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August 29th 04, 07:06 PM

In article ,
Paul Burridge wrote:
On Sun, 29 Aug 2004 00:46:11 +0000 (UTC),
(Ken Smith) wrote:

If this isn't the slow modulation case, for a single device RF output
stage to work as a linear you need to bias it to about the point where gm
is 1/2 the "mid current" gm. This makes the stage class AB.

If you want to go towards class B or C there are things you can do in the
slow modulation case. Basically as the RF input increases, you move into
class C and as it decreases you move back towards AB. Moving to class C
lowers the output power for large signals a bit so the AB class's standing
current can be reduced by some amount.

Can't agree. If you're running any amount of standing current, then it
ain't class C.

If the conduction angle is less than 180 degrees it is class C. Note that
I said "you move into class C" this means the amplifier isn't always
running as class C. It only runs in class C with large signals. Since
the large signal case is were the losses really matter, this sort of
design will work quite a bit better than a simple class AB circuit.

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