Patent ID: 7439819

Claim:
A piezoelectric oscillator having a structure obtained by connecting an amplifier, an external frequency adjustment circuit, and a piezoelectric element in series, wherein the external frequency adjustment circuit is a variable capacitance circuit using a MOS capacitance element and voltage, and has a configuration for supplying a reference signal having a constant voltage value to a back gate electrode of the MOS capacitance element and supplying a control signal around the reference signal to a gate electrode of the MOS capacitance element, the MOS capacitance element is a channel transistor of a second conductivity type formed in a well region of a first conductivity type, the second conductivity type being opposite to the first conductivity type, and a bias voltage is supplied between an extraction electrode of the second conductivity type formed in source and drain regions of the channel transistor of the second conductivity type and an extraction electrode of the first conductivity type formed in the well region of the first conductivity type.