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I don't understand this argument

First off, what are we optimizing for? Maximum Class A output? Minimum distortion at low input signal levels? Minimum distortion at higher input signal levels? Thermal stability? Bandwidth? Something else?

Biasing the device so that the Q point is in the middle of load line results in the maximum Class A output, but that isn't necessarily the optimum for anything else.

Suppose you want to pick a different Q point, one that gives you a little better linearity at low signal levels. If this happens to be at a higher bias than the midpoint, you're increasing quiescent current dissipation and reducing maximum output. Some designers might make that trade, but most probably won't. On the other hand, suppose this optimum Q point is lower than the midpoint. If the circuit is single-ended, you're reducing maximum output. If the circuit is push-pull, you're reducing maximum output in Class A , but potentially increasing maximum output overall because one device can still amplify when the other hits cutoff.

That's just one possible rationale for choosing a Q point. Other factors that might play into the decision are thermal stability, limits on quiescent current dissipation imposed by the thermal design and operating environment, longevity, distortion at higher signal levels, and in the case of transistors, bandwidth/response time. And most of all, what sounds the best to the designer.

Class A and Class AB are modes of operation, not types of amplifiers. You can't categorically say that one amp which is operating up to 10W in Class A is better than another amp operating up to 10W in Class A.


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