The class-AB amplifier

When a class-A amplifier is working properly, it has low distortion. But class-A operation is inefficient. (Amplifier efficiency will be discussed later in this chapter.) This is mainly because the bipolar transistor or FET draws a large current, whether there is a signal input or not. Even with zero signal, the device is working hard.
For weak-signal work, efficiency is not very important; it’s gain and sensitivity that matter. In power amplifiers, efficiency is a significant consideration, and gain and sensitivity are not so important. Any power not used toward generating a strong output signal will end up as heat in the bipolar transistor or FET. If an amplifier is designed to produce high power output, inefficiency translates to a lot of heat.

When a bipolar transistor is biased close to cutoff under no-signal conditions, or when an FET is near pinchoff , the input signal will drive the device into the 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 the no-signal current that flows in a class-A amplifier. This is called class-AB operation.
With class-AB operation, the input signal might or might not cause the device to go into cutoff or pinchoff for a small part of the cycle. Whether or not this happens depends on the actual bias point, and also on the strength of the input signal. You can visualize this by imagining the dynamic operating point oscillating back and forth along the curve, in either direction from the static (no-signal) operating point.
If the bipolar transistor or FET is never driven into cutoff/pinchoff during any part of the signal cycle, the amplifier is working in class-AB₁. If the device goes into cutoff pinchoff for any part of the cycle (up to almost half), the amplifier is working in class-AB₂. In a class-AB amplifier, the output waveshape is not identical with the input waveshape.
But if the wave is modulated, such as in a voice radio transmitter, the waveform of the modulations will come out undistorted. Thus class-AB operation is useful in radio- frequency power amplifiers.