The invention relates to a power amplifier and a modulator thereof. The power amplifier comprises a circuit filtering unit, a frequency generator, a quantized unit and an output stage module. The frequency generator provides a signal with reference frequency for the circuit filtering unit and comprises a logic circuit, a current array and a virtual load. The virtual load is coupled with the current array and stands for a load circuit in the circuit filtering unit, wherein the equivalent capacitances of the virtual load and the load circuit are proportional. The current array generates the signal and the virtual signal according to a logic signal and outputs the signal and the virtual signal to the circuit filtering unit and the virtual load respectively. By adopting the frequency generator which can adjust the frequency automatically, the influence of a passive device in the power amplifier on the characteristics of the power amplifier owning to process variation is reduced.