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Reply-To: "Fred Bartoli"
From: "Fred Bartoli"
Subject: Re: Op-amp virtual earth not 0V
Date: Tue, 17 Sep 2002 11:54:29 +0200
X-Newsreader: Microsoft Outlook Express 5.00.2919.5600
Organization: Guest of ProXad - France
NNTP-Posting-Date: 17 Sep 2002 11:52:10 MEST
"Nghia" a écrit dans le message news:
> 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?
Because opamps are not perfects.
At frequencies above a few Hz (or 10s Hz, more for some high speed ones)
opamps behave like integrators (gain rolling off 6db/octave).
Hence the differential input voltage (the one you observe at the virtual
ground node in an inverter scheme) has to be the derivative of the output
voltage divided by the opamp gain.
For an output sin wave, you'll have had a cos wave, for square wave,
something like dirac pulses and so on...
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