[sdiy] What makes a transistor "low noise"?

[James Patchell]

Noise in transistors is a complicated subject.  I took a quarter long 
course in this when I was in college...and even then I would say we only 
scratched the surface...

Some things that affect noise in transistors:

Processing
Geometry
Bias Current...there is an optimal bias current for any given source impedance.

Processing is probably one of the most important...type numbers mean very 
little.  I once got a batch of 2n4403's (Motorolla) in plastic (TO92) cases 
that were noisier than all get out.  I got the same part in a TO5 package 
(again from motorolla), but they were also mil spec parts, and they were 
extremely quiet.  Things I don't know is if the same die was using in both 
parts.  I suspect, however, that back then, the mil spec parts were not 
processed on the same line, at the very least.

At 08:31 AM 11/5/2004 -0800, Metrophage wrote:
>As I BEGIN to understand how transistors work, I wonder about this. I
>often see op-amps labelled as being less noisy than general-purpose
>types, but typically not for transistors. Still, I have heard that some
>are noisier than others.
>
>What is it about transistors which would indicate that they would or
>would not distort signals somehow? How are such factors reflected in
>the specs which are usually offered?
>
>Through my time of synth reading, I feel I have a rough idea of what
>makes some op-amps more or less suitable for a given application. Not
>yet so with transistors though! I'd be psyched to hear some info and
>opinions on this.
>CJ
>
>
>
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         -Jim
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