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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Op-amp virtual earth not 0V
Date: Wed, 18 Sep 2002 07:41:52 GMT
NNTP-Posting-Date: Wed, 18 Sep 2002 00:41:52 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> Hi all,
> I've just done an experiment with an unknown op-amp where we put in a
> triangle wave (1 volt) at 100Hz into an inverting voltage gain circuit,
> gain of -10. The op-amp saturates at +-15V.
> Now when observing the virtual earth point I noticed that its not zero
> but a square wave of very low amplitude, also at 100Hz. Can't remember
> how much was zoomed in so can't remember the exact value.
> I've searched everywhere on the web and haven't found anything that
> could help me explained this. Why isn't it zero volts?
I have not bothered with looking at the other responses.
The opamp has a finite open loop gain; it needs an error voltage
across the inputs to drive it enough to produce the output that one
Specifically, an un-disturbed opamp will have a differential input
related to the output divided by the open loop gain.
This can be made less simplistic if one considers gain as a function
Please note that your measurement scheme, no matter what it was,
disturbed and altered the opamp circuit and the results are not
Good old Heisenberg rules again....
You may also note that there is a small DC voltage difference between
the inputs; this is due to the input offset voltage and input offset
current flowing thru the input resistances.
This also should not be measured directly; the results are not
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