Capacitive measuring device with mosfet

Abstract From the Canadian patent

A capacitance measuring device comprises a MOS transistor having a source drain, and gate; a first capacitor C1 connected between the gate and the drain so that charge is coupled from said drain onto said gate; and a second capacitor C2 connected to a source of gate voltage VG and to the gate. One of the first and second capacitors has a known capacitance and the other has an unknown capacitance. A DC voltage is supplied between the source and drain to cause a saturation current to flow therebetween. The ratio .delta.VG/.delta.Vd for the saturation current, where VG is the applied gate voltage and Vd is , the drain voltage, is measured and the unknown capacitance derived therefrom.