Patent ID: 7327195

Claim:
A PLL frequency synthesizer comprising: a voltage control oscillator for changing an oscillation frequency, depending on a potential of an oscillation frequency control signal; a frequency divider for dividing an output signal from the voltage control oscillator with a predetermined frequency division ratio; a phase comparator for receiving an output signal from the frequency divider and an external reference signal, detecting a difference in phase between the output signal and the reference signal, and outputting a phase difference signal; a charge pump circuit for causing a constant current to flow in or out, depending on the phase difference signal from the phase comparator; a loop filter for filtering out a high frequency component of an output of the charge pump circuit, converting the current flowing into or out of the charge pump circuit into a voltage, and outputting the voltage as the oscillation frequency control signal; and a linearization circuit for controlling a gain of the charge pump circuit so as to compensate for nonlinearity of a loop gain of the PLL frequency synthesizer with respect to the oscillation frequency control signal, wherein the linearization circuit has a plurality of transistors for receiving the oscillation frequency control signal from the loop filter and changing flowing currents, depending on a potential of the oscillation frequency control signal, and the gain of the charge pump circuit is continuously controlled, depending on a sum of the currents flowing through the plurality of transistors.