Miniature broadband branch-line coupler in silicon-based integrated passive device technology

In this work, a miniature broadband branch-line coupler is proposed. By implementing the quarter-wavelength lines in a three-stage branch-line coupler using bridged T-coils, the circuit size can be largely reduced with no reduction in bandwidth. Specifically, a proposed broadband branch-line coupler with a center frequency f₀ = 2.2 GHz is implemented using a silicon-based IPD process. The circuit size is 6.07 mmx2.89 mm, which is only about 0.045λ₀×0.021 λ₀ at f₀. The measured bandwidth for 1-dB amplitude imbalance and 10-degree phase imbalance is from 1.65 to 2.87 GHz, which corresponds to a 54% bandwidth. To our knowledge, this is the smallest branch-line coupler with a larger than 50% bandwidth ever reported.