Variable reactance circuit including differentially-connected transistor device providing a variable reactance input impedance

DETAILED DESCRIPTION Accordingly, an object of this invention is to provide a variable reactance circuit which is free of the above disadvantages of the prior art.
Another object of the invention is to provide a variable reactance circuit in which its capacitance may be small so as to make the circuit suitable for formation as an integrated circuit.
Still another object is to provide a variable reactance circuit, as aforesaid, which may be conveniently provided with a wide range of variation of its reactance.
A further object is to provide a variable reactance circuit, as aforesaid, in which the reactance can be varied without any variation in DC level.
According to an aspect of the invention, a variable reactance circuit comprises a differential amplifier formed of first and second transistors; a reactance element, such as, a capacitor, connected between the collector and base electrodes of the first transistor; a source of DC potential supplied to the base electrode of the second transistor; and a control signal source connected to the connection point between the emitter electrodes of both the transistors so that a reactance component corresponding to the level of the control signal source is obtained as the input impedance of the first transistor.
With the foregoing circuit according to the invention, it is not always necessary to provide the same with a very large output impedance, so that a resistor can be used as a load and accordingly the circuit can be easily formed as an integrated circuit.
Further, with such variable reactance circuit no DC level variation accompanies changes in the reactance, so that the circuit can be connected directly to a following stage, for example, an oscillator.
The above, and other objects, features and advantages of this invention, will be apparent in the following description when read in conjunction with the accompanying drawings.