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megasat16 24th March 2011 07:11 PM

Pure Class A Push Pull
I know there are many Class A amps using Push Pull Output transistors (BJT or MOSFET).

I am asking coz I am increasing seeing manufacturers use the term "Pure Class A" for the Class A amps with Push Pull output stage.

Traditionally and for purists, "Pure Class A" always apply to SE type and PP always is just called Class A (neither pure or otherwise in front of Class A). I have a few real (means not using clever biasing scheme such as Sliding Bias, etc.) Class A Push-Pull amps that were made in the late 70s and 80s.

The question is can a Push Pull always remain CLASS A (never leave Class A) till it reaches max output and never transaction into Class B but to clip?

And while maintaining the Class A, can the push-pull arrangement produces more current as the load impedance decreases?

Please discuss! :D:devily:

GoranB 24th March 2011 07:38 PM

I think that with enough bias current an Push-Pull amplifier will stay in class A without clipping.

megasat16 24th March 2011 07:51 PM


Originally Posted by GoranB (
I think that with enough bias current an Push-Pull amplifier will stay in class A without clipping.

I have no doubt about it and agreed.

The problem is with the word "Pure" when Class A can become a Class AB with the high input signal.

lineup 24th March 2011 07:57 PM

I think you can call all true Class A for 'pure'

Regardless if it is SE or PP

megasat16 24th March 2011 08:01 PM


Originally Posted by lineup (
I think you can call all true Class A for 'pure'

Regardless if it is SE or PP

That I also agree. It's True Class can be Pure Class A.

But when the True Class A leaves from Class A into Class B under some conditions, then, it's no longer True Class A.

So, how do we limit to the True Class A?

GoranB 24th March 2011 08:03 PM

You can overdive any A class push-pull amplifier using a high level input signal. But under normal conditions, with normal level input signal and good PS there shouldnt be any clipping.
About word "Pure" (A class)...I think its used for better selling of audio devices.

DF96 24th March 2011 08:18 PM

"Pure Class A" is a marketing term, not a technical term. An amp is either class A or not. Peak clipping does not violate Class A, but cutoff does. Class A can be either SE or P-P.

This is what I understand.

megasat16 24th March 2011 08:19 PM

Yes! But in Pure Class A PP design, we don't want to go into Class B or leave Class A at all.

Otherwise, it would be like what is already stated. "Pure" is used for marketing the audio devices and nothing more. I guess in Pure Class A PP design, you can limit the over driven signal with the NFB and auto adjusting the overall gain or using rails voltage to clip after the max Class A watts are reached so the output devices doesn't go into Class B when the input signal is higher than the max bias Class A bias current.

When input over driven the PP amp out of Class A, the push-pull Class A is no longer a Class A but a Class AB. And it certainly doesn't fit the "Pure" or "Class A" bill under that condition.

So, in order to stay Pure (or True) Class A, the gain needs to be auto limited with FB network, and rails voltage and power supply design.

GoranB 24th March 2011 08:25 PM

Maybe im wrong but i dont think that an class A amplifier can leave class A and work in class B or AB. It can clip, but it will stay in class A, no matter of distorted signal. You also dont need NFB to prevent it of clipping (under the normal conditions).

DF96 24th March 2011 08:46 PM

NFB does not limit a signal. If the output clips NFB will push the signal even harder against the stops - the opposite of what might be wanted.

Over-driving does not push A into AB. Excessive (i.e. too low resistance) loading might do it, but that is not the amps fault.

In Class A design, the aim is not to go into cutoff. There is not some stronger aim of really really really not wanting to go into cutoff, which is "Pure Class " or "True Class A". It is either Class A or not, under specified conditions.

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