Now that I've covered the preamps lets talk a little about power amplifier used in the last stages of an amplifier. For now lets discuss class A class B and Class AB topologies and I'll post a different one on the class D topology.
-Class A - since the active element in a class A mode is always conducting more or less an efficiency levels of around 25% are possible at best with a transformer impadance matching. But always working in the linear region does not only waste power. That would mean that best results are to be expected (high quality of audio). The shortcome is that for every 1W of musical power another 4W must be spent in heat dissipation.
Class B - this provides no voltage amplification because the output is located at the two emitters of the transistors therefore voltage gain is a little less than unity. The way it operates is that there are two parts of it one that handles the upper part of the waveform and one that covers the bottom one. This means thats the consumption is for only one cycle. Therefore efficiency levels of up to 50% can be accomplished. With these amps there is a certain problem that must be dealt with. If there is no bias than the two transistors will not operate in the first 0.7V of the waveform. That causes the so called crossover distortion.
Class AB - it ia biased in such way that one of the transistors will carry more than half the cycle of the wave meaning that to some point the amp will work in a class A mode providing low distortion and at and since there the operating point is low power one the efficiency levels wont matter much. Once the power goes above that level the other transistor also kicks in providing class B way of operating but since the power levels have risen the crossover diatortion has decreased. Considered a good compromise between class A and B it offers a little lower efficiency than class B but a higher quality of audio.
