The class-B amplifier

When a bipolar transistor is biased exactly at cutoff, or an FET at pinchoff, under zero-input-signal conditions, an amplifier is working in class B. In class-B operation, there is no collector or drain current when there is no signal.
This saves energy, because the circuit is not eating up any power unless there is a signal going into it. (Class-A and class-AB amplifiers draw current even when the input is zero.) When there is an input signal, current flows in the device during exactly half of the cycle. The output waveshape is greatly different from the input waveshape in a class-B amplifier; in fact, it is half-wave rectified.
Sometimes two bipolar transistors or FETs are used in a class-B circuit, one for the positive half of the cycle and the other for the negative half. In this way, distortion is eliminated. This is called a class-B push-pull amplifier. A class-B push-pull circuit using two NPN bipolar transistors is illustrated in Fig. 1. This configuration is popular for audio-frequency power amplification. It combines the efficiency of class B with the low distortion of class A. Its main disadvantage is that it needs two center-tapped transformers, one at the input and the other at the output. This translates into two things that engineers don’t like: bulk and high cost. Nonetheless, the advantages often outweigh these problems.
The class-B scheme lends itself well to radio-frequency power amplification. Although the output waveshape is distorted, resulting in harmonic energy, this problem can be overcome by a resonant LC circuit in the output. If the signal is modulated, the modulation waveform will not be distorted.
You’ll sometimes hear of class-AB or class-B “linear amplifiers,” especially in ham radio. The term “linear” refers to the fact that the modulation waveform is not distorted by the amplifier. The carrier wave is, as you’ve seen, affected in a nonlinear fashion, because the amplifiers are not biased in the straight-line part of the operating curve.

Fig. 1: A class-B push-pull amplifier.

Class-AB₂ and class-B amplifiers take some power from the input signal source. Engineers say that such amplifiers require a certain amount of drive or driving power to function. Class-A and class-AB₁ amplifiers theoretically need no driving power, although there must be an input voltage.