[Part 1 offers an overview and introduction to the sources of distortion in audio power amplifiers. Part 2 focuses on distortion in the audio amplifier input stage. Part 3 examines distortion mechanisms in the voltage amplifier stage (VAS). Part 4 focuses on distortion in the audio power amplifier output stage. Part 5 continues the discussion of distortion in the power amplifier output stages. Part 6 looks at some of the remaining distortion mechanisms in audio power amplifiers. Part 7 examines frequency compensation in audio power amplifiers, with some examples from actual designs.]
There are two salient facts about Class A amplifiers: they are inefficient; they give the best possible distortion performance. The quiescent dissipation of the classic Class A amplifier is equal to twice the maximum output power, making massive output power impractical. But the nature of our hearing means that the power of an amplifier must be considerably increased to sound significantly louder.
It is well known that power in watts must be quadrupled to double sound pressure level (SPL), but this is not the same as doubling subjective loudness; this is measured in Sones rather than dB above threshold, and some researchers have reported that doubling subjective loudness requires a 10 dB rather than 6 dB rise in SPL, implying that amplifier power must be increased tenfold, rather than merely quadrupled.1 This may help to put worries about amplifier size into perspective ...
There is an attractive simplicity about class A. Most of the distortion mechanisms studied so far stem from class B, and we can thankfully forget crossover and switchoff phenomena (Distortions 3b, 3c), non-linear VAS loading (Distortion 4), injection of supply-rail signals (Distortion 5), induction from supply currents (Distortion 6), and erroneous feedback connections (Distortion 7). Beta-mismatch in the output devices can also be ignored.
The art of compromise
The only real disadvantage of class A is inefficiency, so inevitably efforts have been made to compromise between A and B. As compromises go, traditional class AB is not a happy one because, when the AB region is entered, the step change in gain generates significantly greater high order distortion than that from optimally biased class B. However, a well-designed AB amplifier will give pure class A performance below the AB threshold, something a class B amp cannot do.
Another compromise is the so-called non-switching amplifier, with its output devices clamped to pass a minimum current. However, it is not immediately obvious that a sudden halt in current-change as opposed to complete turn-off makes for a better crossover region. Those residual oscillograms that have been published seem to show that some kind of discontinuity still exists at crossover.2
A potential problem is the presence of maximum ripple on the supply rails at zero signal output; the PSRR must be taken seriously if good noise and ripple figures are to be obtained. This problem can be simply solved by the measures proposed for class B designs.