With the development of multi-band wireless communication and the increasing data transmission rate, the circulator as an antenna interface must be able to work in multiple frequency bands and provides large bandwidth. It presents a high challenge to the design of circulators, especially the active quasi-circulators. In this survey, we review the representative active quasi-circulators and summarize three different techniques and the corresponding structures to show an incremental improvement of the isolation and bandwidth of the active quasi-circulators. In addition, we also compare the performance of several state-of-art active circulators, and analyze their advantages and disadvantages. Finally, we conclude the future trend of the active quasi-circulators.

中文翻译：

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有源准循环器综述

随着多频段无线通信的发展和数据传输速率的不断提高，作为天线接口的环行器必须能够工作在多个频段并提供大带宽。这对环行器的设计提出了很高的挑战，尤其是有源准环行器。在本次调查中，我们回顾了有代表性的有源准环行器并总结了三种不同的技术和相应的结构，以显示有源准环行器的隔离度和带宽的增量改进。此外，我们还比较了几种最先进的有源循环器的性能，并分析了它们的优缺点。最后，我们总结了有源准环行器的未来趋势。