Re: [Help-gnucap] Yet another newbie question

[al davis]

On Wednesday 09 July 2008, Johannes Bauer wrote:
> Now I wanted to use a diode, for example. However I have to
> specify a model as a value - now one really silly question
> is: How do I find out what models are available for diodes? I
> tried some standard values like 1N4001 and 1N914 none of
> which were recognized.

If you need to ask that question, you probably can get by with 
just guessing.  Just make a model statement taking the 
defaults:

.model 1n4001 d

If you need a little more accuracy than that, adjust the 
parameter IS to get the expected forward voltage.

As you get more advanced, you can look on the web for specific 
models.  Google for "1n4001 spice model" and see what you find.  
It should ultimately lead you to something of the form:

.model 1N4001 D  (and a bunch of name-value pairs)

You could just use that, but if you want that kind of detail you 
should look at several to see how much they vary.

Usually, most Spice models work with gnucap, but it cannot be 
guraranteed.

There are so many devices, from so many vendors, that it is 
impossible to keep track of them all.

> Is there also a way to use operational amplifiers? I'd guess
> that would be the "U: Logic Device" family in analog mode,
> but again - the problem with knowing which models are
> supported. Tried the LM324, but no luck again.

No .. the "U" is not an op-amp.

Op-amps are more complex devices, so you need to decide what 
kind of model you want.

If you need to ask, you can probably get by with a very simple 
model.  Use a voltage controlled voltage source (VCVS, type E).

Eop (out1 0  in+ in-) 10000

If you need to model clipping, use the "tanh" behavioral 
modeling function to specify open loop gain and clip voltage.

Eop (out1 0 in+ in1) tanh(10000, 12)

If you need something a little better, make a subckt with two 
controlled sources and a dominant pole.  If you don't know how 
to do this, the VCVS is probably all you need.

If you need better, google for a spice model.  (or in the 
future .. Verilog-A model) ..   Op-amp models are usually 
subcircuits, designed for a particular variant of Spice.  Some 
work, some don't.  The only way to know is to try it.  These 
models have lots more detail.  If you are just beginning, you 
are better off without it.