Re: AM Modulation

On 3/19/06 7:25 PM, in article cb7s121qu8340pg4j63asp6vfa45c2k4a0@xxxxxxx,
"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sun, 19 Mar 2006 12:55:00 -0800, "RST Engineering \(jw\)"
<jim@xxxxxxxxxxxxxxxxxx> wrote:

Somebody in this thread said something I'd like to check in on. THe comment
was made that you can't use high-level AM modulation (collector, drain,
plate ...) on a class A (or other linear class) output stage. I'm having
trouble wrapping my head around that one. I've never tried high level AM on
anything but class C, so I can't argue from experience. But...as Clem
Kaddiddlehopper said, "It just don't seem right; it just don't seem right".

Any comments?

Jim


If we define a class A stage as having the active device always
conducting, as most people do, the output (drain, plate, collector)
signal swing is pretty much determined by input signal level,
transconductance, and output load. So unless transconductance changes
with Vcc, output amplitude is independent of Vcc. Gm *does* change
with Vcc for triodes, but not a whole bunch, and changes very little
for fets and bipolars.

A class C device is overdriven, saturates on the negative swings and
cuts off on the positive ones. So p-p output into a tuned plate load
is approximately 2*Vcc, which is why a class C stage makes a good,
pretty linear modulator. Not to mention efficient.


Yes, it's hard to beat.

It certainly generates what has been defined as "linear sidebands," but the
Class C stage itself is not linear. This confuses those who don't know the
difference.


As Clem would say, if you swing Vcc far enough to really modulate a
class A stage, it jest ain't class A any more.

John


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