A compact quadrature hybrid MMIC using CMOS active inductors

A miniaturization technique for the quadrature hybrid is proposed for monolithic-microwave integrated-drcuit applications. By using active inductors for the circuit implementation, a significant area reduction can be achieved due to the absence of distributed components and spiral inductors. Using a 0.18-μm bulk CMOS process, a 4.2-GHz quadrature hybrid is implemented for demonstration. The chip size measures 0.72 × 0.64 mm² where the hybrid circuit only occupies active area of 0.4 × 0.2 mm². With the enhanced Q factor provided by the active inductors, the fabricated circuit has an insertion loss less than 0.4 dB for -3-dB coupling while maintaining excellent return losses and port isolation in the vicinity of the center frequency. Owing to the reconfigurable capability provided by the CMOS active inductors, the center frequency of the quadrature hybrid can be varied from 3.6 to 4.7 GHz by adjusting the bias currents, exhibiting a tuning range of 26% at C-band.