>Download a copy of SwitcherCAD III from Linear Technology
>so you have a nice free SPICE engine with GUI to play with.
>That can help you understand what is happening (the theory,
>but not always the implementation) in an amplifier (BJT,
>CMOS, FET, op amp, tube, whatever) circuit.

There is also a very helpful advice group at
http://groups.yahoo.com/group/LTspice/ where the files section has a fair
number of things that are not provided with the package as provided, and the
members give lots of help on using other manufacturers models with
SwitcherCad.

I went to a presentation by the guy that wrote and maintains SwitcherCad,
and they make no bones about providing it for free, and using other
manufacturers models with it.

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