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From: firstname.lastname@example.org (carltons)
Subject: Re: OP Amp Output Resistance
References: <3D8D30C9.email@example.com> <3D8D4F02.991FC7CA@webaccess.net> <3D8FD91D.2FB63DA8@nospam.com>
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Tue, 24 Sep 2002 09:45:45 GMT
Date: Tue, 24 Sep 2002 09:45:45 GMT
In article <3D8FD91D.2FB63DA8@nospam.com>, Fred Bloggs
> carltons wrote:
> > You are absolutely right. The output is non linear. I have answered this
> > question many times with non-engineers who don't seem to understand how
> > the output impedance can go down with something simple like feedback. I
> > always say with confidence that the feedback causes the output voltage to
> > try to stay at the same level no matter what the load, which would not
> > happen without the feedback and all I get are blank stares.
> Why don't you try explaining that the feedback amplifier is amplifying
> the scaled error between input and output in such a way as to drive it
> to zero, and then since the feedback tap is at the output, the voltage
> drop due to output resistance becomes part of the error and is therefore
> nulled too. You don't need calculus to understand this basic concept.
I've tried that also to no avail. You can show the offset voltage and how
it is amplified by the huge gain of the amp and you will still get that
weird stare in response. I do believe that you have the right approach
here, but I think that most of the people don't get it until long after it
is explained, which is okay with me because I want to explain things and
if there's a time lag, so what. The goal is to educate, after all.
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