A voltage controlled oscillator (VCO) is provided which can minimize the frequency shift and phase disturbance caused by noise in the input voltage to the VCO. The VCO includes a voltage-to-current converting unit, receiving the input voltage, for generating a bias voltage. A ring-oscillating unit including an odd number of cascaded ring oscillator stages (ROSC) is coupled to the voltage-to-current converting unit for generating an oscillating signal. A bias unit is coupled to the ring-oscillating unit for generating a stable DC bias voltage for the ring-oscillating unit. A current adjustment unit is coupled to the voltage-to-current converting unit and the ring-oscillating unit, which can generate a working current in response to the bias voltage for the ring-oscillating unit. By feeding the bias voltage and the DC bias voltage to the current adjustment unit, a current feedback loop is formed which serves as a current-sampling shunt-mixing circuit producing a stable current output that allows the VCO to produce an oscillating signal with a stable frequency.