a variable reactance circuit is composed of a differential amplifier having first and second emitter - coupled transistors , a reactance element , such as a capacitor , connected between the collector and base electrodes of the first transistor , and a control signal source connected to the emitter electrodes of the transistors by which the input impedance of the first transistor is effective as a variable reactance which is changed in accordance with the level of the control signal . such variable reactance circuit may be used with an oscillator or resonance circuit to constitute a frequency modulator controlled by the control signal source .