Chapter 5. Filter Design and Approximation

Summary

The subject of network synthesis became highly refined in the twentieth century due to the work of some very capable electrical engineers. To work through that body of knowledge would be beyond the present purpose of reviewing filter synthesis techniques. This chapter will therefore concentrate on basic techniques and solutions to the filter approximation problem and implementations that have come about by special requirements. At this point in time, operational amplifiers have not made a significant inroad to the RF frequency range, so active filters will not be treated here. Filters and impedance matching circuits are vital in the design of transistor amplifiers. The particular values of impedances needed for an amplifier depend on the device type and its orientation. Formulas for these impedances are summarized in Appendix D.

The solutions to the filter approximation problem and implementations that have arisen because of special requirements are treated in this chapter. The treatment is further limited to filters that perform certain tasks on continuous time varying analog signals by way of linear time-invariant circuit elements. Such filters are called analog filters. These analog filter functions are frequency selective circuits, and they can be analyzed in the time domain or in the frequency domain using Fourier transforms and Laplace transforms.