TABLE OF CONTENTS In earlier sections of this book we looked closely at the distortion produced by amplifier output stages, and it emerged that a well-designed Class-B amplifier with proper precautions taken against the easily-fixed sources of non-linearity, but using basically conventional circuitry, can produce startlingly low levels of THD. The distortion that actually is generated is mainly due to the difficulty of reducing high-order crossover non-linearities with a global negative-feedback factor that declines with frequency; for 8 ? loads this is the major source of distortion, and unfortunately crossover distortion is generally regarded as the most pernicious of non-linearities. For convenience, I have chosen to call such an amplifier, with its small signal stages freed from unnecessary distortions, but still producing the crossover distortion inherent in Class-B, a Blameless amplifier (see Chapter 3).
Page 145 suggests that the amount of crossover distortion produced by the output stage is largely fixed for a given configuration and devices, so the best we can do is ensure the output stage runs at optimal quiescent conditions to minimise distortion.
Since it is our only option, it is therefore particularly important to minimise the output-stage gain irregularities around the crossover point by holding the quiescent conditions at their optimal value. This conclusion is reinforced by the finding that for a Blameless amplifier increasing quiescent current to move into Class-AB makes the distortion worse, not better, as gm-doubling artefacts are generated. In other words the quiescent setting will only be correct over...
