The class-AB  amplifier

When a class-A amplifier is working properly, it has quite low distortion. But class-A operation is inefficient. This is mainly because bipolar transistor or FET draws a large current, whether there is or is not signal input. Even with zero signals, device is working hard.For weak signal work, efficiency is not significant; it is gain and sensitivity which matter. In power amplifiers, efficiency is an important consideration, and gain and sensitivity are not significant. Any power not used toward generating the strong output signal will end up as heat in the bipolar transistor or FET. If the amplifier is designed to generate high power output, inefficiency translates to large heat.

When the bipolar transistor is biased close to cutoff under the no signal conditions , or when an FET is near pinchoff, the input signal will drive the device into nonlinear part of the operating curve. A small collector or drain current will flow when there is no input, but it will be less than no-signal current which flows in a class-A amplifier. This is called as class-AB operation.

With class-AB operation, input signal may or may not cause the device to go into cutoff or pinchoff for the small part of cycle. Whether or not this happens depends on actual bias point, and on the strength of input signal. You can visualize this by imagining dynamic operating point oscillating back and forth along with the curve, in either direction from static operating point.

If bipolar transistor or FET is never driven into cutoff during any part of signal cycle, the amplifier is working in class-AB1. If device goes into cutoff pinchoff for any part of cycle, the amplifier is working in class-AB2.In a class-AB amplifier, output waveshape is not similar to input wave shape. But if the wave is modulated, such as in voice radio transmitter, waveform of the modulations will come out undistorted. Therefore class-AB operation is quite useful in radio-frequency power amplifiers.