I teach my students that prior to analysis of an electrical, electronic, or
EM network/system one must ask and answer a critical question. The question
is: Is the network/system linear, close enough to linear for engineering
purposes, or not linear?

If linear, or essentially linear, one brings into play linear analysis.
Thevenin equivalents, which are only equivalent as far as what they do to
the outside world, are a part of linear analysis.

Most RF power amplifiers that deliver more than one or two watts are
non-linear circuits. Typically, the active device conducts for only a
fraction of each cycle. How else could one get DC power to RF power
efficiencies of over 50 %? Great care must be taken in modeling such
circuits.

A simple example: Consider a transformer fed bridge rectifier (very
non-linear) that is connected to an (old fashion) series L, shunt C filter.
In steady state, if L is large enough, one may model the rectifier as a
series of series connected voltage sources with harmonically related
frequencies (and a DC source). It is left as an exercise for the student to
decide on the sizes, frequencies, and phases of the sources. (Because of
the LPF properties of the LC network, one does not need many harmonics.)
Then one may apply superposition (the essence of a linear process) to
estimate the ripple on the load. However, the model just described is
invalid if L is too small or if L is non-linear. The model is insufficient
to predict the losses in the rectifier. This example is not likely to be
found in current electronic texts, but we all know for whom they are
written.

Techniques exist for dealing with many non-linear networks. They must be
used with great care. If one holds one's nose, one might find an
"equivalent" for a transmitter that suffices for describing what happens
outside of the transmitter, but not inside of the transmitter. Please do
not make conclusions about the "equivalent" itself. Please discriminate
between linear and non-linear networks.

Thus ends the lecture. 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
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