Abstract: Based on the HBT process of SANAN, a mobile phone power amplifier chip suitable for 4.5-5.0 GHz(N79) in the 5G communication frequency band is designed. In order to obtain higher gain and output power, the amplifier circuit of the power amplifier adopts a three-stage amplifying structure. In order to optimize the return loss and increase the gain and output power, a transformer matching is used between the third stage and the second stage. Convert one output signal of the first stage into two differential signals, and the transformer on the substrate synthesizes the two differential signals of the third stage into one signal. The power amplifier adopts adaptive active bias civcuit technology and double bias circuit technology, which can effectively improve the linearity of the output power and the stability of the static working point. The simulation results show that the return loss of the power amplifier is effectively optimized. In the entire N79 operating frequency band, the return loss of the power amplifier is less than -10 dB, the gain is 33-34 dBm, the output power 1 dB compression point is 37 dBm, and the output power is the highest power added efficiency at the 1 dB compression point is 45%.The power amplifier obtains higher gain, higher output power, and higher power-added efficiency.