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Re: [Help-gnucap] Re: using a pipe to make gnucap accept input from othe
Re: [Help-gnucap] Re: using a pipe to make gnucap accept input from other program
Thu, 11 Jan 2007 03:22:28 -0500
On Thursday 11 January 2007 02:33, Hugo Coolens wrote:
> al davis wrote:
> > The reason Spice does it the way it does goes back to the
> > small computers of the 70's, where you needed overlays to
> > fit a large program into small memory. It is a multi-pass
> > system. The "raw file" is the interface between the
> > simulator engine and a post-processor. Computers are
> > bigger now, so we don't need to do it that way anymore.
> I understand what you mean but wouldn't it be nicer to have
> gnucap do itself the task of choosing the right statement
> order when using batch-mode?
It would be very easy to do, but I prefer it the way it is.
You might also ask another question, .... why is "op" required
before "ac", or why does "ac" not give the same results every
If you don't do an "op" immediately before "ac" you will get the
operating point of whatever the most recent transient, dc, or
op finished with. This also is deliberate. It is this choice
that allows you to evaluate the ac perfomance with a variety of
operating conditions. Think about a class-B amplifier with
negative feedback, that has a stability problem that shows as a
glitch in the waveform at a particular level. Only gnucap lets
you home in on that point and do a detailed AC analysis there,
at an instant in time. Run the transient analysis to the
glitch, stop it there, then run AC. You can run several
sweeps, move the probes, and run again, and see lots of detail
at that point.
But back to the original comment. I might want to do an overall
analysis,see the results, then home in on some detail. Let's
take that same class-B amplifier. I want to see a plot of gm
of the driver stage, in time, with signal, or the nonlinear
input capacitance, which does strange things as the output
stage goes through its crossover region. This kind of detail
is only available when you have fine control that gnucap
Even in batch, maybe I want to try several variants of the
circuit. First, a basic circuit, then maybe add an emitter
follower, and see how that works, then change it to a
complementary pair and run again, all in one batch file. Can
Spice do that? Can Spice show me, in one batch file, how the
AC performance for positive signals which use the NPN
transistor is different from the AC performance for negative
signals that use the PNP? in and near the crossover region?
There is also the possibility that "batch" mode isn't really
batch, but an interaction with another program, perhaps an
optimizer, perhaps it is doing a more complex analysis like
periodic steady state, distortion of an oscillator.
All this is only available when you, the user, have detailed
control. This is lost when there is any automated rearranging.