Tue Nov 4 22:50:44 EST 2014

Resistors and current sources

Spice book arrived.  Let's have a closer look at this resistors +
current sources approach.

Reading chapter 2, all matrices that appear are symmetric.  This can't
be a coincidence.  See next post.

Reading on, it talks about spice linearizing diodes as current sources
in parallel with resistors.  I=I0+GV

Same for dynamic elements, which I find surprising.  This seems to be
a clear difference from the approach I was thinking about.  Or maybe
not?  See figure 2.14

Actually it is not that different, except for:
- change of variables (GV = I)
- multiple iterations for a single time point for NL els
- several integration methods

The GV=I form seems better as it reduces the number of variables: only
node voltages.  Currents are derived from voltages.

The idea to "rotate" nonlinear elements into time should still work,
making the computation at a particular time step linear.  Since the
circuit is symmetric and sparse, CG can be used if it's positive
definite (probably is.. indeed i found some mention online. but why?)

It might be good to find out if using V only (instead of both V and I)
creates bad numerical condition.  The spice book already mentions
diodes with high currents (small dV -> large dI).  Maybe in such cases
a change of variables makes sense?  ( eigencircuit? see next posts )

[1] http://books.google.com/books/about/Inside_SPICE.html?id=oOVSAAAAMAAJ