Re: gEDA-user: LED in reverse

[John Doty <jpd@xxxxxxxxxxxxx>]

On Nov 23, 2006, at 2:17 AM, Karel Kulhavy wrote:

> > > My application is noise-critical between 1MHz and 10MHz. Would  
> > > BC547C be
> > > better than 2N3904?  I assume it's not a switching transistor.
> > Rather use BC549 or BC550, they have tighter spec on noise.
> > Or use BF240 if gain has to be higher.
> > PNP transistors can have lower noise due to lower base resistance.
>
> Does the gold increase only 1/f noise or also the broadband (white)  
> noise?

Broadband. The main effect is to reduce beta by promoting  
recombination. That increases base current shot noise: every  
additional recombination event is one electron's worth of charge  
noise variance.

This isn't a huge effect: an expert can usually get better results  
from things like 2N3904 than a non-expert can get from anything.  
Noise performance depends less on the parts than on how you use them.  
For a good start here, study the part of Horowitz and Hill's "The Art  
of Electronics" that describes low noise design.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxx




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