Abstract: An automatic gain control (AGC) circuit is applied to control a margin voltage of an oscillator. The margin voltage is the voltage difference between a high-level output and a low-level output of the oscillator. The AGC circuit of the present invention includes a comparator and a processing unit. Wherein, the comparator compares the margin voltage of the oscillator and a reference voltage. Based on the output of the comparator, the processing unit outputs a ripple code to determine the value of a driving current output from a current generator. The oscillator generates an oscillation output to the comparator based on the driving current.