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The only thing that differentiates the different classes of amps (A,B,AB) is the idle current of the output device. In class A (blue) the idle current is always set at 1/2 the Icmax, so the output transistor is always dissipating 1/2 the maximum power. That's why it only has 50% efficiency and that's a theoretical value. Often it's much lower, and sometimes when you're listening to music on low volume it can fall down to single digits. The maximum theoretical efficiency can be reached with transformer coupling. These amps sound the best because the output transistor is always in a conductive state e.g. it always stays in the linear part of its characteristics. Now class B is a whole other story. It contains two putput devices, one for each half cycle (negative positive). When no signal is applied it doesn't draw any current and that's why it has much higher efficiency (78.5%). But as it is with class A, that value is rarely reached. These amps have an inferior quality compared to the class A counterpart, because one of the output transistors is always off, thus introducing non linear distortion. Moreover class B amps have a so called crossover distortion, that occurs around the 0V point, when neither transistor is conducting. That can be fixed by slightly biasing them (a few mA is enough) and through a negative feedback. Now class AB (yellow) as the name stands combines the best of both. It has the quality of class A amps with the efficiency of class B amps. That's done by biasing a class B amp deeper into the class A region. For instance instead of a few mA idle current a class AB amp can be biased at a 100mA idle current. This way, at low audio levels, the amp will work in class A mode, and after you've reached the 100mA threshold it'll take it to class B mode, but at higher power levels where the crossover distortion is negligible. Just remove the signal sources from the amps and observe the idle current which they draw.
Now for the other classes which i haven't displayed.
Class G and H are not widely acknowledged by the scientific community, because they're a variation of the other classes. For instance class H is actually a class B amp modulating the supply rails for class A amp. The efficiency isn't great but is much improved.
Class D amps are a whole other story. They do not use analog amplification. They use PWM as the driving component for an LC filter which restores the signal. The main reason why they're so efficient is because the switching component has either ni voltage drop with maximum current through it, or a maximum voltage drop, with no current through it. It's tough to explain, but as you know the Power is IxU, which means either 0VxImax or 0AxUmax, so the dissipated heat is ideally nonexistent. But in reality efficiency levels of 95% are normal. The active devices can be viewed as switches
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