[SI-LIST] Re: Active capacitance canceling circuitry

Gary,
    Many years ago I saw a circuit by perhaps Robert Pease that has low 
input C.
It used a 2N4416. This device has roughly equal Cgd=Cgs.

J1      drn  gte  src  2N4416
Rdrn  drn  Vcc  1k
Rsrc   src  Vee  10k
N1     col  bse  emt  2N3904
Remt  emt Vee   1K
Cbs    emt  drn    0.1u

    Cbs bootstraps the Cgd to follow Cgs. With Cin ~ (Cgs - Cgd * Av) where 
Av ~0.9
With Cgs = 2.2pF then Cin (gte) ~0.25pF < 100MHz.

Alan


----- Original Message ----- 
From: "Gary Morrell" <gmorrell@xxxxxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, August 29, 2005 4:00 PM
Subject: [SI-LIST] Active capacitance canceling circuitry


This is more of an analog question but I'd thought I'd pick the collective 
wisdom here.

I'm working on recovering AC signals from some very small capacitors, 
typically 100 to 300fF (Yep, femtoFarads, 1 X 10^-15F). The capacitor is 
driven by a small amplitude RF sine source, the other terminal is loaded 
with a high value resistor, typically selected to be about equal to the 
reactance of the capacitor under test at the RF excitation frequency. Simple 
circuit, right? Problem is, anything I connect to the junction between the 
capacitor-under-test and the resistor load has waaaay more stray capacitance 
then the capacitor of interest, the result being that the stray C in 
parallel with the load resistor load drops the effective load impedance, and 
the SNR goes kaput in a hurry. Even op-amps with very high bandwidth can 
have differential and/or common mode input capacitance of 2 to 4 pF.

I've used active parasitic capacitance canceling circuits on MOS microprobes 
and some active O'scope probes, but has anyone here actually seen one of 
these active capacitance cancellation circuits? MOS microprobes typically 
have <40fF of input capacitance, 10^-14A of input leakage and bandwidth to 
300MHz; I'd love to get my mitts on a circuit that would perform even close 
to these specs, build it and play with it some.

Regards,

Gary M.

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