K-Band MMIC Quasi-Elliptic Filtering Amplifier Isolator

This paper presents the design and experimental validation of a tri-functional K-band GaAs MMIC filtering amplifier isolator (FAI) that achieves a quasi-elliptic filtering response in the forward direction and high isolation in the reverse one. It is based on a hybrid combination of passive LC resonators and a unilateral frequency-selective gain stage (UFGS), in which an attenuated bandstop feedback network provides both frequency selectivity and strong reverse isolation. The quasi-elliptic response is obtained using transversal-resonator-based topology and an additional cross coupling to enhance selectivity. A comprehensive design method is also developed to guide the design of the tri-functional RF component. For experimental verification, UFGS and FAI were manufactured using a 0.15-μm GaAs MMIC process and tested. The FAI demonstrates the following key RF performance characteristics: quasi-elliptic transfer function with fractional bandwidth: 20%, gain: 12 dB, maximum directivity: 58 dB, noise figure: 4 dB, and output third-order intercept point: 14.3 dBm.