In article , gwhite wrote:
Cecil Moore wrote:
[...]
"The concept of 'output impedance' breaks down
for large signal devices.

Motorola seems to disagree with you. Perhaps more importantly, I disagree
with you. The large signal impedance of a transistor working into a tuned
load still applies quite well.

For example, what is
the output impedance of a class C or D amp taken
when the transistor is on or off?"

When dealing with an RF output section you don't deal with just one part
of the cycle. You can tell if the real part of the impedance is matched
if:

(1) If you increase the resistance of the load does the power
decrease?

AND

(2) If you decrease the resistance of the load does the power decrease?


The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.

What do you mean by "maximum signal swing" in this context. I can get a
bigger swing by leaving the output completely unloaded and hence causing
the actual efficiency to be zero.

The reactive component issue is still there too. Reactive loads cause
increased currents in the output stage without delivering any power to the
load so they still need to be reduced as much as practical.




--
--
forging knowledge