Re: gEDA-user: geda-user Digest, Vol 54, Issue 32

[John Doty <jpd@xxxxxxxxx>]

On Nov 11, 2010, at 9:58 PM, clif@xxxxxxxxxxxxx wrote:
>> 
>> You can configure ngspice to support POLY. See http://www.brorson.com/gEDA/SPICE/x496.html.
> 
> Yes but there isn't much in the way of examples on how to use them.
> 
>>> Are POLYs really that much better at solving convergence problems to be worth the extra trouble? Is there a good into or howto on how to use them?
>> 
>> There is a vast literature on the subject of constructing polynomial approximations to functions. Google for "polynomial approximation". You'll never be able to digest it all.
> 
> True, but that dosn't help with for exmaple how to make models for multiple variables. Apparently you can say something like POLY(2) or POLY(3) but then how do you list the values? In Gnucap it seems you list the coeficent of X^0 (1) but a lot of models start with X^1. It would be helpfull to have a howto that described the best practices for all of this with some example problems.

http://newton.ex.ac.uk/teaching/CDHW/Electronics2/userguide/secC.html

> 
> Also some discussion on when to use them vs arbituray expressions as in Bxxxx sources. Naively one might assume that becuase they should be well behaived functions that each point would have a usable first or even second derivative, and this would help convergance. Though here is a quote from the NGspice manual Chapt 12.2.7 BSIM1 model (level 4):
> 
> "A known problem of this model is the negative output conductance and the convergence problems, both related to poor behavior of the polynomial equations."

Common problems with polynomial approximations:

1. They rapidly become useless outside a limited domain. BSIM models are indeed prone to unphysical behavior at operating points outside the domain of their approximations because they use polynomial adjustments for some computations.

2. It takes a lot of terms to do a decent approximation of a function whose shape isn't very "polynomial like".

> 
> So it's not even obvious that they will perform better. Are they there
> just to support legacy models, or are there times when they are the best tool we have?

Most special function libraries use polynomial approximations heavily, so if you're using Bxxxx you're almost certainly using polynomials implicitly. The advantage of the library functions is that they've generally been designed to use a given polynomial only within the domain in which it behaves well.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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