Amplifier White Noise

[Don Tillman]

   Date: Fri, 20 Aug 99 09:35:27
   From: Scott Gravenhorst <chordman at flash.net>

   I just put together a 2 transistor preamp and I noticed
   that there is some white noise, even with a shorted input.  The input
   transistor is an MPF-102 biased with a 1 meg resistor to ground.  Is
   the noise a result of the FET's reverse biased gate?  When I put a signal
   thru it, there is enough gain that the noise becomes unnoticable, I'm
   just curious as to where it comes from and what I might do to reduce
   it.

I think the basic design is causing the problem.  The FET source
follower looks fine, but then you attenuate the signal by a factor of
five or so and then make up that gain and then some with a transistor
stage.  Also, the transistor stage is very sensitive to temperature
and device variations and it runs with a very high output impedance.

Generally you want your voltage gain first so subsequent stages
won't add much noise.

A single common source FET stage would do you much better.  Perhaps a
second source follower or emitter follower after that if you need
lower output impedance.

Also, it's a good idea to use an FET spec'd for low noise, say a J201
or similiar.  The MPF102 has no noise spec.

  -- Don