Nanoamp current detection circuit based on 100M

This circuit can be used as a sensitive current detector or a high input impedance voltage detector. Adjust R1 so that the voltage at point A is 1/2-3.1V lower than the ignition voltage of 2N494C. Only 40nA input current is needed to charge C2 and bring the gate level high to the ignition level. Once the 2N494C is turned on, capacitors C1 and C2 discharge through the 27Ω resistor, thus generating a positive pulse with sufficient amplitude to Trigger a semiconductor controlled rectifier or other pulse sensitive circuit.