A 0.6–0.8 GHz CMOS Frequency-Tunable Nonmagnetic Circulator With Digital Predistortion-Based Linearity Enhancement for in-Band Full-Duplex Radios

In this article, a 0.6–0.8 GHz CMOS frequency-tunable nonmagnetic circulator employing a hybrid-capacitor-based C–L–C transmission line and digital-predistortion (DPD)-assisted linearization is presented for advanced in-band full-duplex (IBFD) radios. Each transmission line uses hybrid-capacitors composed of metal-insulator-metal (MIM) capacitors and varactors, providing voltage-controlled tunability and enabling multi-band operation across 5G New Radio (NR) low-band frequencies. An N-path gyrator provides non-reciprocity, while the hybrid-capacitor structure extends the 90° phase-shift bandwidth for adaptive frequency control. To mitigate the intrinsic nonlinearities of the CMOS switches, a DPD algorithm based on the memory-polynomial (MP) model and indirect-learning architecture (ILA) is applied for the first time to a CMOS circulator. Fabricated in a 65-nm CMOS process, the proposed DPD-assisted circulator achieves transmitter and receiver insertion losses of 3.1–3.7 dB and 2.6–3.0 dB, respectively, while maintaining > 30 dB isolation across 600–800 MHz. With a 1024-quadrature amplitude modulation (QAM) 5G NR modulation signal (20 MHz bandwidth, peak-to-average power ratio of 11.6 dB) at +10 dBm input power, the DPD improves the error vector magnitude (EVM) from –24.1 dB to –42.8 dB and the adjacent-channel leakage ratio (ACLR) from –26.1 dBc to –44.6 dBc. The circulator consumes 18.1 mW and occupies an active area of 0.34 mm².