Class C is where the output device(s) are conducting less than 180 degress (less than half) of the input waveform. I think class C is mostly used in RF stuff.

Class H is: (longer explanation) Picture a power amp with +/- 50v supplies for the output stage (plus 50v and minus 50v). For a given voltage gain of the amp, the amp can correctly amplify increasingly larger input signals untill the output transistors are fully turned on. The amp. is on the verge of clipping at this point. The output voltage magnitude will be a little less than the +/- 50 volt supplies because the output devices have a minimum resistance and any current passing through this minimum resistance will result in a minimum voltage drop. If the input signal gets too large, the output signal will no longer increase, the output waveform will stay where it is (assuming a well behaved amp with response down to DC and ideal supplies for this discussion) untill the input signal comes down to a value that results in an output signal that is comfortably within the +/- 50v of the output supply. In class G, before the output gets too big, the output voltage is switched from the +/-50v supplies to larger supplies, lets say +/- 100v. Now, when the input signal is big enough to reqire an output of more than what the +/- 50v can supply, the larger supplies take over. If the switchover time is quick and smooth enough, the output waveform is disturbed minimally by the switchover from one set of supplies to the other.

The advantage of this is in the reduced heat produced by the amp compared to one that used the big output supplies all of the time.

The instantaneous heat power dissipated by an output transistor is the current through it multiplied by the voltage across the output transistor at any given instant. If we are using an amp with small output supplies we will produce less heat than an amp with big supplies. Of course, this amp with small supplies will deliver less maximum power to the speaker. If we switch in big supplies only when we need them, we will be producing "big supply heat" only when the output waveform is big enough to demand the big supplies. When we are playing music at a level that does not need the big supplies, we only use the small supplies and only produce "small supply heat".

So, a class G amp, when playing "quietly" produced "small supply heat". When playing loudly, produces a combination of "small supply heat" and "big supply heat" which, at the WORST case (say, producing full output square waves) will be about equal to "big supply heat" but will much more likely be closer to the "small supply heat" example when playing MUSIC.

A variation of the class G is where, instead of switching from small supplies to big supplies and back as needed, the supplies, when needed, vary as needed to allow the amp to put out an unclipped wave form. The output supplies track the input signal (above a certain level). The output transistors have enough voltage to produce an unclipped output but not a lot extra to waste as heat. This variation can result in even less heat but it is more complicated and tricky to execute properly.

Paul

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the 1derful1