___________________________________________________________________________ Transistors
Class B Amplifier Operation
2-56. Amplifiers biased so that collector current is cut off during one-half of the input
signal are classified class B. The DC operating point for this class of amplifier is set up so
that base current is zero with no input signal. When a signal is applied, one half cycle will
forward bias the base-emitter junction and IC will flow. The other half cycle will reverse
bias the base-emitter junction and IC will be cut off. So, for class B operation, collector
current will flow for approximately 180 degrees (half) of the input signal (see Figure 2-17,
view (C)). The class B operated amplifier is used extensively for audio amplifiers that
require high-power outputs. It is also used as the driver- and power-amplifier stages of
Class C Amplifier Operation
2-57. In class C operation, collector current flows for less than one half cycle of the
input signal (see Figure 2-17, view (D)). The class C operation is achieved by reverse
biasing the emitter-base junction that sets the DC operating point below cutoff and allows
only the portion of the input signal that overcomes the reverse bias to cause collector
current flow. The class C operated amplifier is used as a RF amplifier in transmitters.
2-58. We already know that the two primary things that determine the class of operation
are the amount of bias and the amplitude of the input signal. With a given input signal and
bias level, you can change the operation of an amplifier from class A to class B just by
removing forward bias. You can change a class A amplifier to a class AB amplifier by
increasing the input signal amplitude. However, if the input signal amplitude is increased
to the point that the transistor goes into saturation and cutoff, it is then called an
OVERDRIVEN amplifier.
2-59. The two terms, used in conjunction with amplifiers, that you should be familiar
with are FIDELITY and EFFICIENCY. Fidelity is the faithful reproduction of a signal. In
other words, if the output of an amplifier is just like the input except in amplitude, the
amplifier has a high degree of fidelity. The opposite of fidelity is distortion. A circuit that
has high fidelity has low distortion. Therefore, a class A amplifier has a high degree of
fidelity; a class AB amplifier has less fidelity; and class B and class C amplifiers have low
or "poor" fidelity. The efficiency of an amplifier refers to the ratio of output-signal power
compared to the total input power. An amplifier has two input power sources (one from the
signal and one from the power supply). Since every device takes power to operate, an
amplifier that operates for 360 degrees of the input signal uses more power than if operated
for 180 degrees of the input signal. By using more power, an amplifier has less power
available for the output signal; so the efficiency of the amplifier is low.
2-60. The class A amplifier operates for 360 degrees of the input signal and requires a
relatively large input from the power supply. The class A amplifier, even with no input
signal, still uses power from the power supply. Therefore, the output from the class A
amplifier is relatively small compared to the total input power. The result of this is in low
efficiency that is acceptable in class A amplifiers because they are used where efficiency is
not as important as fidelity. Class AB amplifiers are biased so that collector current is cut
off for a portion of one alternation of the input which results in less total input power than
the class A amplifier. This leads to better efficiency. Class B amplifiers are biased with
little or no collector current at the DC operating point. With no input signal, there is little
wasted power. Therefore, the efficiency of class B amplifiers is higher still. The efficiency
of class C is the highest of the four classes of amplifier operations.
23 June 2005
TC 9-62
2-19
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