Design And Optimization Of Monolithic Spiral Transformers And Baluns

Along with the rapid development of wireless communications, the rising demands of small size, low-power and high reliable radio frequency integrated circuits (RFICs) have generated tremendous interests in monolithic passive components. Owning the advantages of mass-production and low cost, CMOS processes have gained a leading place in very large scale integrated circuits (VLSI), so full-CMOS monolithic integrated communication systems become the most viable scheme.The lack of high quality passive devices has become the bottleneck in CMOS RFICs. Transformers are one of the important elements in RF applications. They can applied in different applications and have been used in almost all RF modules such as low noise amplifiers (LNAs), voltage control oscillators (VCOs), power amplifiers (PAs) and filters. They have special importance in differential circuits and low-voltage low-power circuits. But the absence of detail designing methods of passive transformers and baluns in CMOS processes has severely limited their applications in RFICs.In this paper, we discuss the design and optimization of monolithic spiral transformers and baluns that are suitable for RF applications. Two types of transformer construction, two-port (1:1 and 1:N turns ratio) and multi-port transformers (i.e.,baluns) are considered in detail.We first discuss the setting and accuracy of the simulation tool used in this paper. Then, the effects of imperfect coupling and parasitics between transformer windings have been presented, according to the lumped-element model. Based on the analysis, the relations between the performance and the layout of the spiral transformers and baluns are found with a lot of simulation and comparison. After that, special optimization about geometry and construction have been made under area reduction schemes and promotion of the coupling. The effects of resonant tuning are also discussed, which can reduce the losses between input and output at the expense of operation bandwidth. A method for estimating the optimized size of a monolithic spiral transformer to meet a given frequency is outlined, and the optimized monolithic spiral transformers and baluns are provided for RF applications such as 2.4GHz and 5GHz WLAN. The accuracy of simulation is established through measurements in two baluns. Finally, the physical separation on the crosstalk between transformers is presented and a brief guild for the design of the guard rings is provided.Although this thesis addresses transformers and baluns fabricated in silicon IC technologies, the results of this study are also applicable to components fabricated on insulating and semi-insulating substrates, such as GaAs.