From: Jim Thompson
Subject: Re: SPICE and amp stability
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Date: Wed, 20 Nov 2002 16:35:29 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 11:35:29 EST
Organization: Cox Communications
On 20 Nov 2002 16:19:13 GMT,
"Mike Engelhardt" ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: SPICE and amp stability",
Wrote the following:
|> > Or is there some more correct way to determine the loop characteristics?
|> 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.
If you're using PSpice, you can download, from my website, my
"Loopgain" part. This performs two passes, one with voltage drive,
and one with current drive, then computes gain and phase; all done
without perturbing the loop.
This *may* be applicable to other "Spices" as well.
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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