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From: "Mike Engelhardt"
Subject: Re: SPICE and amp stability
Date: 20 Nov 2002 18:56:59 GMT
Reply-To: "Mike Engelhardt"
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
analog, Jim, myself, Kevin, and Walter wrote:
>>>>> 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 LoopCutter.sss example:-).
>>> 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.
> Er, shouldn't that be a floating voltage source inserted
> in series between a very low impedance source and the
> finite impedance branch to which it was formerly connected?
> (Or the dual method with a current source, of course.)
> This is general method that works regardless of the relative
> impedances at the insertion point within the loop to be measured
Of course I'm aware of the general case! I just
wanted to present the manifestation that I felt
was most likely directly applicable to the original
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