In this article, we provide a comprehensive review of past and present efforts in the field of microwave nonreciprocity with an emphasis on the most commonly used nonreciprocal devices, namely gyrators, isolators, and circulators. After discussing the origin and history of such efforts, we elaborate on the pros and cons of four distinct approaches to break reciprocity which include magnetic biasing of ferrite materials, intrinsic nonreciprocity of solid-state devices, nonlinearities with geometric asymmetries, and finally, periodic spatiotemporal variation. We subsequently pay due attention to the spatiotemporal approach and show that the recently emerging proposals to achieve magnet-free nonreciprocity with linear, periodically time-varying circuits can compete with traditional ferrite devices and even outperform them in several metrics, thus opening the door to exciting venues for miniaturized low-cost and high-performance nonreciprocal devices with numerous applications ranging from full-duplex communications and quantum computing to biomedical imaging and radar systems.

Microwave Nonreciprocity

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.

Design of Active Circulators Using High-Speed Operational Amplifiers

With two inverting or noninverting active hybrid networks in a wave structure, wave-active, filters can be constructed without the need for additional inverting amplifiers. One-to-one correspondence exists between the dynamic elements of the active and passive networks. The hybrid has one bidirectional and two undirectional ports. Tandem connection at the undirectional ports leads to S-parameter realizations such as the ladder. Series and parallel connections at the bidirectional ports, while not strictly wave-active, can be used to reduce the number of amplifiers or to derive realizations for such networks as parallel ladders. Gyrator- C realizations in which the gyrator is a loop of an inverting and a noninverting hybrid become a special case of the composite filter. Experimental results show agreement with the analytic conclusions and confirm the low sensitivity.

The composite active filter: A combined wave-active and gyrator-C filter

A review of the two types of circulators using Operational Amplifiers (OA) with detailed comparison is given. Novel active circulator circuits using Current Conveyors (CCII) and Current Feedback Operational Amplifiers (CFOA) and Differential Voltage Current Conveyor (DVCC) are introduced. The proposed CCII circulator circuit uses six CCIIs and three floating resistors. Two different circulator types using the CFOA are given. A circulator circuit which uses three DVCCs and has the advantage of using three grounded resistors is also introduced. Spice simulation results using 0.5 μm CMOS transistors are included to support the theoretical analysis and demonstrate comparisons among the different types of circulators.

Active circulator circuits using oa, ccii, cfoa and dvcc

It is shown how filters requiring floating inductance and constant-resistance allpass networks may be made with loss-less circulators and capacitors; the filters, if matched in the passband, are insensitive to small changes in the capacitances.

Circulator-capacitor networks

A method is described for making multiport circulators with active elements which are directly coupled; each port requires one differential amplifier.

P.E. Greenaway

Papers

Directly coupled active circulators