Variable impedance circuits

摘要

A variable impedance circuit comprises a differential current circuit having a diode and a first transistor connected so as to operate differentially with respect to each other, and a current control circuit for controlling current flowing through the differential current circuit, whereby variation of the operative resistance of the differential current circuit causes variation of impedance thereof. The current control circuit comprises a second transistor having a collector connected with the differential connection of the differential current circuit, and an emitter grounded via a resistor, and a third transistor having a base applied with a control voltage and an emitter connected with the emitter of the second transistor. A fourth transistor is also provided having a collector connected to the base of the second transistor, a base connected to the collector of the third transistor and an emitter applied with source voltage independently from the current flowing through the differential current circuit. The current control circuit is connected with the differential current circuit through only the collector of the second transistor to establish a high impedance of the current control circuit as seen by the differential current circuit.