# Design of a Broadband and High-Isolation CMOS Active Quasi-Circulator
## 2 Circuit Design and Implementation  
The suggested design of the ultra-broadband quasi-circulator consists of an active balun that is built using a distributed amplifier with a cascading structure. It also includes a phase inversion channel to promote isolation, as depicted in Figure 1. The distributed balun can be seen as a series of interconnected distributed amplifiers without a termination at port 2. The incident signal from port 1 travels over the gate terminal of the distributed balun with a sectional phase constant. The voltage, Vgs1, across each gate capacitor of the field-effect transistor causes a current to flow into the drain terminal. By carefully choosing the FET dimension and circuit parameter, the phase velocities of the gate and drain terminals can be regulated to be almost equal. This leads to the sum of the forward direction as they reach port 2. The intrinsic phase inversion characteristic of the common-source field-effect transistor (FET) allows for the attainment of the antiphase output from port 2. In addition, the drain current generated by the cross voltage, Vgs2, flows into the drain terminal of the cascading stage and subsequently to the forward combination at port 3. The double-phase inversion produces a non-inverting phase output at port 3. Therefore, the balun can achieve equal output power and antiphase relationship between ports 2 and 3. The quasi-circulator is defined by the condition that \(\mid\)S21\(\mid\) and \(\mid\)S32\(\mid\) are both equal to 1, assuming all other scattering parameters are set to zero. This situation indicates that the transmission path of the distributed balun from port 1 to port 3, also known as \(\mid\)S31\(\mid\), will be unnecessary or superfluous. In order to resolve this problem, the quasi-circulator design has implemented a phase cancellation approach. In Figure 1, a supplementary distributed amplifier was linked in parallel to ports 1 and 3, and it is identified as the phase cancellation path. This transmission path follows the phase inversion characteristic and is similar to the S21 path of the distributed balun. By adjusting the circuit settings, it is possible to control the phase difference between port 3 of the distributed balun and the output port of the phase cancellation path to a precise angle of 180\({}^{\circ}\). Ultimately, the two output ports can be merged to achieve a higher level of \(\mid\)S31\(\mid\) isolation for the distributed quasi-circulator. In terms of the transmission path of \(\mid\)S32\(\mid\), if a signal is applied to port 2, it can cause the distributed balun's cascading stage to produce a combination of reverse traveling waves at port 3. Furthermore, the one-sided nature of FET impedes the transmission of the incoming signal from port 2 through port 1. In addition, the proposed distributed arrangement allows for achieving exceptionally low magnitudes of \(\mid\)S12\(\mid\), \(\mid\)S13\(\mid\), and \(\mid\)S23\(\mid\) while taking the same factors into account. The flow chart of the proposed quasi-circulator is shown in Figure 2. The whole quasi-circulator consists of three stages: the first buffer stage, the second buffer stage, and the phase cancellation stage. The first and second stages are equal to an active balun. The active balun is composed of two common-source field-effect transistors (CS FETs) of a cascade architecture. The first stage circuit consists of three transistors, which can be equivalent to M1, and the second stage circuit consists of two transistors, which can be equivalent to M2. The phase cancellation stage transistor M3 is used for phase cancellation. Figure 1: Signal flow direction in a quasi-circulator. The quasi-circulator functions by means of the transmitting channel denoted by the symbol S31. This path is simultaneously formed by the cascading structure that consists of M1 and M2, as well as the CS FETs M1 and M2, which ultimately result in the transmission paths denoted by the symbols S21 and S32, respectively.