Also known as a push-pull
amplifier, the most basic design
consists of two matched transistor
s 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 gate
s 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.