Re: Why is limexp() an analog operator.

No, the LRM does not say that limexp is clamped to linear extrapolation.
The LRM actually says that the per-iteration change of its output is
internally limited.  (The pnjlim algorithm from Spice is shown in
Section 10.9 with the $limit function, if you're curious.)

-Geoffrey



Jonathan David wrote:
> limexp is an exponential (analog operator) clamped to linear extrapolation for large values.. 
> I've never heard of pnjlim (not a spice user ,nor compact model developer) 
> the point of limexp is to provide an exp that would NOT lead to convergence issues in an analog solver.. (assumed to be spice like)
> in NON-analog contexts one would just use exp , as its the desired function, and there is no convergence issue to avoid.. 
> 
> the point of limexp is NOT linear extrapolation (we have table model and other ways to do that.) its just an exponential "limited" to a linear function for large input values.
> Jonathan
> 
> 
>  
> Jonathan David
> j.david@ieee.org
> jb_david@yahoo.com
> http://ieee-jbdavid.blogspot.com
> Mobile 408 390 2425
> Work:
> jbdavid@scintera.com
> http://www.scintera.com
> 408 636-2618
> 
> ----- Original Message ----
> From: Geoffrey.Coram <geoffrey.coram@analog.com>
> To: David Miller <David.L.Miller@freescale.com>
> Cc: Verilog-AMS LRM Committee <verilog-ams@eda.org>
> Sent: Wednesday, October 10, 2007 8:05:02 AM
> Subject: Re: Why is limexp() an analog operator.
> 
> The old spice pnjlim is still a powerful tool!
> 
> If you want linear extrapolation, then you can write your own
> analog function to do that, and then you can use the function
> wherever you want.
> 
> In my experience, pnjlim is better.
> 
> -Geoffrey
> 
> 
> David Miller wrote:
>> I am wondering why we treat limexp() as an analog operator? Do we really 
>> need to store previous history of limexp() (My understanding is this is 
>> what the old SPICE pnjlim does.)
>> Can't we just perform a linear extrapolation when the input goes out of 
>> bounds?
>> Having limexp() as an analog operator imposes lots of restrictions on 
>> where it can be used.
>>
>> Dave
> 

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.