5-25. The efficiency of an amplifier refers to the amount of power delivered to the output
compared to the power supplied to the circuit. Since every device takes power to operate, if
the amplifier operates for 360 of the input signal, it uses more power than if it only
operates for 180 of the input signal. If the amplifier uses more power, less power is
available for the output signal and efficiency is lower. Since class A amplifiers operate
(have current flow) for 360 of the input signal, they are low in efficiency. This low
efficiency is acceptable in class A amplifiers because they are used where efficiency is not
as important as fidelity.
Class B Operation
5-26. A class B amplifier operates for 50 percent of the input signal. Figure 5-4 shows a
simple class B amplifier. In the circuit, 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. Even though only half the input
signal is desired in the output, you would use a class B amplifier instead of a simple
rectifier because 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.
5-27. Class B amplifiers are twice as efficient as Class A amplifiers since the amplifying
device conducts (and uses power) for half of the input signal. Where exactly 50 percent of
the input signal must be amplified, a class B amplifier is used. If less than 50 percent of the
input signal is needed, a class C amplifier is used.
Figure 5-4. Simple Class B Transistor Amplifier
Class AB Operation
5-28. If the amplifying device is biased in such a way that current flows in the device for
51 to 99 percent of the input signal, the amplifier is operating class AB. Figure 5-5 shows a
simple class AB amplifier.
23 June 2005