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From: "Mike Engelhardt"
Subject: Re: SPICE and amp stability
Date: 20 Nov 2002 21:12:56 GMT
Reply-To: "Mike Engelhardt"
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>>>> Or is there some more correct way to determine the loop
>>> You have to look at the *loop* gain. This is the open loop gain
>>> around the *feedback* path. This involves setting up a circuit such
>>> that the opamp has a closed loop for dc (or very, very low
>>> frequencies), but open loop for the main plot. This is so the amp
>>> will bias correctly. The easist way to see how this is done is to
>>> run my SuperSpice
>>This is a poor way of doing this. All you have to do insert a floating
>>AC voltage source in series with the feedback at in front of a high
>>impedance point of the circuit, typically the opamp input. Then do your
>>.AC analysis, and plot the ratio of complex voltages to either side of
>>the voltage source. This method also is a good one for bench use,
>>called either a frequency response analysis or Venable.
> It is a method that is used very often and does indeed work
> in practise, despite not being exact. A poor method would be
> something that basically don't work.
Well, don't take it personally, but I thought it was a poor
suggestion because it's more complicated(not even accounting
for having to download that piece of junk product of yours
you were trying to promote with your followup). Anyway, on
the bench your method doesn't work when the opamp has too
much noise times gain to keep the output from hitting the
rails. I have used your method years ago and for some opamps
it worked and for others it didn't. Always the outer loop
needed to be adjusted for the opamp being tested and the opamp's
output stage was never at the bias point of the closed loop
circuit of interest. The method I and others suggest gives
the open loop response from the exact bias point of the closed
loop system. This Frequency Response Analysis/Venable method
just seems like a more suitable approach to me.
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