This paper presents an ultra-compact transformer-based quadrature generation scheme, which converts a differential input signal to fully differential quadrature outputs with low passive loss, broad bandwidth, and robustness against process variations. A new layout strategy is proposed to implement this 6-port transformer-based network within only one inductor-footprint for significant area saving. A 5 GHz quadrature generation design is implemented in a standard 65 nm CMOS process with a core area of only 260 μm by 260 μm, achieving size reduction of over 1,600 times compared to a 5GHz λ/4 branch-line coupler. This implementation achieves 0.82 dB signal loss at 5 GHz and maximum 3.8° phase error and ±0.5dB amplitude mismatch within a bandwidth of 13% (4.75 GHz to 5.41 GHz). Measurement results over 9 independent samples show a standard phase deviation of 1.9° verifying the robustness of the design.