Controlling Active Load-Pull in a Dual-Input Inverse Load Modulated Doherty Architecture

Mathematical analysis of Doherty amplifiers have assumed many simplifications. Most notably, the peaking amplifier does not contribute power into the load and the peaking stage has an observed impedance of infinity. This paper will show that these simplifications impair the performance of a single-input Doherty amplifier and that phase tuning for compensation is needed to improve the overall system performance. The dual-input Doherty amplifier is capable of overcoming the limitations of power-dependent phase imbalance and phase compensation lines at the input of the peaking stage; however, the characterization of such an architecture is not straightforward. A new measurement technique is proposed to measure dc current, dc voltage, and output power levels to allow unique characterization of a dual-input Doherty amplifier. Phase compensation lines at the input of the peaking amplifier will be shown to be not required, as long as correct offset lines are calculated for both the carrier and peaking stage and that the lambda/4 transmission-line length is not necessarily required for active load-pull. Results of a dual-input inverse load modulated Doherty amplifier are presented where the peaking stage delivers 10 dB less of maximum output power than the carrier, while still maintaining Doherty behavior. The peaking stage can therefore be implemented with a smaller device than the carrier.