Newsgroups: sci.electronics
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From: whit@milton.u.washington.edu (John Whitmore)
Subject: Re: Impedence matching...
Message-ID: <1991Mar30.001621.4822@milton.u.washington.edu>
Organization: University of Washington, Seattle
References: <1991Mar29.150314.23177@e2big.mko.dec.com> <2532@umriscc.isc.umr.edu>
Date: Sat, 30 Mar 1991 00:16:21 GMT

In article <2532@umriscc.isc.umr.edu> robf@mcs213k.cs.umr.edu (Rob Fugina) writes:
>I'm confused as to the importance of impedance matching...  Since I may
>not know the whole scope of this subject, I'll say I'm referring to things
>like stereo line-level, headphone, microphone, antenna inputs and outputs.
>I can see that if you have a high impedance source, you need to have a high
>impedence load or input so as not to load down the source.  So what is the
>need, when you have a low impedence source, to have a low impedence load?

	The power transferred to the load is greatest when the impedances
are matched (to be accurate, when the load impedance is the complex
conjugate of the source impedance).
	So, if you don't want to waste the gain of the previous stage
of the amplification chain, you should match impedances.  IF you have
gain to burn, however, you might deliberately mismatch impedances
for some sort of effect (like making a voltmeter input higher impedance
than the circuits it measures, or making an ammeter input much lower
impedance than the circuits it measures).
	Try a few numbers; if a 1000 Ohm output carries 1 Volt of
signal, the voltage level is 1V(no load), or 0.5V (1000 Ohm load),
or 0.001V (into a 1 Ohm load).  So the load power is 0 (into no load...)
or 0.25 mW into 1000 Ohms, or 0.001 mW into 1 Ohm.
	In a very real sense, the power transferred is always competing
with some noise power; higher power transfer minimizes the degradation
of the signal due to noise.

	Lastly, some common elements of a circuit (the long wires) have
odd frequency-dependent properties UNLESS a particular impedance 
drives (and receives) the signals.  Usually RF cables are connector-
coded so that the wrong impedance wiring literally cannot be connected
where it doesn't belong.  

	John Whitmore