Broadband and compact 3-dB MMIC directional coupler with lumped element

This paper proposed a broadband and compact 3-dB MMIC edge-coupled directional coupler. The proposed coupler consists of eight stepped-impedance coupled sections and a tiny capacitor. The capacitor is constructed by using the MIM capacitor process and is located on the center of the coupler. The capacitor and stepped structure effectively realize 3-dB coupling and enhanced amplitude/phase characteristics. A millimeter-wave prototype MMIC coupler is fabricated on the 100-μm thick GaAs substrate by using the GaAs pHEMT process. The total coupler length is only 440 μm. The fabricated coupler achieves a coupling loss of -4dB±0.5dB from 30 GHz to 69.3 GHz. The output amplitude and phase imbalances are less than 1.4 dB and 90 degrees ± 5 degrees at the same frequency range, respectively. The proposed MMIC coupler promises very compact and broadband MMIC.