Matched transistors question

[Don Tillman]

   From: johns at oei.com
   Date: Fri, 19 Apr 96 16:04:26 

   I have been following all the VCF/VCO control talk lately and once in 
   a while somebody mentions using matched transistors.  I always thought 
   that transistors found in transistor array ICs were closely matched by 
   default (as well as thermally matched).  Is this really true?   

Yes.
								   If it 
   is true, why would anybody go to the trouble of trying to match 
   transistors then?

Either the transistor array wasn't available, or the design required a
specific configuration of transistors that wasn't avaiable in an array.

   Better yet, when I review the popular ladder filter scematics, I 
   always see that the last rung of the transistor "ladder" always uses a 
   transistor IC pair whereas the other transistors are always discrete.  
   Wouldn't it be easier just to use a single transistor array IC 
   instead?

Except for the top pair and the diff amp pair, the ladder transistors
don't need to be matched.

  -- Don