Linearity-optimized power tracking GaN HEMT Doherty amplifier using derivative superposition technique for repeater systems

This paper presents the linearity-optimized power tracking Gallium Nitride (GaN) high electron mobility transistor (HEMT) Doherty power amplifier (DPA) for wide-band code division multiple access (WCDMA) repeater systems. Also, we analyze effects of the drain bias of the carrier amplifier on the linearity and efficiency of the DPA. To improve linearity of the DPA without extra linearization techniques, the derivative superposition technique (DST) is employed by using higher drain bias of carrier amplifier than that of peaking amplifier and the optimization of the gate bias of peaking amplifier. The optimum drain and gate biases are adaptively controlled by the power tracking bias supply circuit. For experimental validation, the DPA is designed and implemented with 25-W peak envelope power (PEP) GaN HEMT and tested with a 1-carrier WCDMA signal of 2.14 GHz. At an adjacent channel leakage ratio (ACLR) of -45 dBc, a Pout of 39.4 dBm (7.6-dB back-off power from Psat) and a power-added efficiency (PAE) of 31.8% is achieved for the power tracking DPA. For the conventional DPA, a Pout of 32.7 dBm with a PAE of 21.4% is achieved at an ACLR of -45 dBc.