MMIC Active Bandpass Filter With Nonreciprocal and Quasi-Elliptic Response

This letter details the design and validation of a monolithic microwave integrated circuit (MMIC) active bandpass filter (BPF) with nonreciprocal and quasi-elliptic-type response. Its architecture is based on cascaded active frequency-selective stages. Each stage is shaped by a resonant cell that introduces one transmission pole or one pole and two transmission zeros (TZs). The overall filter exhibits a three-pole/two-TZ response. Each stage is comprised of two FETs with a passive resonant cell connected between them. The FETs introduce gain to counteract the loss of the MMIC components. Due to their near-unilateral behavior, a nonreciprocal transfer function is obtained. Thus, the proposed device functions as a co-designed BPF/isolator. The operating principles of the active BPF are characterized through simulated examples. For validation, a GaAs prototype was developed at X-band. It demonstrates a variable amount of gain in the forward direction of transmission and a highly isolated reverse transmission with >60 dB of isolation.