Active circulators—The realization of circulators using transistors

An approach in realizing circulators using three transistors is described. The authors propose two circuit configurations. In the one configuration, the transmission efficiency has some relations to the impedance level and the efficiency is theoretically less than unity under the matched termination condition, but the backward leakage can be null in the frequency range limited by the frequency characteristics of the transistors. Experimentally, the efficiency ηv=0.71, the forward/backward transmission ratio is 53 dB, and the frequency range extends from 100c/s to 3Mc/s with the input resistance of 200Ω (40 dB at the limit frequencies). In the other configuration, the efficiency is essentially unity independent of the impedance level. A circulator with ηv=1.0 and the maximum transmission ratio of 70 dB with 40 dB at 300 kc/s is obtained in experiment.