tempco resistors

[gstopp at fibermux.com]

     Yes you're right, I was over-simplifying a little... I was trying to 
     figure out how not to open the matched-transistor can of worms. The 
     tempco resistor is there to cancel out a smaller secondary effect left 
     over after the primary temperature dependency is compensated for, like 
     you say.
     
     Ya know while we're on the subject, I'd be interested if somebody 
     *would* open the matched-transistor can of worms! I don't think I'm up 
     to it.
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: Re: tempco resistors
Author:  don at till.com at ccrelayout
Date:    5/1/96 11:00 AM


   Date: Wed, 01 May 96 09:39:02 PDT
   From: gstopp at fibermux.com
     
   It's not a stupid question - it took me years to understand this and 
   I'm still not sure I got it right. But I'll try...
     
That explanation seems a little confusing, Gene.  I'd say that the 
crux of the biscuit is that the transistor junctions responsible for 
exponential control are completely temperature sensitive, and, while a 
second matched transistor junction at the same temperature can be 
employed to compensate for the temperature sensitivity, there's a more 
subtle effect (it shows up in scaling accuracy) that the tempco 
resistor takes care of.
     
Thus, exponential converters that employ thermostatically-controlled 
heaters don't need tempco resistors.
     
  -- Don