Active three-way circulator using transistor feedback network

This letter presents an active three-way circulator with both theoretical analyzes and experimental verifications. The conventional active circulators have their limitations on power handling and noise figure (NF), however, this proposed
structure maintains both performances, which is formed by three 2-port transistor feedback networks (TFNs). This TFN is formed by an FET together with a feedback inductor. Experimental results show that this circulator achieves the isolation more than 35 dB, the insertion of around 2 dB, and the return losses
better than 10 dB at each port at 1.8 GHz. The output power is linearly proportional to the power operation and the NF is around 5 dB. This compact structure, therefore, is suitable for use in the transmit/receive front end with a sufficient isolation, power handling, and NF for time division duplex operation.