Circuit arrangement for capacitance or admittance measurement

Abstract

The circuit arrangement is used for capacitance or admittance measurement by the principle of apparent current measurement in that to the capacitance or admittance to be measured an AC voltage of fixed magnitude and fixed frequency is applied and the AC current flowing through the capacitance or admittance is used as a measure of the measured value. The test object whose capacitance or admittance is to be measured lies in the emitter circuit of a transistor connected as voltage follower, to the base electrode of which the AC voltage is applied. The emitter terminal of the transistor connected to the test object is connected to the corresponding pole of the operating voltage source via an adjustable current source. The collector connection of the transistor is connected via an impedance of high resistance for alternating current to the other pole of the operating voltage source. Connected to the collector is a rectifier circuit which converts the alternating current applied thereto to a measuring DC voltage proportional thereto. The direct current flowing via the adjustable current source is set in dependence upon the measuring DC voltage such that it is always slightly greater than the peak value of the alternating current flowing through the test object. Instead of a transistor, another amplifier element with controllable current path may also be used, for example a field-effect transistor. The impedance of high resistance to alternating current is preferably a second adjustable current source which is so constructed that it is controlled automatically to synchronize with the first adjustable current source.