Switched-Current Filters

Abstract

In this chapter, the design of switched-current filters, employing the CMOS building blocks and circuits developed in previous chapters, is considered and the results of two integrated test filters presented. The chapter starts by considering the synthesis of switched-current filters based on biquadratic sections. Sections based on either integrators or differentiators are described and measured results from an integrated integrator based lowpass section are presented to demonstrate the technique. For the realisation of high-order or high-g filters, the ladder approach is very popular because of its low sensitivity properties. This approach is described, with the aid of an illustrative integrated bilinear fifth order elliptic lowpass filter example, which demonstrates the practical application of the technique. The next three filter synthesis approaches; FIR filters, wave active filters and the transposition of switched-capacitor designs, are only briefly discussed as detailed descriptions can be found in Chapter 10, 11 and 9, respectively. Finally, this chapter proposes a new approach to the realisation of tunable filters on pure digital processes. The proposed switched-transconductance (ST) technique takes advantage of the fact that in many applications switched current circuits need a voltage interface, and hence also a linear transconductor; by incorporating this transconductor with the switched current circuits tunable impedances with good inherent accuracy can be realised.