Simple Audio Power Amplifier 60W by K1058, J162

mosfet amplifiers there is no clear distinction between class-B and class-AB. For a bipolar output stage it is common to call a design class-B if the quiescent current, Iq, is adjusted for minimum crossover distortion, and class-AB if the current is increased well above this optimum level so that operation remains in class-A for small signals. (Not everyone agrees on this convention but it seems a sensible choice, for audio amplifiers at least, because these two classifications have rather different design problems. For example ‘class-B’ usually requires far more accurate quiescent current setting and thermal compensation for best results.) Class-AB causes higher distortion than class-B at higher signal levels where device switching starts to occur. For mosfets the distortion is generally highest at low quiescent current and reduces as Iq is increased, so there is not a similar optimum value giving minimum distortion. My choice of 100mA for the circuit presented here is because the mosfets used have zero temperature coefficient around this value, but with adequate heatsinks the current could be increased. With reduced supply voltage the circuit can even be biased into class-A for the lowest distortion, and here mosfets have some advantage because of their nearly square-law characteristics. If they were perfectly square-law then the peak output current in class-A would be 4 x Iq instead of 2 x Iq for typical bipolar output stages, and up to this level the distortion would be zero for ideal accurately matched complementary devices. Unfortunately well matched pairs are unobtainable, and they are not very accurately square-law, and they have non-linear capacitances, so even class-A still has its problems.

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