A ring oscillator includes a first delay stage generating a first phase signal and a second delay stage generating a second phase signal. Each of the first and second delay stages includes variable resistance circuit. A phase comparator circuit performs a phase comparison between the first and second phase signals to generate a phase error signal. An amplifier circuit generates a control signal from the phase error signal. A feedback loop applies the control signal to control the resistance of the variable resistance circuits in the first and second delay stages.