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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. Home: |
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