High-Directivity Enhancement With Passive and Active Bypass Circuit Techniques for GaAs MMIC Microstrip Directional Couplers

• Published in: IEEE transactions on microwave theory and techniques 2011-12, Vol.59 (12), p.3095-3107
• Main Authors: Yamamoto, K., Kurusu, H., Suzuki, S., Miyashita, M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Bypasses; Circuit properties; Circuits; Circuits of signal characteristics conditioning (including delay circuits); Couplers; Couplings; Design. Technologies. Operation analysis. Testing; Directional couplers; Directivity; Electric, optical and optoelectronic circuits; Electronic circuits; Electronic equipment and fabrication. Passive components, printed wiring boards, connectics; Electronics; Exact sciences and technology; Gallium arsenide; Gallium arsenides; Heterojunction bipolar transistors; heterojunction bipolar transistors (HBTs); Integrated circuits; Joining; Microwave circuits, microwave integrated circuits, microwave transmission lines, submillimeter wave circuits; MMICs; monolithic microwave integrated circuit (MMIC); Noise levels; power detectors; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices
• ISSN: 0018-9480; 1557-9670
• DOI: 10.1109/TMTT.2011.2169982

This paper describes monolithic microwave integrated circuit microstrip directional couplers based on a new directivity enhancement technique. This technique utilizes a bypass circuit-a phase shifter and an attenuator-placed between the coupling and isolation ports to cancel out backward wave leakage to the coupling port, thereby enhancing directivity of the couplers. The bypass circuit can be implemented with a simple passive LCR circuit or an active heterojunction bipolar transistor (HBT) phase-inversion attenuation circuit. Edge-coupled-type microstrip spiral couplers using the passive and active bypass circuits were fabricated in a GaAs HBT process. Measurements are as follows. The coupler with the passive LCR bypass circuit delivers a -21-dB coupling factor and a 0.14-dB insertion loss (IL) at 2.6 GHz while keeping enhanced directivity of more than 30 dB with a 23.1% relative bandwidth. This corresponds to more than 21-dB improvement of directivity at the same frequency, compared to the coupler without the bypass circuit. For the coupler with the active attenuator, a peak directivity of 46 dB and more than 30-dB directivity with a 120% relative bandwidth are achieved while a -21-dB coupling factor and a 0.13-dB IL are delivered at 2 GHz. Additional measurements of HBT power detectors integrated with the couplers show that the couplers with passive and active bypass circuits can suppress detection errors of less than ±0.20 and ±0.10 dB, respectively, under 4:1 voltage standing-wave ratio load mismatching conditions, thus proving the effectiveness of the bypass techniques.