# Class B or class A/B?



## MarkZ (Dec 5, 2005)

You commonly hear amplifiers in car audio being referred to as having class A/B output stages. The thought is that since you need to apply a voltage to a push-pull transistor configuration to overcome the Vbe of the transistors, this applied voltage means it's class A/B.

For those who aren't aware, a bipolar transistor requires a certain forward voltage to conduct (Vbe), much like a diode. Therefore, the transistor won't switch "on" for low voltage portions of the input wave, so the output would remain zero which generates "crossover distortion"...










Class A/B is traditionally defined as a stage with a forward bias designed to push these transistors well into their "on" regions so that crossover distortion doesn't arise, and also to account for the temperature variations of the transistors' Vbe. So, for a portion of the wave, BOTH transistors in the push-pull stage are conducting.

But amp designers ideally don't want that either, mostly for two reasons. It makes things inefficient (and therefore hotter than they need to be), so there's a lot of wasted current going through the output stage. And second, because it generates something usually referred to as gm-doubling distortion. 

So, the goal is to create a bias circuit that can accurately estimate the Vbe of the push-pull transistor pair. That way you're not overbiasing into class A/B, and you're not underbiasing into crossover distortion. This would be a class B configuration, not class A/B. The problem is that transistor Vbe changes with temperature, so the bias circuit should be designed in a way to mimic the transistors' temperature variation. This is still (usually) class B.

Anyway, this whole thing seems to have evolved into meaning that all amps are class A/B because there's a bias applied. But, realistically, class B is what manufacturers should be striving for. And many of them do (at least attempt to) add the thermal tracking component to the bias circuit. But they'll still call it class A/B. Why?

Moral of the story: buy class D and you'll know what you're getting. 

In the meantime, I'm going to continue to refer to all of them as _class B_. The lazy design is to overbias the output stage to make it class A/B to bias the circuit in a way to eliminate crossover distortion (and the potential for thermal runaway) for all possible values of Vbe. But that's not really class A/B -- that's lazy class B! Much in the same way that slightly underbiasing the output stage would generate crossover distortion and make it behave similar to class C -- we don't call it class C! We call it class B not implemented correctly.


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## FoxPro5 (Feb 14, 2006)

Excellent tutorial!


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## Torquem (Jun 27, 2009)

MarkZ said:


> Moral of the story: buy class D and you'll know what you're getting.


is that good or bad? lol
Good write up. I learned something!


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## sigma6 (Mar 28, 2006)

Thanks Mark. Interesting subject.


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