The three basic power amplifier topologies. Class A Class B and Class AB. Each presents certain features that the others dont,like class A works in an always conductive mode which gives off a lot of heat (a quarter is useful the other gets dissipated) which means its efficiency is 25% at best. Its mostly a voltage amplifier with a pinch of current amplification. I would like to point out that this class gives absolutely the best sound quality compared to the other ones and is widely prefered by the audiophiles. The second amplifier (Class B) also known as push pull stage is only a current amplifier and has a voltage gain less than unity. (Umax-base voltage drop) It has an overall efficiency of 75% due to the fact that each transistor operates at only one half of a cycle which not only increases efficiency but causes the transistors to dissipate less heat. Its not used in the preamp stages because it has a certain crossover distortion which is present when the transition from one part of the cycle switches to the other (due to the 0.7v transistor voltage drop) this could be minimised with the proper biasing techniques and a negative feedback. The 3rd amp topology is a mixture of the previous two and is called Class AB amplifier. It presents trades from both class A and class B amplifiers and is considered a good compromise from both. Its efficiency is around 50% and works in such way that when the output level is below a few watts the class A stage works giving nice sound quality and a little heat dissipation. When the output level rises beyond a certain threshold the class B topology kicks in and also gives a nice sound quality because the sound level has become high enough to neglect the crossover distortion. So to sum it up class A gives best quality results but has lower efficiency than the push pull amplifier and to compromise for the class a inefficiency class AB is being used. There are other kinds of amps like class D which operates in the digital domain and uses PWM and filters to restore the original waveform. These kind of amps have a 100% theoretical efficiency but do not offer the best sound quality.
