Re: Impedance matching CAD analog circuit function?

> Why?

Three reasons:
1. I have a low power design that I'm trying to develop, and think
matched impedances will provide most efficient results.
2. The simulation model shows this odd transient (dampened ringing)
that I suspect is due to mis-matched impedances, which are not seen in
the design reference circuit.
3. Desire to understand basics of how to match them given source and
load situations/stages, how it translates into better design
performance, and what the manifestations/problems/characteristics are
of badly matched impedances.

For #3 I have read that when designing a communications bus, for
example, mismatched impedance results in standing waves, and
reflections. Although I am dealing with an analog design that has
nothing to do with a bus, clearly the importance of impedance matching
makes a big difference given these types of problems. I realize this is
only anecdotal, and therefore not necessarily meaning anything from a
technical point of view.

If an analog circuit is cut in half with the left side viewed as an
output impedance and the right with an input impedance, my
understanding is the max power possibly transferred is -3 db. I arrived
at this by looking at some texts and thinking in terms of voltage
dividers. The real design I have has a square wave input, and the
Fourier transform (I believe) is therefore infinite. So matching the
characteristics of the input transfer function to derive an output
transfer function such that -3 db is achieved is no trivial
accomplishment. I think that some simplification takes place when
trying to discover the optimal values of the R, L, C network to best
hit the -3 db target by neglecting the higher order terms. Again this
is all intuition, and I realize not a strong technical argument.
However I have several texts in front of me that I am reviewing
further.

>For a low noise bipolar design, the optimum noise is given by around
>about re=Rs/sqrt(hfe), and since ri=hfe.re, one can argue that there is
>a matching being done.

What is re, Rs, if by hfe you mean DC Current Gain (or Small-Signal
Current Gain?). Also can you please elaborate on the meaning of
'ri=hfe.re'.
Thanks.

.