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From: email@example.com (carltons)
Subject: Re: OP Amp Output Resistance
References: <3D8D30C9.firstname.lastname@example.org> <3D8D4F02.991FC7CA@webaccess.net>
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sun, 22 Sep 2002 13:51:40 GMT
Date: Sun, 22 Sep 2002 13:51:40 GMT
In article <3D8D4F02.991FC7CA@webaccess.net>, Chuck Simmons
> Stephen wrote:
> > All op amps have a non zero output resistance that is not noticable due
> > to feedback unless varying current demand exceeds the bandwidth of the
> > amplifer. I need to know how linear this resistance is.
> Not very linear at all. You are talking about the power bandwidth and
> that depends on the output structure of the opamp. Generally, opamps
> become highly nonlinear in that regime and the output may even be
> asymmetric for a symmetric input.
> > I also would like to know if the output resistance is not specified, if
> > it can be deduced by how output voltage swing specification varies with
> > output current.
> No. Not unless the specification is determined open loop with low output
> voltage level.
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. I could
throw in some delta volts versus delta amps, but I try to keep it simple.
I guess I'll never be a good teacher without my Calculus. :-)
Steve Carlton WB4CZR
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