Bifilar Wound Balun Transformer

[Jamie]

It's cool to stick a micro-ohm resistor somewhere in a circuit to
sample current, and follow that with a 1e6 gain amp with infinite
CMRR.

Micro ohms?, we use micro ohms for measuring current but they are kind
of large in mass. Or are we not talking about copper?

Jamie

 

[Tim Williams]

Spice is all double floats, no? 1e6 isn't a very big number. A current
sampler is open-loop so won't have convergence problems.

What the numbers are doesn't matter, the solver uses iterative algorithms
that save inverting the whole N x N matrix. Matrix operations are O(N^3),
so a modest circuit with 100 nodes takes about 10^6 more operations than a
basic battery and lightbulb circuit. And that's if it guessed the correct
timestep the first time, which very rarely occurs. SPICE throws away a
*lot* of computation.

Obviously, you've never simulated anything of significance, or you'd know
this. The degree of these approximations is controlled by the simulation
parameters, which experienced users understand must be optimized for a
given model to avoid slowing to a crawl (with default settings, LTSpice
likes to wander off into the picoseconds, eating glue) or generating
errors (most often "timestep too small").

Tim

 

[Bill Sloman]

BillSlomanwrote:



I get the idea that a simulator is the only source for any inclinations
you come up with..

Congratulations on coming up with an idea. You don't do it often. For
extra credit come up with an idea that makes some kind of sense - you
haven't yet, but while there's life there's hope.

 

[Bill Sloman]

Spice is all double floats, no? 1e6 isn't a very big number. A current
sampler is open-loop so won't have convergence problems.

LT Spice sometimes does weird things with time steps, especially if a
circuit has radically different time constants here and there, or
mysterious stuff inside a part model. Real transistors seem to stress
it, too. Somebody should write a book or article about all that.

It's been done. I think I referenced a text on the subject in my Ph.D.
thesis back in 1969.