[sdiy] Quad transistor sources

[WeAreAs1 at aol.com]

In a message dated 6/27/04 5:42:43 PM, cfmd at bredband.net writes:

<< The MAT-04 gives you four transistors. The MAT-02 is basically the same
silicon but with a different metal layer hooking them up in two-and-two
(in parallel) such that temperature gradients is cancelled (to some
degree). >>

I've often wondered if there might be some practical advantage to doing this 
with discrete transistors.  That is, connecting several of the same type 
together in parallel so that their individual differences are kind of cancelled out 
(or rather, averaged out).  I imagine such a thing could work, but who knows 
how many you'd need to parallel in order to get the desired result.  I once 
read that the National "super-matched" precision transistor pair (LM394) is made 
this way, and that it has 100 paralleled transistors on each side of the 
"pair", for exactly this purpose.  Is this true?  When you parallel a bunch of 
trannies, does the internally paralleled Base input require more overall current 
in order to properly drive the transistor?  (that is, does the effective 
resistance of the Base junction change when you link all the internal transistors 
together?)

I saw a schematic once for a mic preamp in an Amek Angela mixing console, and 
they had several (maybe eight or ten) discrete transistors paralleled 
together in ther preamp's differential input "pair" (but no explanation why in the 
service manual)  One can only guess that if the transistor pair is better 
matched, then its common mode rejection and noise cancelling function might be 
better (especially if you also use matched resistors in the circuit).  I wasn't 
aware that this was a problem in typical two-transistor/one-opamp mic preamps -- 
most of them just use an unmatched pair of fairly cheap devices.  Any thoughts?

Mike B.