Re: low noise amplifier for high impedance source

archiees wrote

I am a student who has been working on a low noise preamplifier
for a high impedance current source. I have put the model of the
detection circuit here:
http://img146.imageshack.us/img146/9748/detectmodelid8.jpg
Its a differential ac current source with an instrinsic capacitance ~
20pF on each side. I use 1M ohm resistors to bias my input JFETs.
The bandwidth I need is only from 10 Khz to couple of MHz.

archiees wrote...

Thanks for your reply Phil.
** Do you really have a current source ???
Yes, to my understanding. The source consists of 2 electrodes
(Penning trap). Charged particles rotate between these
electrodes and induce an image charge. This image charge is
allowed to flow in an external circuit (like resistors) to
constituting differential image currrent out of the electrodes.

The schematic you give in your .jpg drawing is

.. |--
.. ,---------+-------------->|-- JFET
.. | +--/\/\-- gnd
.. | ICT +---||--- gnd
.. | cs |--
.. '-(->)----+-------------->|--
.. +--/\/\-- gnd
.. +---||--- gnd

where the resistors are 1M, and the capacitors 20pF. The
signal voltage developed across a low-value resistor like
1M will likely be much less than you like, as Phil implied.
Resistor noise density is i_n = (4kT/R)^1/2, so you'll want
a high R, like 100M or higher. The load capacitance will
start reducing the signal (including the resistor noise)
voltage above a frequency fc = 1 / 2pi R C, which is only
8kHz for 1M, and 80Hz for 100M. So clearly you want to
lower value of the load C if you can.

One point, a portion of the JFET's capacitance, Crss, will
be added to C, unless you employ tricks to prevent this.

Once you know the signal voltage vs. current, as a function
of frequency, you can compare it to e_n, the JFET's voltage
noise at that frequency. You will find low e_n JFETs have
high capacitance. Just a little gotcha from Mother Nature.

For example, a classic 2n4392 has en = 3nV and Crss = 3pF.
Compare this to a bf862 with en = 0.9nV and Crss = 1.9pF.
Ooops!! Actually, that's a bad example, contrary to Mother
Nature, as well as Murphy's Law, because the Philips bf862
JFET is far, far better than it should be! :-)


--
Thanks,
- Win
.

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