What is the definition of Class A?

Bricolo · 17th November 2002, 03:07 PM

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 · 17th November 2002, 03:14 PM

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

Vivek

Bricolo · 17th November 2002, 03:49 PM

ok

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

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

JoeBob · 17th November 2002, 05:10 PM

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

Bricolo · 17th November 2002, 06:17 PM

360 means a complete phase?

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

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 · 17th November 2002, 09:31 PM

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 · 18th November 2002, 09:35 AM

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/

17th November 2002, 03:07 PM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	What is the definition of Class A? 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

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17th November 2002, 03:14 PM	#2
Vivek diyAudio Member Join Date: Nov 2000 Location: Bangalore, India	Transistors conduct current for the full 360 degrees of the cycle in Class A operation. Vivek

17th November 2002, 03:49 PM	#3
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	ok but does any relation exist between Vcc and Vds, Vcc and Vg...? I don't clearly understant "conduct for the full 360°"

17th November 2002, 05:10 PM	#4
JoeBob diyAudio Member Join Date: Nov 2001 Location: Montreal, Canada	The 360 degrees of the sine wave. Class-B would be 180 degrees, half a cyle, while Class-A is the full cycle.

17th November 2002, 06:17 PM	#5
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	360 means a complete phase? So, in a single mosfet amp (like a szekeres like this) 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?

17th November 2002, 09:31 PM	#6
P.Lacombe diyAudio Member Join Date: Jul 2001 Location: Orleans, France	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.

18th November 2002, 09:35 AM	#7
Pjotr diyAudio Member Join Date: Nov 2002 Location: Netherlands	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/