1-8
Class B Operation
As was stated above, a class B amplifier operates for 50% of the input signal. A simple class B
amplifier is shown in figure 1-6.
Figure 1-6.—A simple class B transistor amplifier.
In the circuit shown in figure 1-6, the base-emitter bias will not allow the transistor to conduct
whenever the input signal becomes positive. Therefore, only the negative portion of the input signal is
reproduced in the output signal. You may wonder why a class B amplifier would be used instead of a
simple rectifier if only half the input signal is desired in the output. The answer to this is that the rectifier
does not amplify. The output signal of a rectifier cannot be higher in amplitude than the input signal. The
class B amplifier not only reproduces half the input signal, but amplifies it as well.
Class B amplifiers are twice as efficient as class A amplifiers since the amplifying device only
conducts (and uses power) for half of the input signal. A class B amplifier is used in cases where exactly
50% of the input signal must be amplified. If less than 50% of the input signal is needed, a class C
amplifier is used.
Class C Operation
Figure 1-7 shows a simple class C amplifier. Notice that only a small portion of the input signal is
present in the output signal. Since the transistor does not conduct except during a small portion of the
input signal, this is the most efficient amplifier. It also has the worst fidelity. The output signal bears very
little resemblance to the input signal.
