An X-Band Quasi-Circulator GaAs MMIC

This paper presents the design of a 8-12GHz quasi-circulator fabricated in a $0.25 \mu \mathrm{m}$ GaAs pHEMT monolithic microwave integrated circuit (MMIC) process. The circuit consists of three Lange couplers which connect gates and drains of three equal gain-matched amplifiers. The coupling factor of the unequal-split Lange coupler is designed to achieve an isolation higher than 20dB over a 40% bandwidth, with a return loss of better than 10dB and an insertion gain of 2.4dB across the band. The layout includes bias networks for the three amplifiers and occupies a 2.5mm $\times 2.5$ mm die. The compact quasi-circulator is intended for use in full-duplex front ends.