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Chapter
31
Power Amplifiers
Topics Covered in Chapter 31
31-1: Classes of Operation
31-2: Class A Amplifiers
31-3: Class B Push-Pull Amplifiers
31-4: Class C Amplifiers
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31-1: Classes of Operation
 The class of operation for an amplifier is defined by
the percentage of the ac input cycle that produces an
output current.
 The class of operation for an amplifier determines its
power efficiency.
 The class also determines how much the input signal
is distorted by the amplifier.
 The classes of transistor amplifiers are
 Class A
 Class B
 Class C
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© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
31-1: Classes of Operation
 Fig. 31-1 illustrates class of operation
for transistor amplifiers in terms of
conduction angle.
 In Fig. (a) two cycles of sine wave
input are shown.
 Fig. (b) shows that collector current, IC
flows for 360° of the input in Class A.
 Fig. (c) shows that collector current, IC
flows for 180° of the input in Class B.
 Fig. (d) shows that collector current, IC
flows for 120° or less of the input in
Class C.
Fig. 31-1
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31-2: Class A Amplifiers
 In a class A amplifier, collector current flows for the
full 360° of the ac input cycle.
 The signal amplitude of any Class A amplifier at the
input should not be large enough to drive the amplifier
into either cutoff or saturation.
 If the signal amplitude at the input is too large, either
or both peaks of the output waveform will be clipped
off (flattened).
31-2: Class A Amplifiers
 Fig. 31-2 (a) shows a common-emitter class A amplifier circuit.
 Fig. 31-2 (b) illustrates the dc load line.
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Fig. 31-2
31-3: Class B Push-Pull Amplifiers
 The collector current, IC, of a transistor in a class B
amplifier flows for 180° of the ac input cycle.
 The main disadvantage of class B operation is that
two transistors must be used to get a linear
reproduction of the input waveform being amplified.
 A class B push-pull amplifier uses two transistors to
get a linear reproduction of the input waveform being
amplified.
 A class B push-pull amplifier has medium efficiency.
31-3: Class B Push-Pull Amplifiers
 Fig. 31-5 (a) shows a
class B push-pull amplifier.
 The transistors, Q1 and
Q2, conduct during opposite
half-cycles of the input
waveform.
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Fig. 31-5 (a)
31-3: Class B Push-Pull Amplifiers
 The dc and ac load line for the circuit in Fig. 31-5 (a) is shown in Fig. 31-
5 (b).
 With no ac input signal, both transistors, Q1 and Q2, are cut off, and onehalf of VCC appears across the collector-emitter region of each transistor.
Fig. 31-5 (b)
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31-3: Class B Push-Pull Amplifiers
 Fig. 31-5 (c) shows the problem with biasing the transistors exactly at cutoff.
 When Vin crosses through zero, Q1 and Q2 are both cut off, resulting in a time
when the output voltage does not follow the input voltage.
 This results in the undesirable effect known as crossover distortion.
Fig. 31-5 (c)
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31-3: Class B Push-Pull Amplifiers
 Fig. 31-6 (a) shows how a
typical class B push-pull
amplifier would be biased.
 This form of bias is called
diode bias.
Fig. 31-6 (a)
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31-3: Class B Push-Pull Amplifiers
 In the circuit of Fig. 31-6 (a),
when Vin is positive, Q1
conducts and Q2 cuts off (see
Fig. 31-6 b).
 Because Q1 acts like an
emitter follower, the ac signal
voltage at the base and
emitter are the same.
The output coupling
capacitor, Cout, is charging
during the positive alternation
of Vin.
 The charging current flows
through RL and the collectoremitter region of Q1.,
Fig. 31-6 (b)
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31-3: Class B Push-Pull Amplifiers
 Fig. 31-6 (c) shows the output when Vin is negative; Q2 conducts and Q1
cuts off.
 Q2 provides a discharge path for the output coupling capacitor, Cout.
 The discharge path is through RL and the collector-emitter region of Q2.
Fig. 31-6 (c)
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31-4: Class C Amplifiers
 Class C amplifiers cannot be used in audio circuitry
because of their high distortion.
 Class C amplifiers can be used as tuned rf amplifiers
where the undesired harmonic frequencies can be
filtered out.
 A class C amplifier is more efficient than either a class
A or class B amplifier; its efficiency approaches 100%.
31-4: Class C Amplifiers
 Fig. 31-8 (a) shows a tuned class C
amplifier.
 The input coupling capacitor, base
resistor, and base-emitter junction form a
negative clamper.
Fig. 31-8 (a)
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31-4: Class C Amplifiers
 Fig. 31-8 (b) illustrates the equivalent input circuit of the tuned class C
amplifier.
 Because of the clamping action, only the positive peaks of the input
signal drive the transistor, Q1, into conduction.
 The RBC time constant is made long with respect to the period of the
input waveform to provide the proper clamping action.
Fig. 31-8 (b)
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31-4: Class C Amplifiers
 Fig. 31-8 (c) shows the peak-to-
peak output voltage from the tuned
class C amplifier.
 The minimum voltage is zero and
the maximum voltage is 2VCC.
 The peak-to-peak voltage available
at the output equals 2VCC because
the tank voltage adds to the positive
value of VCC during the positive
alternation of the output voltage.
Fig. 31-8 (c)
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31-4: Class C Amplifiers
 Fig. 31-8 (d) shows a graph of
frequency versus voltage gain for
the tuned class C amplifier.
 At the resonant frequency, fr, the
impedance of the tuned LC circuit is
maximum.
 The tank impedance, Ztank, is
purely resistive at fr.
Fig. 31-8 (d)
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