Optimized load modulation in a Doherty amplifier using a current injection technique

The classical Doherty configuration suffers from a lack of accurate control of the load impedance performed by the peaking stage. The result is a reduction in the maximum obtainable efficiency and linearity performance for a given input power level. By driving the carrier and peaking power amplifiers independently, it is possible to present the optimum load to the carrier stage for either maximum efficiency, linearity or anywhere in between for any input power level. This paper proposes the injection of a controllable current into the load to create the required active load-pull. Simulations in ADS using a 10W GaN HEMT device are presented for maximum efficiency. Two different Doherty power combining networks are used and the required current for each network is explained. Measurements of a class-F amplifier in a Doherty current injection architecture are taken and drain efficiency versus output power is plotted showing a potential of 10dB dynamic range of output power with a tracked drain efficiency between 65-70%.