Experimental investigation of bias current and load modulation effects in phase distortion of GaN HEMTs

The relationships between phase distortion, bias point and load modulation in power amplifiers are experimentally evaluated for two different GaN HEMT technologies. Measurements on devices with fixed and modulated load, to emulate the dynamic working conditions of the main stage of a Doherty amplifier, have been carried out through an advanced source/load-pull setup. Almost flat phase distortion against output power can be obtained by the proper bias point when the load remains constant, whereas it is shown that this possibility is not compatible with load modulation, inherently present in Doherty power amplifiers.