Analysis and design of a wideband transformer balun in GaAs process

This paper presents a new design approach for wideband spiral transformer balun in GaAs process. By using even and odd mode method, analytical solution for ideal amplitude and phase balance conditions are derived. A first order equivalent circuit of transformer is employed to illustrate the approach utilizing the parasitic capacitances of transformer at high frequency for balun design. Based on this approach, a transformer-based balun covering 6 to 18 GHz is designed, which demonstrates an amplitude balance less than ±1.5 dB from 4 to 24 GHz and phase balance less than 10 degrees from 2 to 22 GHz. The simulation and experimental results show a good agreement. The chip size is 0.7×0.7 mm².