A control unit includes an input terminal and an output terminal for a signal to be controlled, a control input terminal and a control output terminal for a control signal, variable capacitors connected in a bridge configuration between the input terminal and control input terminal, between the input terminal and control output terminal, between the control input terminal and output terminal, and between the control output terminal and output terminal, capacitances thereof being changed by the control signal, and a differential signal-controlled power source in which the control signal is applied across the control input terminal and control output terminal in a differential mode with a pair of signals having the same absolute value and mutually opposing polarities. Voltage or current of the signal to be controlled is controlled by the control signal changing the capacitances of the variable capacitors in the bridge configuration.