Load and Frequency Independent Doherty Power Amplifier Back-Off Extension

This article presents the principle of extending the back-off efficiency of a three-stage Doherty power amplifier (DPA) by substituting the second load modulation stage using a load transformation stage. Thereby, the DPA output combiner is split into two independent circuits so that the second peaking PA, peaking II, can act as a reflector for the DPA instead of additionally loading the combiner. The derived design equations introduce the flexibility to maintain the load of a stand-alone DPA and, therefore, the frequency response. The independence between the DPA and peaking II offers extremely large output power back-off (OBO) implementations up to 20-dB OBO while maintaining the bandwidth of a two-way DPA. With a detailed analysis of the design considerations and limitations, a design space for the DPA and RDPA combiner as well as a pure S-parameter-based bandwidth prediction and circuit design method is derived. Thereby, an ultracompact DPA combiner with an lambda(0)/20-length impedance inverter is implemented. To further validate the proposed design technique, a 19-dB OBO RDPA with 80-W peak envelop power (PEP) and 35% relative bandwidth from 2.1 to 3 GHz is fabricated. Moreover, the RDPA concept can also be applied to double the OBO of the already existing DPA implementations.