9.1 Introduction

A filter is a twoport that shapes the spectrum of the input signal in order to obtain an output signal with the desired frequency content. Thus, a filter has passbands where the frequency components are transmitted to the output and stopbands where they are rejected. Traditionally, such circuits working in the continuous-time domain have been designed as resistively terminated lossless LC filters ^[1, 2, 3 ^] where resonance could be employed to achieve complex poles and the desired steep transition regions between passbands and stopbands. With the growing pressure towards microminiaturisation, inductors were found to be too bulky so that designers started to replace passive RLC filters by active RC circuits where gain, obtained from operational amplifiers (op amps), together with resistors and capacitors in feedback networks, was used to achieve complex poles. Active filters based on the ubiquitous op amp have found wide acceptance throughout the industry over the last two or three decades and their mature development and technology guarantee their continued popularity in many applications.

One disadvantage of op amp-based active RC filters is the limited frequency range over which these circuits can be used: the finite bandwidth of op amps usually constrains the applications to be below 100 kHz, with performance deviations becoming increasingly worrisome and difficult to control as operating frequencies increase ^[3, 4, 5 ^].

Analog filters, even on-chip in fully integrated form, for operation in the megahertz range are beginning to...