Design of GaN HEMT based Doherty amplifiers

In this paper the design of high efficiency unsymmetrical Doherty power amplifiers (PAs) will be presented. The Doherty PAs were designed to achieve high average efficiency for digitally modulated signals with high peak to average power ratio (PAR) used in third (3G) and fourth (4G) generation systems. The Doherty amplifiers have been designed using two unequal sized GaN devices for the main Class-AB and peaking Class-C amplifiers. In the first narrow band design, a 50 W Doherty amplifier has been implemented at 2.5 GHz. A drain efficiency of 54% (48% power added efficiency (PAE)) has been measured at its maximum output power of 47 dBm. The efficiency remains constant up to the 6 dB output power back-off. Moreover, the Doherty PA has been linearized to achieve an Adjacent Channel Leakage Ratio (ACLR) of -46 dBc at 10 MHz offset for a 10 MHz (UMTS-LTE) signal with a PAR of 8.5 dB, and an average output power of 40 dBm. 44% PAE was measured for this signal constellation. In a second design, the status of a wideband unsymmetrical Doherty PA with 31% fractional bandwidth will be reported.