Re: low noise amplifier for high impedance source

If I understood correctly, you apply potential across plates and measure
current through your experiment? If that's the case, what is potential
value? It will be very hard to measure so low signal and have wide
bandwidth. I would suggest having a tuned circuit for frequencies of
interest and in addition, lockin amp after fet preamp. Reduced bandwidth
would help with noise problem.

Hi Sinisa,
thanks for the suggestions. The induced current is in higher pA range.
I agree that using tuned circuit will help if i am trying to detect
signal due to few ions (infact Gerald Gabrielse does use tuned circuits
to pick up signals induced by single protons over at Harvard). However
for my application which is a mass specectrometer you only know a range
of frequencies where your unknown sample ions will have - in essence we
want to do a broadband (10KHz to 1MHz) detection.

There are amps from Burr Brown (TI) that work
in sub pA range, although questionable if it would work on 4K. Could keep
them warm by heating PCB.
Yes, i was suggested using JFETs and warm these up to 70 kelvin (just
to turn them ON) inside the cryogenic vacuum system. I am going to try
it as i realize, getting bigger GaAs MESFETs (with low 1/f noise) which
work fine at 4 kelvin is not working out.

You could use differrent preamp boards for
different experiments or make it adjustable so you could tune it for
frequency of interest. In addition, you could match impedance of your amp to
experiment.
We did thought about it, if i can find a way to tune the preamplifier
impedance which sits in a vacuum system, externally. Still, it will be
something i want to try in future, for now i want to see what is the
best i can do in broadband detection mode using low noise JFETs.
thanks again.
Regards,
Arch

.

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