EMRFD Message Archive 8831

Message Date From Subject
8831 2013-07-28 05:02:29 Chris Howard w0ep impedance matching in audio circuit
I have an op-amp based audio filter (Hy-Per-Mite from 4 State QRP club)
and I am looking at integrating that into my old HW-8.

I can see a few ways to do it.

I was wondering about impedance matching to the input of the new
filter, depending on where I put it in the audio signal path.
Wouldn't the driving circuit do a cleaner
job if I attempted to match the impedance?

Or, maybe there is some magic in the op-amp thing that I'm not seeing?
I thought op-amps were high impedance on the input.

I was thinking that a resistor of the appropriate size from signal
to ground would give the driving circuit the impedance it
needs to be happy.


Chris
w0ep
8836 2013-07-28 14:47:05 Andy Re: impedance matching in audio circuit
Impedances are rarely ever matched in audio circuits, since about 75 years
ago. The main exception is with passive unbuffered LC filters. Very long
transmission lines (miles) also need impedance control, which you might
think of as impedance matching but the reasons are different. And things
like detectors also need a particular load impedance to work right.

Ordinary op-amp circuits are "never" impedance matched. The output of a
closed-loop op-amp has an output impedance approaching 0 ohms, but wants to
see a moderately high impedance load. That reduces the current that the
op-amp needs to supply. The less current, the better ... the happier it is.

Wouldn't the driving circuit do a cleaner
> job if I attempted to match the impedance?
>

Possibly the opposite. Adding a load requires the previous stage to supply
more current, probably making it more nonlinear (more distortion). Yes the
negative feedback tends to right that; but why do things to hurt it?


> I thought op-amps were high impedance on the input.
>

They are; but nobody uses op-amps by themselves. The input impedance
depends on the feedback & resistors external to the op-amp. The input
impedance of the complete op-amp stage is usually made high (kilohms or
more).


> I was thinking that a resistor of the appropriate size from signal
> to ground would give the driving circuit the impedance it
> needs to be happy.
>

That impedance is (almost always) close to infinity ... unless the circuit
that is driving it is a passive filter of some sort, which needs to have a
specified load on it.

Andy


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