Practical Load Compensation Networks in Chireix Outphasing Amplifiers Using Offset Transmission Lines

This paper describes a load compensation network in an outphasing system, realized using series transmission lines instead of conventional shunt reactive components. In particular, it is shown that using transmission lines of different lengths, here referred as Offset Transmission Lines (OTL), can provide load compensation effects at the intrinsic plane of a real device, close to those proposed by Chireix when using shunt reactance in an ideal case. The concept is analysed and demonstrated using a dedicated outphasing circuit, based on GaN HEMT packaged devices. At 2 GHz, the prototype amplifier maintains an efficiency greater than 50% over an output power back-off range in excess of 8 dB. Drain efficiencies of more than 65% and 40% are maintained over 5.5dB and 10dB output power ranges respectively, while achieving a maximum combined output power of 44.5 dBm.