# Class A to class AB transition power rating

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#### maggiesnmacs

I've read that a class AB stage can put out twice the bias current in class A before becoming class AB. So for example an amplifier with 1A bias current can put 2A out in class A so 32 watts output into 8 ohms. Is this correct? What is the mechanism behind this and what happens to the current flow as the amplifier transitions from class A to class B?

#### Lazy Cat

When one half of the output stage goes to 0 A, the other half faces 2 A via load.

#### maggiesnmacs

Ok that makes sense. But that is peak current correct? So in this example of 1A bias then 32 watts peak output is the maximum, and therefore 16 watts RMS output with a sinewave?

I just find this interesting since Nelson Pass is usually fairly accurate with his measurements and in this page he notifies that his amplifier with 1A bias outputs 30 watts class A into an 8 ohm load. Would that be with a square wave?

https://passlabs.com/articles/leaving-class-a

#### Lazy Cat

Yes correct, 2 Ap before Class B, meaning 16 Wrms/8 Ohm in Class A, if we speak about sine wave signals.

#### bwaslo

Some circuits can stay in "class A" when peak output current is more than double the bias current. Class A just means that neither output transistor cuts off (goes to zero current conduction) completely, it doesn't really say anything about how linear the device is as it goes from bias level to cutoff. Most devices are disgustingly nonlinear down near cutoff, actually.

#### Lazy Cat

If the word goes about Class A, not Class AA or other nonswitching tech, then it's like this, otherwise can be as you pleased.

#### mjona

I've read that a class AB stage can put out twice the bias current in class A before becoming class AB. So for example an amplifier with 1A bias current can put 2A out in class A so 32 watts output into 8 ohms. Is this correct? What is the mechanism behind this and what happens to the current flow as the amplifier transitions from class A to class B?

Have a read of Pass A-40 circuit https://www.passdiy.com/project/amplifiers/the-pass-a-40-power-amplifier where there are answers from the horses mouth.

For more on single ended design check out Pass's earlier article "Construct a Class A Amplifier".

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