Published online by Cambridge University Press: 17 December 2010
Analog filters, essential parts in many different electronic systems, can be implemented in CMOS using switched-capacitor (SC), switched-current (SI), active-RC, MOSFET-C, or OTA-C techniques. Owing to their considerable importance in analog CMOS circuits, Chapter 10 will focus on SC circuits. In this chapter, we study the MOSFET-C and OTA-C techniques, which have been the most important ones for the realization of continuous-time (CT) integrated filters in CMOS technologies. Applications of CT filters include anti-aliasing and reconstruction filters, read channels of disk drives, data-communication circuits, and high-speed data links. A disadvantage of CT filters is that the filter coefficients are sensitive to process and temperature variations and aging. Therefore, tuning of the components that determine the frequency response is required. In this chapter, we will concentrate on the fundamental aspects of filter design at the component level and on the basic building blocks for each technique. We will describe some of the main limitations of the MOSFET-C and OTA-C techniques and the concepts of some tuning schemes used for keeping the filter transfer function close to the nominal specifications. The synthesis of high-order filters, which can be found in more specialized texts such as, is not the subject of this chapter.
Basics of MOSFET-C filters
The MOSFET-C (MOSFET-capacitor) is a mature technique for the integration of CT filters in CMOS technologies. MOSFET-C circuits are similar to the active-RC topologies composed of operational amplifiers, resistors, and capacitors.
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