[sdiy] Quad transistor sources

[ASSI]

On Monday 28 June 2004 04:22, WeAreAs1 at aol.com wrote:
> I've often wondered if there might be some practical advantage to
> doing this with discrete transistors.

I can't see an advantage, but it certainly is doable and might even 
make sense when using SMT. The thermal coupling would still be bad, but 
you could either save the hand matching of individual transistors into 
pairs or achieve even better matching (relative to a pair made of 
individual transistors) when using pre-matched components. I can't see 
how this could be better than a competently designed monolithic 
integrated pair, however.

> 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?)

As long as you remember to scale up all currents so that you use each 
individual transistor at the original current density there should be 
no first order differences. Some parasistics won't scale, but that is 
likely not a problem for audio. With the better matching circuit 
performance at lower current density might actually improve.

> 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).

This reduces noise and was a fairly common technique for phono preamps. 
Matching is not required for this application.


Achim.
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