Mostly there are two methods for setting the output stage bias current of a transistor amplifier to make sure sufficiently less distortion levels. One is optimal class B, a setting where the transconductance of the output devices at idle is the mutual of the degeneration resistors. At this bias setting open loop output impedance and gain are maximum constant and distortion is the minimized.

Amplifiers that use this approach are normally referred to as class-A amplifiers. The other means of setting the bias current is class-A. Here the output stage transistors are biased quite strongly therefore the bias current is larger than with the optimized class-B setting.

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Both output transistors conduct the output current crosses the bias current. An output stage can be said to be operating in class A until that point. When an amplifier is said to class A, that under normal operating term it is not allowed to reach into class B.

A class-A bias setting was accepted for a number of reasons, because there is no normal way to assure that an well class-B will also maintain its optimum setting. Especially a quick change of the die temperature of the outer transistors caused by a changing dynamic load will cause for the setting to shift.

Times New Roman]The effects of this shift will not immediately show up in harmonic bend in measurements, but will clearly be visible in intermediation distortion measurements. Secondly, although the distortion of an optimized class-B can be kept less, it will have mainly of high harmonics, whereas class-A has mainly 2nd or 3rd order harmonics.