[sdiy] Quad transistor sources

[James Patchell]

The reason to parrallel the transistors in the mic preamp was to reduce the 
noise.  The voltage noise goes down by the square root of the number of 
transistors you connect in parallel.  So, you hook up four transistors, you 
get 1/2 the voltage noise.  16 transistors gives you 1/4 the noise.  One 
problem doing this is the current noise goes up by the same amount, so you 
need to find the optimum number of transistors to parrallel, and a given 
bias current, for the source impedance you are trying to amplify.

Connecting 100 transistors up in parallel will cause not cause base current 
problems, as the composite transistor will act as a single transistor.  So, 
from the outside, you will never know.  You apply base current, it will be 
multiplied by the Beta of the transistor and reflected in the collector....

At 10:22 PM 6/27/2004 -0400, WeAreAs1 at aol.com wrote:

>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.

         -Jim
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