What is the definition of Class A?

[Bricolo]

I haven't found anything precise about this



what is defining if an amp is class A?
are there some equations, ore other requirements?


thanks

 

[Vivek]

Transistors conduct current for the full 360 degrees of the cycle in Class A operation.

Vivek

 

[Bricolo]

ok

but does any relation exist between Vcc and Vds, Vcc and Vg...?

I don't clearly understant "conduct for the full 360°"

 

[JoeBob]

The 360 degrees of the sine wave. Class-B would be 180 degrees, half a cyle, while Class-A is the full cycle.

 

[Bricolo]

360 means a complete phase?

So, in a single mosfet amp (like a szekeres like this)

An externally hosted image should be here but it was not working when we last tested it.

It means that
-At Vin=0, Vg is at a value where the mosfet conducts
-At Vin=max, Vg is still at a value where the mosfet conducts
-At Vin=-max (negative peak), always the same...



Am I right?


What are the maximum peak values of a device's source line output?

 

[P.Lacombe]

Yes, you are right. A device (transistor, tube...) which operates in class A is biased so that a current flow anytime across it. This is not the case in class B, because usually positive and negative inputs of the source signal are amplified separately, in symmetric configuration : each of the two devices conducts about half of time. Other classes are unusable for audio, or more complex.

Regards, Pierre Lacombe.

 

[Pjotr]

Hi,

Simply stated:

"Class A amplifiers use one or more transistors that conduct during both the full positive and negative cycles of the signal."

However class A is not limited to SS amps. The early Tube ones were all class A.

A simple introduction on various classes:

http://www.norh.com/docs/amps/

😉