Hello. I need some help. I've realized a circuit in which I sample extremely low currents (<100 pA) using charge transfer to a 3.3pF cap, through a low leak FET (2N4117 reverse gate current about 1 pA). The problem : Instead of progressively charging the cap, this one seems to be charged and discharged during the same pulse sent to the gate. Technical data : gate pulse from -4 V (rest level) to 0 V during 400 ns (Fet switched On) , each 50 µs. Where is the error ? You will find here enclosed a very simplified schema.