Self-Conduction of the Auxiliar Amplifiers in Load-Modulation based PAs

In load-modulation-based power amplifiers (PAs), a spontaneous early turn-on of the auxiliary PAs is commonly observed, i.e., the devices start to dissipate power even if no power is injected at their input, and, consequently, a significant degradation of the overall efficiency in the back-off region is observed; this is especially true when shallow class-C biases are employed. This work explains the mechanisms behind this effect, demonstrating that the self-turn-on of the auxiliary PAs is due to the presence of an input voltage that results mainly from the interaction between a feedback current through C
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(excited by the voltage at the combiner, imposed by the main PA) and the source impedance presented to the auxiliary PA. This is then experimentally shown by performing source-pull measurements on the peaking device of a 20W Doherty PA.