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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: SPICE and amp stability
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Date: Wed, 20 Nov 2002 19:42:22 -0000
NNTP-Posting-Date: Wed, 20 Nov 2002 19:42:33 GMT
Mike Engelhardt wrote:
> Kevin wrote:
>>> 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.
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
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.
Although this is more exact, it can be a bit more messy when you are
probing about the schematic. You need to set up the v1 divide v2 all the
time. I personally, cant usually be bothered to use this approach.
There are other methods as well that use duplicate circuits.
> Then do your .AC analysis, and plot the ratio of complex voltages to
> either side of the voltage source.
Oh, and someone already beat you to the desription of this one.
> This method also is a good one
> for bench use, called either a frequency response analysis or Venable.
Actually it can be a bit tricky to do in practise on the bench due to
equipment limitations. You need to get the floating source. If you
actually know that the errors due to the amp input and output are low,
it can still be more convenient to use a more approximate set-up.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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