Foundations and Techniques of Active Inductors Design in CMOS RF Circuits

Recently, active inductors have emerged as an appealing alternative to conventional on-chip spiral passive inductors within radio-frequency (RF) integrated circuits. Unlike passive inductors, which occupy large silicon area and are fixed, active inductors emulate the real inductive behavior using active devices such as MOSFETs and capacitors, thereby realizing compact size, electronic control, and compatibility with standard CMOS processes. The theory of active inductors is reviewed in this chapter, starting from the very basics of inductance and comparing the passive versus the active implementations. Amplifier-based, current conveyor-based, and gyrator-C topologies of active inductors are reviewed along with their relative merits and trade-offs. Emphasis has been placed on the gyrator-C structure since it is one of the two broad classes of topologies highly commendable for tunability and ease of integration. Then, design considerations for both the single-ended and differential topologies followed by presenting various techniques to improve performance figures of merit, which include quality factor (Q), inductance tuning, noise, linearity, stability, and power consumption, are presented. Combining theoretical insight with practical design strategies, this chapter provides the foundation for applying active inductors in RF and microwave systems, especially in filters, oscillators, and impedance-matching networks.