Optimized Design of a Highly Efficient Three-Stage Doherty PA Using Gate Adaptation

We demonstrate an optimized design of a highly efficient three-stage Doherty power amplifier (PA) for the 802.16e mobile world interoperability for microwave access (WiMAX) application at 2.655 GHz. The "three-stage" Doherty PA is the most efficient architecture among the various Doherty PAs for achieving a high peak to average power ratio (PAPR) signal. However, it has a problem in that the carrier PA has to maintain a saturated state with constant output power when the other peaking PAs are turned on. We solved the problem using a gate envelope tracking (ET) technique. For the proper load modulation, the gate biases of the peaking PAs were adaptively controlled, and the peak power and maximum efficiency characteristics along the backed-off output power region were successfully achieved. Using Agilent's Advanced Design System and MATLAB simulations, the overall behavior of the three-stage Doherty PA with the ET technique employed was fully analyzed, and the optimum design procedure is suggested. For the WiMAX signal with a 7.8-dB PAPR, the measured drain efficiency of the proposed three-stage Doherty PA is 55.4% at an average output power of 42.54 dBm, which is an 8-dB backed-off output power. Digital predistortion was used to linearize the proposed PA. After linearization, a -33.15 dB relative constellation error performance was achieved, satisfying the system specifications. This is the best performance of any 2.655-GHz WiMAX application ever reported, and it clearly shows that the proposed three-stage Doherty PA is suitable as a highly efficient and linear transmitter.