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Okay, there are 3 known amplifier classes - class A,B,AB. There are others but they're either an improvement of these or digital. Anyway our focus will be on class B and its inherent zero point switching distortion. It occurs due to the fact that the transistor is not an ideal switch and it has a turn on voltage of around 0.8V (Si) and 0.3V (Ge). Any time the input wave is under this voltage, the transistors are not conducting and the output is not following the input. These distortions are more audible at lower power levels. There are several ways to get rid of them. The first way is to introduce some idle current on the active devices. Even a few uA (like in this case will do). This way we ensure that the transistors are always conducting (no matter how little in some moments). They in fact won't ever completely turn off this way, even though they'd be conducting very little in idle state. No matter what the textbooks say, class B amps always have some low idle current. This is done by prefixing the base voltages with the two diodes I've used here. This way we shift the bias point to a higher, almost conducting state. The second way to do it is by modifying the sinewave at the middle, causing it to skip the +0.8V and -0.8V very fast. This can be done either manually (lower left circuit) or can be done by a negative feedback (upper circuit) which automatically makes the needed adjustments to match the output to the input. This is actually the best way for doing it. Most of the time these ways are used in conjunction to have a more prominent effect.
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