Re: [ng-spice-devel] a useful device


To ng-spice-devel@ieee.ing.uniroma1.it
From Al Davis <aldavis@ieee.org>
Date Thu, 22 Feb 2001 20:15:06 -0800
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On Thu, 22 Feb 2001, mcmahill@mtl.mit.edu wrote:
> you know what would be useful to have?  A voltage source like
> device which you could specify to only be present for an operating
> point analysis.  The application is where you want to have
> something connected to get a circuit biased correctly and then
> vanish during an AC analysis.  I often times just use a very large
> inductor, but it might be nice to actually have a device for this.

ACS has that, close enough to do what you want.

As you know, Spice lets you give AC, DC, tran values for sources.  
So, you can give a DC value, and it becomes like a short for AC.

What you want is for it to become an open for AC, not a short.  In 
ACS, all passive components can have separate AC, DC, etc.. values, 
so use a resistor.  Give it a low value for DC and a high value for 
AC.

R123 (1 2) dc 1 ac 1g


Another thing that ACS lets you do is to run a transient analysis to 
a point, then stop it and do an AC analysis there.  I have used this 
in analyzing audio amps in the frequency domain at various points on 
the waveform.  

al.

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