Re: gEDA-user: device and sub-circuit models

[al davis <ad151@xxxxxxxxxxxxxxxx>]

On Sunday 07 December 2008, vsrk sarma wrote:
> I am facing difficulties in including device/sub-circuit
> models into gnucap simulation.
> Specifically for transistors & analog ICs such as opamps.
> It appears to me that application package does not have any
> and we have to create. Am I right?

Models for specific devices are not included.  You need 
to "Google for it", or make your own.

> Is it possible to use available spice models? If so how to
> make them compatible with gnucap.
> how to ensure pin compatibility of imported model & my
> circuit.

Gnucap mostly accepts spice models.

The reason I say "mostly" is that there is so much variation in 
Spice models.  Is a model written for HSpice?  PSpice?  Each 
one is a little different.  You need to try it.  The one 
closest to gnucap is HSpice.

The development snapshot has more features, so if you get a 
model that is not compatible with gnucap-0.35, it is likely it 
will work with the development snapshot.

Pin compatibility is not an issue.  Gnucap uses the same syntax 
and pin lists as Spice.

If you are using J-Fets, you must use the development snapshot.

> Apologies for very basic questions (I have not used Spice
> earlier )

If you are just getting started, it is usually better to make 
your own models.  You probably don't need the detail provided 
by the published models.  You will benefit if you understand 
the models you use.

For an op-amp, you can usually use a voltage controlled voltage 
source.

For transistors, usually you can just add a ".model" statement, 
and specify "BF" (beta) and "IS" (saturation current).

For MOSFETS you can usually specify "level=1" for the simplest 
model and specify "VTO" (threshold voltage) and "KP" 
(transconductance parameter).  You need to know this to design 
your circuit anyway.

Sometimes you need to experiment with the parameters.  Here's a 
reference that might help with that:
http://wiki.gnucap.org/dokuwiki/doku.php?id=gnucap:experimentally_finding_model_parameters

Everything I said here applies equally to Spice.  The models are 
the same.



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