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Subject: Re: OP Amp Output Resistance
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Date: Tue, 24 Sep 2002 10:47:26 GMT
NNTP-Posting-Date: Tue, 24 Sep 2002 12:47:26 MET DST
"carltons" schrieb im Newsbeitrag
> In article <3D8FD91D.2FB63DA8@nospam.com>, Fred Bloggs
> > carltons wrote:
> > > You are absolutely right. The output is non linear. I have answered
> > > question many times with non-engineers who don't seem to understand
> > > the output impedance can go down with something simple like feedback.
> > > always say with confidence that the feedback causes the output voltage
> > > 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.
I found it useful explaining OpAmps to start with an inverting configuration
and ideal opamp with infinite input resistance and gain and zero offset,
noise and output resistance:
Since the +in is gnd, the opamp will keep the -in also to gnd.
So if a pos voltage is applied to the input resistor, the output will go in
negative direction, until both currents are equal, since no current can flow
into the -in pin.
so we get the input current= Uin/Rin =! -Uout/Rfeedback.
give a few values for the resistors and let everyone understand this basic
principle, let the pupils solve a couple of different conditions (Rin, Rfb
varies) for pos and neg Uin etc.
Then only introduce the non-inverting configuration with the voltages
varying on both inputs and a voltage divider from output to the -in.
Again questions about minimal gain, how to adjust gain, graph input vs.
output plot etc.
Now slowly you can begin with offset voltage, how to compensate, how big
and so on...
Remember it is your skill if they understand or not. First you will have to
absolutely master the subject yourself. I think here lies the real problem,
There is the book:Tietze, Ulrich Advanced electronic circuits
which teaches theory and circuit design, it might be helpful.