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 .