Input stage mess

Well, I've finally got doing that low noise preamp: the target is
200pV/rtHz, bandwidth from 0.1Hz (with provision for 1/f noise) to 1MHz.

Lots of interesting pbs to solve. One remaining thing is some stability
issue wrt to cable/generator impedance.
The input stage will be 4 to 6 paralleled Interfet's IF3602. They'll work at
a low servoed 1V VDS to minimze thermal noise problems.
Under those bias condition the JFet will show about 300pF Cgs and 180pF Cgd.
The Jfet is servo-cascoded so that amounts to and equivalent 480pF Cgs.
The closed loop preamplifier show an input impedance that has a painfull
negative real part input admittance.
Admitting a first order response, the input admittance is:

Cgs w^2
Yin = -------------- with WT = 2 pi GBW and beta= feedback network
attenuation
WT beta + j w

This translate to an equivalent parallel network:

2 2 WT beta
Rin(w) = - ------------ - ------------
Cgs WT beta Cgs w^2

1 Cgs w^2
Cin(w) = --- . -------------------
2 w^2 + WT^2 beta^2

Now the figures:
designing for a 10 loop gain (WT beta) at 1MHz this give a low -12R for the
real part at high frequencies and will give a nice oscillation with the
input cable impedance (estimated between 300nH & 500nH).
As the generator impedance is low, the easy way to deal with this could be a
serie RC in parallel with the input (sort of 2-5 nF and under 12R resistor).
Unfortunatly a 10R resistor is 410pV/rtHz and 41pA/rtHz which translates to
about 130pV/rtHz across the 0.5uH cable inductance at 1MHz. A bit more than
I would like.
More, this 10R is dangerously close to the -12R value, which didn't
accounted for additionnal parasitic poles and will probably be lower.
Plus cable resonance with the input capacitance will rise the noise level.

I can't insert a damping resistor in series with the gate connexion for
noise reasons (200pV/rtHz is a super low 2.5R noise resistance).

I've thought of a lot of schemes for neutralizing Cgs, but found nothing
practical.

I also can't run the input stage open loop, which would solve this issue but
will raise some others.

Any idea?


--
Thanks,
Fred.


.

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