An active quasi-circulator integrated circuit relying on a differential central core is presented. Measurements on the quasi-circulator reveal that the forward transmission coefficients, S 21 and S 32 , are both close to 24 dB in the range of 2.5 to 6 GHz. The isolation between the ports is greater than 20 dB and can reach up to 59 dB in the case of S 13 . The chip was fabricated on a standard 130 nm CMOS process and the core circuit area measures 0.105 mm 2 , making it the smallest active quasi-circulator known to date.Introduction: A considerable number of active quasi-circulator circuits have been demonstrated to date in both hybrid and RFIC form [1][2][3][4][5][6].A key reason to use an active quasi-circulator compared to a passive ferrite-based one is to reduce cost. Hybrid and RFIC implementations have their own respective advantages in relation to cost and which circuit is seleted is dependent on the end-use of the quasi-circulator. For medium-power applications, for example, a hybrid circuit might be a better solution than an RFIC implementation because relatively inexpensive packaged power transistors can be easily found. Of course, one drawback of the hybrid circuit is its large area. For smallsignal applications, a general-purpose RFIC quasi-circulator can be the more cost-effective choice. In this Letter, we report a quasi-circulator CMOS RFIC that relies on a novel differential circuit topology. A pair of transconductor circuits are used to convert differential voltage signals to current signals which can be summed to provide isolation between two of the ports. This quasi-circulator has the smallest active area known to date, measuring only 0.105 mm 2 .

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