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From: Fred Bloggs
X-Mailer: Mozilla 4.7 [en]C-CCK-MCD EBM-Compaq1 (Win95; U)
Subject: Re: OP Amp Output Resistance
References: <3D8D30C9.firstname.lastname@example.org> <3D8D4F02.991FC7CA@webaccess.net> <3D8FD91D.2FB63DA8@nospam.com>
Date: Tue, 24 Sep 2002 14:16:52 GMT
NNTP-Posting-Date: Tue, 24 Sep 2002 07:16:52 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> 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.
Well- operational amplifier theory and understanding is intimately tied
to feedback theory in general. You might try illustrating feedback
principles with some of the more easily understood historical examples
from servo mechanism theory. There is a wealth of interesting circuits
that were actually used at one time involving positioning systems using
batteries and potentiometers with feedback driving wipers for voltage
null. These types of circuits are always interesting, ingenious, and
readily grasped- as long as you avoid minute details like stiction,
contact resistance non-linearities, large excursions, and time
constants. Static graphical analysis is always fun too and is a very
powerful way of illustrating the linearizing effect of the feedback.
Your presentation will be on the sterile side if you omit the historical
background which drove the development of the theory and devices to
solve some very important practical problems.
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