EveryCircuit – Transistor Class AB Audio Amplifier

From Dino’s Hackaweek :
The class B amplifier uses two transistors. One handles the positive side of the waveform and the other handles the negative side of the waveform. Each transistor is in an off state until the waveform crosses over the zero point and then it must turn on. This takes about 0.7 volts so there is a small amount of time that the transistor is not amplifying the signal. This lag time at the crossover point can cause distortion. There is a way around this however, the class AB amplifier.
In the class AB amp the transistors are biased in such a way so as to never fully turn off. They are kept on by two biasing diodes which allow a small amount of collector current to flow even when there is no signal present. This means then that the transistor will be “ON” for more than half a cycle of the waveform but much less than a full cycle giving a conduction angle of between 180 to 360o or 50 to 100% of the input signal depending upon the amount of additional biasing used. The amount of diode biasing voltage present at the base terminal of the transistor can be increased in multiples by adding additional diodes in series.
http://hackaweek.com/hacks/?p=332