Reflectionless Design in RF Passive and Active Circuits: Recent Advances and Emerging Trends

RF-signal power reflections produced by impedance mismatches remain a critical limitation in high-frequency systems, degrading their operational performance, robustness, and RF-signal integrity. Typical RF-protection mechanisms, such as inter-RF-stage attenuators, isolators$\boldsymbol {/}$circulators, or RF-signal-cancellation loops, often impose penalties in terms of increased in-band insertion-loss levels, size, cost, DC-power consumption, and design complexity. To overcome these challenges, reflectionless or absorptive RF circuits have emerged as an alternative paradigm. They intrinsically absorb undesired stopband$\boldsymbol {/}$non-transmitted RF signals, thereby providing broad-band impedance matching. This overview paper reviews the latest advances on RF reflectionless circuits, covering their fundamental RF design principles and more-recent developments. Beyond the well-established case of RF reflectionless filters illustrated through several state-of-the-art implementations in 3-D and planar technologies, other emerging designs are also addressed. They include reflectionless beyond-transmission-band-flat-group-delay single-ended bandpass filters and balanced lowpass filters for high-data-rate digital communications in planar and integrated RF platforms, and on-chip two-port-absorptive bandstop filters. Furthermore, as key examples of reflectionless RF active devices, millimeter-wave input-absorptive RF amplifiers for beyond-5G wireless scenarios are presented. Cutting-edge applications in antennas, electromagnetic compatibility (EMC), and quantum engineering are also discussed, highlighting the versatility of the reflectionless design philosophy.