The disclosure relates to technology for power supply for a voltage controller oscillator (VCO), where the power supply has a closed loop mode and an open loop mode. In closed loop mode, a peak detector circuit determines the amplitude of the output for the VCO, which is compared to a reference value in an automatic gain control loop. An input voltage for the VCO is determined based on a difference between the reference value and the output of the peak detector circuit. The peak detector circuit can be implemented using parasitic bipolar devices in an integrated circuit formed in a CMOS process. While operating in the closed loop mode, a controller monitors the input voltage and, when the input voltage is stabilized, the controller uses this input voltage value determined in open loop mode.