Patent ID: 7301409

Claim:
An oscillator comprising: a phase-locked loop circuit that generates an oscillation frequency control voltage based on a phase difference between an output oscillation signal and a reference signal; and a differential amplifier circuit that is applied the oscillation frequency control voltage and the reference voltage to a pair of input terminals provided corresponding to two current paths connected to a common current source and takes out a pair of currents flowing through each of the current paths as a differential output current, the oscillator controlling an oscillation frequency of the output oscillation signal based on the differential output current, wherein the phase-locked loop circuit comprises a phase comparator that detects the phase difference; a first switch element that is connected between a output terminal of the oscillation frequency control voltage and a first common power source; a second switch element that is connected between the output terminal and a second common power source; a regulator that is provided between the first common power source and the first switch element, and thereby generates a predetermined limit voltage based on the first common power source; and a switch control circuit that allows the first switch element and the second switch element to selectively become ON state in response to the phase difference, and wherein the limit voltage is set in response to a voltage near the first common power source out of upper limit voltage and lower limit voltage of a linear region of the differential output current with respect to the variation of the oscillation frequency control voltage.