# Design of a Broadband and High-Isolation CMOS Active Quasi-Circulator
## 1 Introduction  
Lately, there has been significant interest in microwave and millimeter-wave communication systems that function in the Ka- to F-band. These systems are valued for their broad operational bandwidth, fast data transfer, affordable production, and excellent performance. CMOS integrated circuits are seen as potential choices due to their circuit implementations, which offer advantages such as low supply voltage, low cost, and high integration [1]. Active circulators are functional components with three ports that are specifically designed for integrated system-on-chip (SoC) applications. These components are appropriate for use with monolithic microwave integrated circuit (MMIC) integration. The relevant literature references for further information on this topic are [2; 3; 4]. These circulators play a crucial role in MMIC integration by allowing signals to flow in a single direction, ensuring optimal impedance at all three ports. Therefore, the transmitter and receiver have the capability to both translate and receive RF signals concurrently, even if they have different frequencies, without the need for a diplexer filter or RF switch. Furthermore, active circulators possess a wide frequency range, minimal signal loss, and compact chip size. Nevertheless, active circulators give rise to issues in transmitting signals from port 3 to port 1. To address the issue of receiver signal leakage into the transmitter port, one can employ quasi-circulators that facilitate just two power flows in the same direction, specifically from port 1 to port 2 and from port 2 to port 3. Various configurations of active quasi-circulator modules have been introduced in microwave and millimeter-wave system applications. One such design involves separating the broadcast and received signals. Historically, the bulk of circulators have utilized ferrite components. These devices rely on the collective physical characteristics of magnetic materials [5]. These devices have a limited bandwidth and are too big to be incorporated into monolithic circuits. Precise fabrication of semiconductor devices can be achieved to meet specific criteria rather than relying on the inherent physical properties of the chips. Moreover, these dynamic circulators are more suitable for SoC applications that utilize compact microcontroller integration. In 1965, there was a proposal to develop active circulators that operated at a frequency of three megahertz (MHz) [6]. A novel active quasi-circulator (QC) module was subsequently designed to enable the incorporation of three ports. This module consisted of transistors equipped with non-reciprocal phase shifters, and its purpose was to attain a level of isolation of 20 dB. Nevertheless, it possesses a substantial chip area and a considerable direct current power consumption [7]. A novel approach was suggested to minimize the utilization of direct current by employing the phase cancellation technique and leveraging the current-reuse topology [8]. A 30 GHz quasi-circulator currently employs the current-reuse technique to minimize power consumption [9]. However, the achieved level of isolation was just 12 dB. The current technology used to decrease chip size and power consumption still has unresolved problems related to narrow band and low isolation [10]. A quasi-circulator employs a power coupler to enhance isolation, although this coupler restricts its bandwidth. In active quasi-circulators, the utilization of common-source/drain designs has been employed to increase the bandwidth, albeit at the expense of higher insertion loss and noise figure (NF) [11]. A study has documented the existence of a quasi-circulator with high insertion gain. However, it should be noted that this quasi-circulator has a limited operating bandwidth. The quasi-circulator, which utilizes phase correction, has successfully attained high port-to-port isolation and wideband operation. Nevertheless, the impedance matching network is intricate and requires a significant amount of space on the device.