Re: gEDA-user: ngspice - problem with AC current source

[John Doty <jpd@xxxxxxxxx>]

On Oct 19, 2010, at 5:46 AM, Fabio wrote:

> Hello,
> 
> I have a simple transimpedance circuit, like this (It's an AOP with a single resistor) :
> http://bit.ly/bhGCeB (.gif)
> 
> Since the transimpedance inverts the current, I'm putting a single idc source "upside down" so that the output voltage is always positive (the dc offset is equal or greater than the ac signal). 
> 
> The value for the idc is: sin(1m 1m 100)
> The result of the simulation is a signal at Vss, but I expected a positive sin.

What happens with a simpler opamp model, like the ones at http://www.gedasymbols.org/user/john_doty/models/opamp/index.html?

> 
> So I switch to two idc sources in parallel. Their values are:
> sin(0 1m 100)
> dc 1m
> 
> And then the simulation is completely different. It's almost ok, it just has a small transient part that I cannot explain:
> http://bit.ly/cmtf6u (.gif)

It looks to me like the simulation is getting stuck at some weird, not physical, operating point. What happens if you just do the operating point calculation ("op" command) by itself?

> 
> Is it OK to use the idc as an AC current source ?

Yes.

> Why is the result completely different (and wrong!) when I use only 1 current source ?

If the simulation is having trouble finding the operating point it can be sensitive to apparently irrelevant details.

> Do you think the small transient part is just an artifact from the simulation ?

Yes. It might help to give the simulator a hint about the initial operating point using ".NODESET" or "IC=".

> 
> Thank you for your attention,
> Fabio
> 
> 
> 
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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