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Class A into Class AB

I’m still a little confused about power amplifiers and integrated amplifiers that are class A & class A/B. Like when they say the first 8 watts are class A then it goes into 400watts class A/B. But the same amplifier can be biased to put out 12watts class A then go into 250watts class A/B. It can be biased again for 18watts class A and 150watts class A/B. etc. Ive read that these amplifiers, ones that can be biased like that...and in general all the "first X amount of watts are class A before going into class A/B"...that those first X amount of class A watts is NOT true class A. If that’s true...what is it then? What’s "kind of" class A mean? What’s the point of a "first X amount of watts are class A" then?

tmac1700

Showing 1 response by erik_squires

erik_squires
15,930 posts
 

The difference in how A achieves zero volts, and how B does it.

With Class A, 0 V is achieved by the + and - transistor banks being fully on.  Because they are balanced, there's no voltage, but full current and a whole lot of heat.

 

B achieves 0V by both banks being off.  No current, no heat. Trouble is that the first couple of volts of a transistor are not linear.  They need to reach a minimum threshold to turn on and respond to the input.

 

Class AB is a compromise where 0V is achieved by both banks being on, but only a little, and when voltage swings enough one bank turns fully off.  High efficiency, low distortion. 

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