Re: gEDA-user: LED in reverse

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Sun, Nov 26, 2006 at 10:19:35AM -0500, John Doty wrote:
> 
> 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.

Is this base current shot noise amplified by the transistor like any
other current that is applied to the base?

CL<
> 
> 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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