Comparative study of planar stacked integrated transformers for MMICs

This paper describes the comparative study of different transformer topologies operating for Monolithic Microwave Integrated Circuit (MMIC). Planar stacked square, octagonal and circular coil topologies were studied to illustrate the good performance. The primary and secondary coils had the same dimensions with 1:1 turn ratio. The simplified physical equivalent circuit was demonstrated to evaluate its components values. Transformers prototype were fabricated using a standard 130 nm CMOS process. COMSOL Multiphysics analysis was carried out to show the electromagnetic and thermal behaviour. Simulations show quality factor improvements for the flipped circular topology. The square topology presents a high temperature and losses due to the right angles. The octagonal topology avoid to concentrates the current density in the angles but allows to increase the temperature due to the great number of segments. Measurements of insertion loss and coupling factor on circular topology demonstrate the good agreement with the proposed model. Simulated and measured magnitudes and phases for different outer diameter and width are identical. The results revealed that the planar transformer with the circular topology offers high performances.