AM/PM distortion in GaN Doherty power amplifiers

The carrier power amplifier (PA) of a Doherty architecture is designed with a load that maximizes efficiency for a particular load-pull ratio. With the increase of the input drive, the peaking PA provides the necessary current to reduce the carrier PA load impedance. From this point on, the carrier PA operates at a constant output voltage, suffering a severe voltage gain compression at its terminals. Since the input Miller reflected C_gd depends on this gain, the device´s total input capacitance changes substantially with drive level, inducing a significant input phase shift. Hence, in this paper we address the impact of this capacitance variation on the AM/PM distortion characteristic of a GaN HEMT based Doherty PA.