From: bill.sloman@ieee.org   
      
   On 11/02/2026 6:39 am, john larkin wrote:   
   > On Tue, 10 Feb 2026 07:49:22 -0900, Christopher Howard   
   >  wrote:   
   >   
   >>> Standard apprach for low-accuracy approximation is diode-resistor   
   >>> network.  But such network gets inpractically large if you need   
   >>> good approximation.   
   >>   
   >> Thank you. Looking at all the suggestions put forward, it seems like —   
   >> for my application — that just getting the AD633 might be best choice.   
   >> My analog computer is short one multiplier for a simulation I wanted to   
   >> play with, and I wonder if there was some easy trick to get a reasonably   
   >> accurate square function without getting a decent multiplier. In a lot   
   >> of basic physics simulations, one has to square velocity to get   
   >> things like drag or lift forces.   
   >>   
   >> Line approximation with a resistor-diode has some appeal. Maybe it could   
   >> be accurate enough for my little educational experiments? I think, to   
   >> pull it off, I would need to use trim pots — two per segment, for the   
   >> biasing and the attenuating.   
   >>   
   >> The schematic from the neurological paper seemed to be a line   
   >> approximation solution with some of the diodes in the op amp feedback. I   
   >> didn't try to get the whole research paper so I'm not sure how one would   
   >> work out the correct resistor values.   
   >>   
   >> I downloaded the information on the MC1496-D and LM13700. I see the   
   >> basic idea is that these chips also produce products of signals, but   
   >> beyond that I can't make any intelligent comments at present. My analog   
   >> computer is usually dealing with signals in the range of a few hundred   
   >> Hz up to a few kHz. I am curious if maybe something could be done with   
   >> the gain control pin on the LM386 chip (I have quite a few of those). It   
   >> looks like, using a series RC network, that the gain can be set anywhere   
   >>from 20 to 200.   
   >   
   > A simple opamp circuit with some resistors and diodes can have a   
   > several-segment approximation to a square root. I did that in a   
   > steamship throttle control once and it worked well enough.   
      
   Didn't you have a DEC digital processor to play with for that gig?   
      
   The DEC PDP-8 I got to play with for my Ph.D. project could be persuaded   
   to do multiplication and division - I used it for signal averaging - and   
   square root extraction shouldn't have been difficult.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)