Characterization of high voltage varactors for load modulation of GaN-HEMT power amplifier

In this work, a semiconductor varactor and a ferroelectric varactor, used for designing load modulation power amplifiers, are compared. The semiconductor varactor shows considerable advantages in terms of tuning range, tuning voltage, tuning speed, and losses in comparison to the ferroelectric varactor, which was a thick-film Barium Strontium Titanate (BST) device. A fixed matching network power amplifier (FMN-PA) was fabricated in order to enable comparisons with a previously designed tunable matching network (TMN) load modulation PA based on BST. Both PAs are driven in class C, operate at 1.8 GHz and use a GaN HEMT. The maximum measured drain efficiencies for the FMN-PA and TMN-PA are 79 % and 69 % at the maximum output power of 43 dBm. At 6 dB back-off output power the efficiencies are 45% and 62% respectively, after controlling the bias voltage for the TMN case. Better performance for the TMN-PA is achievable using the semiconductor varactor according to the obtained characterization results of this work.