V- Through W-Band GaN Active Circulator

This paper presents an active circulator designed to operate from 50-110 GHz implemented in a 40-nm HEMT GaN MMIC process. Three amplifiers are connected with Lange couplers in a ring architecture. The travelling-wave amplifiers use three 4× 25$\mu$m HEMTs, with a simulated small signal gain of 5 dB at 80 GHz designed to be well matched over the octave frequency range. The circulator is stable and demonstrates 20 dB of isolation and 6.3 dB of insertion loss at 88 GHz in full-duplex mode with all amplifiers biased on. The isolation with only one amplifier biased on is referred to as half-duplex, and shows similar isolation over a broader bandwidth. The measured gain compression agrees well with simulations and demonstrates good linearity up to the available 20 dBm input power at 92 GHz. The presented circuit can be bias-switched between full and half-duplex operation.