Re: gEDA-user: ngspice shot noise

[Al Davis <ad82@xxxxxxxxxxxxxxxx>]

On Thursday 29 September 2005 08:45 pm, Greg Cunningham wrote:
> I'm not an engineer, but I recollect years ago Elekto mag
> published a very lo-noise mic amp by parallelling a
> dozen-or-so gp bipolars.  Their argument was that the
> randomness of the noise sources produced cancellation, while
> summing the in-phase signals added.

The correct argument is that by paralleling devices, the 
effective noise source resistance is lowered, which lowers the 
noise voltage.  Making the transistors bigger has the same 
effect.   This will only work if you have a low source 
impedance.  It will make the noise worse if you have a high 
source impedance.

An almost correct argument, that is closer to what you said is 
that when you sum coherent (in-phase) signals, you get a 6 dB 
increase.  When you sum incoherent signals, you get a 3 dB 
increase, hence a 3 dB improvement in signal to noise ratio.

But actually, it can only go so low.  If the microphone itself 
has an impedance of 150 ohms (which is typical) that determines 
the theoretical best you can do.  The practical best is 1 or 2 
dB worse.  How much worse than theoretical is called the "noise 
figure".