Also known as a
push-pull amplifier, the most
basic design consists of two matched
transistors in series, an
NPN tied to the upper
rail, and a
PNP tied to the lower rail, with the junction balanced at about
ground. The same input signal controls the
gates of both transistors. When the signal goes
high, the NPN transistor activates, and more power is fed into the
load. When the signal goes low, the PNP transistor starts to
sink current from the load.
Unlike the inefficient Class A amplifier, the quiescent class B amplifier draws no power. However, there is some distortion introduced, due to the fact that the transistors both need about .7 Volts of offset before they activate. This creates a deadband effect, where small signals do not cause any output at all. The solution to this is to create a bias voltage for both transistors, ususally using some diodes that can offset by the same .7 volts as the transistors. This is sometimes called a Class AB amplifier, but not by me.
Class B amplifiers are usually what are found in your stereo.