A calibration circuit for body biasing includes a phase detector, first and second voltage generators, and first and second voltage regulators. The phase detector has an input terminal configured to receive an oscillation signal from a ring oscillator. The phase detector provides output signals indicative of phase differences between the oscillation signal and a reference signal. The first voltage generator provides a first reference voltage using the output signals from the phase detector, and the first voltage regulator provides a first biasing voltage using the first reference voltage. The second voltage generator provides a second reference voltage using the first reference voltage, and the second voltage regulator provides a second biasing voltage using the second reference voltage. The first biasing voltage is used to bias P-wells of transistors in the ring oscillator, and the second biasing voltage is used to bias N-wells of transistors in the ring oscillator.