gEDA-user: ngspice - problem with AC current source

[Fabio <mailinglists@xxxxxxxxxxxx>]

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.

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)

Is it OK to use the idc as an AC current source ?
Why is the result completely different (and wrong!) when I use only 1 current source ?
Do you think the small transient part is just an artifact from the simulation ?

Thank you for your attention,
Fabio



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