Distortion mitigation in RF transmitters with cascaded nonlinear and efficient power amplifiers

The requirements on the linearization algorithm stemming from the use of a nonlinear amplifier to drive the power amplifier is investigated. It is theoretically shown under which conditions the nonlinear driver amplifier does not create additional nonlinear dynamic effects that have to be compensated. Experimental results using two cascaded Doherty amplifiers show that this cascade setup creates nonlinear distortion which the standard memory polynomial (MP) cannot properly linearize. A solution in the form of the vector-switched MP is shown to properly linearize the system. Experiments also show that it is possible to bias the driver Doherty amplifier's main branch in deeper class-AB without loosing linearity when an appropriate linearization algorithm is used.