Patent ID: 8106723

Claim:
A resonant circuit, comprising: a substrate; a MEMS resonator including a fixed electrode and a movable electrode formed above the substrate and having a first terminal and a second terminal, the movable electrode having a movable portion opposing at least a part of the fixed electrode; a first input-output terminal connected to the first terminal connected to one of the fixed electrode and the movable electrode of the MEMS resonator; a second input-output terminal connected to the second terminal connected to an other one of the fixed electrode and the movable electrode of the MEMS resonator; a voltage applying unit supplying a potential to at least the first terminal to apply a bias voltage between the first and the second terminals; and a variable capacitance connected between the first terminal and the first input-output terminal to allow a capacitance value to be changed by a change in a potential difference between opposite ends of the variable capacitance; wherein the variable capacitance is characterized by at least partially having a voltage-dependent region where the change in the potential difference between the opposite ends of the variable capacitance causes the capacitance value to change monotonously and an increase in the bias voltage causes the capacitance value to be reduced monotonously.