Amplifier White Noise

[Ben Nise]

The 1 MEG resistor will generate 18 uVrms of noise in a 20KHz bandwidth
according to:

mean squared voltage noise = 4kTR*BW

where:

R is the resistance
k is boltzman's constant
T is temp in Kelvin 
BW is bandwidth in Hz

You will hear this noise as long as the input is open circuited.  As soon as
you
connect a signal to the input, the lower input impedance will "short" out
the noise
source and the overall noise level will appear to go down.

Ben Nise
bnise at sonici.com

-----Original Message-----
From: Scott Gravenhorst [mailto:chordman at flash.net]
Sent: Friday, August 20, 1999 3:35 AM
To: synth-diy at mailhost.bpa.nl
Subject: Amplifier White Noise


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.

The circuit is at http://www.flash.net/~chordman/ContactMicPreamp.html

-- Scott Gravenhorst
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