Ian White, G3SEK wrote:



I agree with every word, George.

Why do folks make this so complicated, Ian?


Mostly by insisting on asking questions that may not even *have* an answer!


The problem is that finding the output impedance
analytically is usually very difficult.

Measurement methods (usually questionable) have
been proposed that "estimate" the output
impedance. These tests can often be manipulated to
get some desired result (for example 50 ohms).

The value of output impedance depends especially
on signal level and also several other parameters,
such as negative feedback.

Is the value of output impedance important?
Sometimes in critical situations it can be. For
example, a lowpass filter connected to the output
of the PA may not be exactly correctly terminated
at the input end. The error slightly affects the
filter response, especially at the filter cutoff
frequency. The passband ripple can also be
affected. Most lowpass filter types can be
designed for unequal values of generator and load
impedances.

If the PA is broadband solid-state a sweep method
can be used to optimize the filter design.

Usually these errors are unimportant, especially
in typical Ham Radio.

Bill W0IYH