A power amplifier, which includes n+1 amplifying units coupled in parallel, where n is an integer greater than or equal to 0, third input ends VR(i) of the n+1 amplifying units are coupled to a power input VL, output ends D(i) of the n+1 amplifying units are coupled to a power input VH, and an output power of the power amplifier is in an increasing function relationship with a capacitance value obtained through accumulation of energy returning capacitors xC of the n+1 amplifying units. The power amplifier changes circuit impedance in a manner of controlling the parallel capacitance value by a switching digital signal, thereby controlling a magnitude of a returned power value and forming different output powers.