cutoff and
saturation. The efficiency is very high.
C
BERB
They are alsocalled Class D
amplifiers.
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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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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
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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
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REVIEW• Amplifier Class• Class A• Class B• Class AB• Class C• Class D