But there aren't many models or even internal schematics on the
internet.
On the contrary, there are thousands upon thousands of SPICE models on
the Internet!
Part of the difficulty is determining which ones are worthy of using
and which ones are not. For a popular IC component like a 741 or a
555, you could probably find literally hundreds if not thousands of
copies of SPICE models for them on the 'net, of which a few dozen are
distinct from one another; and then your job is to choose which one(s)
to run with.
Internal schematics, now there you may be correct that they are hard
to find on the Internet. It used to be (when an op-amp was about as
complex an IC as you could get) that equivalent schematics were often
included on the data sheet for the part. Of course the schematic
didn't show you the characteristics of the transistors and diodes, and
maybe omitted resistor values too, but at least it was a starting
point.
And I have seen and used a number of vendor-supplied SPICE models that
included schematics in the documentation that accompanied those
models.
Regards,
Andy
