Bob Drzyzgula wrote:
[snip]
> In impedance matching, one is concerned about the transfer
> of an electrical property across a boundry of some sort,
> commonly a power supply and device, or a signal line and
> a reciever...
[snip] 

Just using your example Bob, nothing to do with what you wrote.

Some people think that matching impedance means to connect two
circuits with equal impedance.  Years ago, I heard an electric
engineer saying that to match the two circuits we were working,
(one had lower input impedance), it was just a matter to install
a series resistor and "voila", both with same impedance.

Never in my life heard bigger stupidity.

The matching impedance means "productive" impedance, not any
other sort of miraculous solution.

Suppose you have an sound amplifier with an output impedance 
of 4 Ohms, a specific waveform is delivering 8 Volts.  
Suppose you have this circuit attaching speakers of 8, 4 and 3 Ohms:

8V --- RINT 4 Ohms-----o-----REXT 8 Ohms---Gnd
Current = 8 / 4+8 = 0.66A,  Power at REXT: R×I» = 3.54W

8V --- RINT 4 Ohms-----o-----REXT 4 Ohms---Gnd
Current = 8 / 4+4 = 1A,     Power at REXT: R×I» = 4W

8V --- RINT 4 Ohms-----o-----REXT 3 Ohms---Gnd
Current = 8 / 4+3 = 1.142A, Power at REXT: R×I» = 3.91W

If the miraculous solution of that "engineer" would used
here in first example, we needed to apply a resistor of
8 Ohms in parallel with the 8 Ohms speaker, so both 
circuits would be matching impedances, right?

8V --- RINT 4 Ohms-----o-----REXT 8//8 (4 Ohms)---Gnd
Current = 8 / 4+4 = 1A, Power at REXT: R×I» = 4W,
but that power would be divided by 2, between the
8 Ohms speaker and the 8 Ohms resistor. So the 
productivity of that solution is at least ridiculous,
even that the amplifier would be delivering the max
possible power, the usage of it would be a disaster.

It means that the "whole" circuit impedance, what appears
at the input needs to match to the other "whole" circuit
impedance, what appears at the output, nothing different.
--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm