On Tue, 6 Jan 2004 09:41:34 -0800, "Tim Wescott"
wrote:

Sorta. The distinction has more to do with whether you're following your
output stage with something that needs to see a specific impedance (like a
filter in a receiver), and whether you're building a power stage. I
couldn't think of a good generalization, so I listed some examples below.
Then I realized that it really boils down to the fact that the impedance
matching rule is a tool for predicing circuit behavior, not a guideline for
life.

RF stage in a receiver: you want to match the output impedance to the
following filter or mixer for maximum power transfer and good filter/mixer
performance. You usually _don't_ want to match input impedance because the
best power transfer impedance is not the same as the best noise figure
impedance.

IF preamplifier (between a passive mixer and a crystal or mechanical
filter): Match both input and output impedances, both to maximize power
transfer and to get good performance from the attached stages.

1st buffer in a VFO: _don't_ match impedances with the oscillator! You want
to purposely refrain from taking much power from the oscillator tank,
because a heavily loaded oscillator is a poor oscillator. You probably _do_
want to match impedances on the output so the following stage won't need so
much gain.

1-transistor transmitter: It's an oscillator, but a power stage too. Load
it (match impedances) enough to get useful power on the antenna, but not so
much that you screw up performance.

Power output (usually): match impedances to the input, don't match impedance
to the output (for all the reasons given earlier). This applies to any
class of stage: A, B, AB, C, D and E (yes, there are class D and E stages).

And even "S" IIRC!
Thanks, Tim. I'm going to have to digest this lot. Might take a
while...
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill