Need explanation about amplifier class.

[nenad88]

Can someone explain me one thing. Often I hear that some amplifier is class A only first few watts then it goes to class AB. I have read about amp classes but dont really understand what happens to amplifier 'to change class' so to say. Thanks 🙂

 

[MarcelvdG]

What is meant with class A up to a power P and class AB above that is that the amplifier's output transistors carry current during the complete signal cycle as long as the output signal power is less than P, while they get (practically) switched off during a part of the signal cycle when the output signal power exceeds P (*). Usually the output signal is assumed to be a sine wave, P refers to its average power and the load is assumed to behave as a resistor.

(*): That is, the device that pushes current into the output turns off in the negative peaks and the devices that pulls current in the positive peaks.

 

[Galu]

I have read about amp classes but dont really understand what happens to amplifier 'to change class' so to say.

Class AB is somewhere between class A (where conduction happens 100% of the time) and class B (where conduction happens 50% of the time).

The switching from class A to class AB is achieved by changing the bias of the two transistors in the amplifier's output stage. Then each transistor will conduct between 100% and 50% of the time depending on the amount of current output.

 

[MarcelvdG]

Depending on what the blue colour means, either the first or the last plot is incorrect.

 

[Galu]

You're right! The first plot should be white, meaning conduction!

I hope this new attachment is clearer. 🙂

 

[nenad88]

I understand that transistors are biased in class A in a way to be just in middle between being off and saturated. In class AB they are switching between on and off, what confuses me is who changes bias after amp reaches certain point. I would understand if there is some Microcontroller but this is pure analog circuit and something is sensing that amp need to switch.

Also how do you choose how much power you allow amp to produce before changing class. Sorry if it is noob question I searched google but couldnt find answer for this. Also if there is literature for this topic I would gladly read it.

 

[vilfort]

The BIAS is what determines the class. In class A, the transistor/tube/MOSFET handles the whole signal swing. Think of the BIAS as a midpoint between + and - signal across the output device. A single ended 300B circuit is a good place to start in understanding this.

In a push/pull arrangement you still can have class A. The PNP/NPN output transistors are each biased for class A - but think of them "overlapping each other" when summed at the amplifier's output terminal.

It is this "overlap" we exploit in class AB. Here each output device is biased so that each output device is still operating in class A for smaller amplitudes - but goes into class AB at higher amplitudes. The BIAS itself does not change. The signal amplitude does.

 

[Galu]

The BIAS itself does not change. The signal amplitude does.

Instead of 'changing the bias', I should have said 'pre-setting the bias'.

 

[Galu]

Also how do you choose how much power you allow amp to produce before changing class. Sorry if it is noob question I searched google but couldnt find answer for this. Also if there is literature for this topic I would gladly read it.

Here's some technical info' from Aiken Amps https://www.aikenamps.com/index.php/the-last-word-on-class-a which denies that the change in class is real:

Here is where the problem comes in: because a class AB amplifier is biased so that the plate current flows for the entire cycle at lower output levels (which is done to reduce crossover distortion), many people claim it is a "class A amplifier at lower volumes". This is simply not true. It is operating in conditions *similar* to class A, but is not a class A amplifier by any means. It is still a class AB amplifier, no matter what you choose to call it.

This source Power Amp Classes states:

Some manufacturers claim that their amp operates as Class-A up to some specified power, and this can certainly be true. However, most amplifiers only operate at very modest quiescent (no signal) current, often as low as 20mA. For an 8 ohm load, that equates to a couple of milliwatts of 'Class-A operation' - hardly worth getting excited about.

 

[PRR]

I understand that transistors are biased in class A in a way to be just in middle between being off and saturated. In class AB they are switching between on and off...

No. B does not "switch between on and off". It modulates from nearly-Off to fully On. Or in your words, "biased in class B in a way to be near the bottom between being off and saturated."

"Near the bottom" is a design choice. It can be "cold" or "hot". A hot B bias has some small class A portion.