[time-nuts] OP-Amps for 10MHz distribution...?
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Wed Feb 29 15:54:48 UTC 2012
Bill wrote:
>Low noise voltage at the cost of noise current which is around 1000 times that
>of low noise JFETs.
Sure, but the noise resistance (input termination at which voltage
noise and current noise contribute equally to the output noise) is
still around 400 ohms -- anything lower and the input current noise
is not much of a factor. For a 10 MHz distribution amp, this
condition is very likely to be met.
>The discussion suggest that opamps contribute to the sideband phase noise
>of the signal. I am interested in the mechanism that adds this phase noise.
>It would have to be a small shift in either gain (changing Miller
>capacitance) or
>an internal capacitance in the opamp.
As you suspected, the primary mechanism is usually modulation of
transistor capacitances by noise. Another is general component
noise. Three good references are:
Walls, et al., Origin of 1/f PM and AM Noise in Bipolar Junction
Transistor Amplifiers;
Ferre-Pikal, et al., Guidelines for Designing BJT Amplifiers with Low
1/f AM and PM Noise; and
Ascarrunz, et al., PM Noise Generated by Noisy Components.
I think Bruce has these linked on his pages at Didier's (KO4BB) site
(http://www.ko4bb.com/~bruce/).
Best regards,
Charles
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