Patent ID: 8085071

Claim:
A phase-locked loop circuit comprising: a phase comparator for comparing phases between a reference signal and a feedback signal and for outputting a phase difference signal indicating a phase difference therebetween; a charge pump for outputting a charge pump current according to the phase difference signal; a low-pass filter that includes a resistor and a capacitor, the low-pass filter smoothing the charge pump current and converting the smoothed current into a control voltage; a voltage-controlled oscillator for generating an oscillation signal with a frequency according to the control voltage; a frequency divider for generating a frequency-divided signal by frequency-dividing the oscillation signal and for outputting the frequency-divided signal to the phase comparator as the feedback signal; and a power supply voltage node, wherein the resistor in the low-pass filter is a variable resistor that is changed according to the control voltage, wherein the variable resistor has a first field-effect transistor having a drain to which the control voltage is applied, wherein the low-pass filter has a first capacitor connected between a source of the first field-effect transistor and the power supply voltage node; and wherein the low-pass filter has a fixed resistor connected between the drain of the first field-effect transistor and the first capacitor.