A power amplifier and a bridge circuit in a power amplifier, thereinto, the power amplifier includes a comparator, a bridge circuit and a low-pass filter. The comparator is adapted to receive a first analog signal, compare the first analog signal with a reference signal and output a square wave signal. The bridge circuit is adapted to amplify the square wave signal and output the amplified square wave signal. The low-pass filter is adapted to convert the amplified square wave signal into a second analog signal. The bridge circuit includes a first MEMS switch and a second MEMS switch. The first MEMS switch and the second MEMS switch turn on alternately when the polarity of the square wave changes, and output a first voltage signal or a second voltage signal respectively. The amplified square wave signal includes the first voltage signal and the second voltage signal output alternately. The present disclosure substitutes the MOS transistors in prior art with surface MEMS switches, so the power consumption, the size of devices and the manufacture costs all can be reduced.