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Old 17th November 2002, 03:07 PM   #1
Bricolo is offline Bricolo  France
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Default 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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Old 17th November 2002, 03:14 PM   #2
Vivek is offline Vivek  India
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Transistors conduct current for the full 360 degrees of the cycle in Class A operation.

Vivek
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Old 17th November 2002, 03:49 PM   #3
Bricolo is offline Bricolo  France
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ok

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

I don't clearly understant "conduct for the full 360"
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Old 17th November 2002, 05:10 PM   #4
JoeBob is offline JoeBob  Canada
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The 360 degrees of the sine wave. Class-B would be 180 degrees, half a cyle, while Class-A is the full cycle.
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Old 17th November 2002, 06:17 PM   #5
Bricolo is offline Bricolo  France
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360 means a complete phase?

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

Click the image to open in full size.



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?
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Old 17th November 2002, 09:31 PM   #6
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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.
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Old 18th November 2002, 09:35 AM   #7
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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/

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