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This paper presents compact CMOS quadrature hybrids by using the transformer over-coupling technique to eliminate significant phase error in the presence of low-Q CMOS components. The technique includes the inductive and capacitive couplings, where the former is realized by employing a tightly inductive-coupled transformer and the latter by an additional capacitor across the transformer winding. Their phase balance effects are investigated and the design methodology is presented. The measurement results show that the designed 24-GHz CMOS quadrature hybrid has excellent phase balance within plusmn0.6deg and amplitude balance less than plusmn 0.3 dB over a 16% fractional bandwidth with extremely compact size of 0.05 mm2. For the 2.4-GHz hybrid monolithic microwave integrated circuit, it has measured phase balance of plusmn0.8deg and amplitude balance of plusmn 0.3 dB over a 10% fractional bandwidth with a chip area of 0.1 mm2 .