Mitigating Phase Variation of Peaking Amplifier Using Offset Line

The effect of the peaking offset line on the Doherty amplifier operation is investigated. The peaking amplifier of the Doherty structure operates at a C-bias condition for the load modulation. With the C-bias, the amplifier has a poor AM-PM characteristic due to the nonlinear input/output capacitances. The nonlinear phase variation of the peaking amplifier influences the load modulation behavior and reduces efficiency of the Doherty amplifier at a high output power region. To mitigate the nonlinear PM characteristic of the peaking amplifier, the length of the peaking offset line should be reduced from the conventional offset line. To validate the offset line design, a 2-stage Doherty amplifier is implemented at 2.655 GHz using GaN pHEMT. Due to the proper load modulation, the Doherty PA delivers a very good performance. For the LTE signal with 20 MHz bandwidth and 7.2 dB peak-to-average power ratio (PAPR), the amplifier delivers a power-added efficiency (PAE) of 49.3% and gain of 25 dB at an average output power of 49 dBm.