Design of Active Circulators Using High-Speed Operational Amplifiers

Design formulas and prototype realizations of an operational amplifier based active circulators operating from dc to 500 MHz are presented. The operational unit of the circulator consists of a chip-resistor based voltage divider and an Op Amp differential amplifier which can be easily fabricated at low cost for general low power applications. An existing widely used circuit is shown to be a special case of the more generalized theory presented in this letter. Our design shows a return loss of greater than 10 dB, an insertion loss flatness within +/- 0.8 dB, and an isolation greater than 20 dB at frequencies below 500 MHz.