Introduction

CMOSspiral inductors and transformers have found a broad range of applications in high-speed analog signal processing including impedance matching and gain-boosting in wireless transceivers, bandwidth improvement in broadband data communications over wire and optical channels, oscillators and modulators, RF bandpass filters, RF phase shifters, RF power dividers, and coupling of high-frequency signals, to name a few. Traditionally, passive inductors and transformers are off-chip discrete components. The need for off-chip communications with these passive components severely limits the bandwidth, reduces the reliability, and increases the cost of systems. Since early 1990s, a signifi- cant effort has been made to fabricate inductors and transformers on a silicon substrate such that an entire wireless transceiver can be integrated on a single substrate monolithically. In the mean time, the need for a large silicon area to fabricate spiral inductors and transformers has also sparked a great interest in and an intensive research on the synthesis of inductors and transformers using active devices, aiming at minimizing the silicon consumption subsequently the fabrication cost and improving the performance.

This chapter looks into the characteristics of spiral and active inductors and transformers, both their advantages and limitations, and the impact of these characteristics on the applications of these devices. Section 1.1 demonstrates the critical need for an inductive characteristic in high-speed applications. The characteristics of spiral inductors and transformers are examined in Section 1.2. In Section 1.3, we investigate the pros and cons of active inductors and transformers. The chapter is summarized in Section 1.4.