From: John Woodgate
Subject: Re: OP Amp Output Resistance
Date: Fri, 27 Sep 2002 22:14:03 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Fri, 27 Sep 2002 21:20:17 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that carltons
wrote (in ) about 'OP Amp Output Resistance', on Fri, 27 Sep 2002:
>Why is the output impedance of an ompamp being questioned in this thread?
It is true that the OP seems concerned about something that is not often
of concern, but such discussions turn up useful insight on occasion. For
example, does the output impedance vary over a cycle of the signal? If
it does, then the output voltage waveform, with a load on the output, is
distorted. But how to verify? Measured distortion might be due to some
other effect in the op-amp internals, and it is not easy to see how to
distinguish one effect from another.
If the op-amp has a common-emitter Class B push-pull output stage (or
reasonable facsimile thereof), then the open-loop output impedance does
vary over the cycle, as the gm of the output devices varies with emitter
current. Then we have to ask whether this is of any practical
significance. It may be device-dependent.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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