This paper describes active circulators for microwave band. Using three dual-gate FETs, a 2 GHz band circulator was constructed. The bandwidth of this circulator is 1.3 GHz, and the insertion loss and the isolation are 0.3 dB and 30 dB, respectively. Further, the circulator for 5 - 18 GHz band is proposed and tested. Three amplifiers for 5 - 18 GHz and three power dividers' are used. The frequency range of this circulator is from 5 to 18 GHz. The circulator exhibit the insertion loss of 0.3 dB and the isolation of 30 dB. The circulator for 5 - 18 GHz band can be used for the input power up to 19 dBm.
"Circulator Using GaAs Dual Gate FETs", Proc. SPIE 1039, 13th Intl Conf on Infrared and Millimeter Waves, (18 November 1989); doi: 10.1117/12.978499; https://doi.org/10.1117/12.978499