Analysis of Common-Mode Isolation on Transformer Based Balun

This paper presents the circuit analysis of a transformer based balun with emphasis on the improvement of Common-Mode Isolation (CMI). Critical parameters are identified based on the initial analysis of the transformer equivalent circuit. Additionally, the influence of the transformers load impedance and the use of Center-Tap (CT) on CMI enhancement are presented. EM simulations of two transformers, a stacked and an interleaved version, designed in 65 nm CMOS technology are presented. The CMI performance of both the transformers is compared to the proposed circuit model and the effect of layout and geometry is also analyzed. Finally, guidelines are presented for the design of transformer based baluns with improved CMI performance.