RF Bandpass Filters with Active Inductors
RF bandpass filters are used extensively in wireless communications. These filters are traditionally implemented using lumped LC, dielectric, and surface acoustic wave (SAW) filters with SAW filters the most widely used, owing to their small size and low power consumption. SAW filters, however, are not compatible with silicon technology. The effort on integrating RF bandpass filters on a silicon substrate is accelerated with the emergence of monolithic spiral inductors and transformers. Two main drawbacks of bandpass filters with spiral inductors and transformers are the low passband center frequency arising from the large spiral-substrate capacitance and the high insertion loss caused by both the skin-effect induced parasitic resistance of the spiral and the substrate eddy-current induced ohmic loss in the spiral at high frequencies. The skin-effect induced loss is proportional to the square root of the frequency of the signal whereas the eddy-current induced loss is proportional to the square of the signal frequency . The former dominates at low GHz frequencies at which most RF bandpass filters operate. Recent RF band select filters are increasingly implemented using Q-enhanced on-chip spiral inductors and transformers to take the advantage of the low-noise and superior linearity of these spiral inductors and transformers and at the same time to minimize the insertion loss [108–112, 107, 23, 113, 24–27]. Active Q-enhancement of spiral inductors is achieved from using a negative resistor that is synthesized using active networks to cancel out the parasitic resistance of the spiral. Because it is difficult to tune the inductance of spiral inductors and transformers in the monolithic implementation of these passive devices, the frequency tuning of these bandpass filters is typically attained using MOS varactors with the downside of a small passband center frequency tuning range. The need for a large silicon area to fabricate spiral inductors and transformers, and the limited capacitance tuning range of MOS varactors greatly increase the cost and limit the robustness of these bandpass filters.
This chapter examines the design of RF bandpass filters using CMOS active inductors. The chapter starts with a brief presentation of a number of important figure-of-merits that quantify the performance of bandpass filters in Section 4.1. These figure-or-merits include bandwidth, 1-dB compression point, third-order intercept point, noise figure, noise bandwidth, spurious-free dynamic range, frequency selectivity, and passband center frequency tuning range. Section 4.2 deals with the configurations of single-ended and differential active inductor bandpass filters. Section 4.3 examines both the circuit implementation and characteristics of a number of published active inductor bandpass filters. The chapter is summarized in Section 4.4.
KeywordsQuality Factor Surface Acoustic Wave Active Inductor Output Buffer Frequency Selectivity
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