Elements of Active Filter Synthesis
Davide Bucci · 2017
This chapter provides an understanding of the problems and the terminology, as well as a design method for simple active filters. It presents some elements of synthesis strategy as well as some tables of filter coefficients useful for active filters. The chapter focuses on all-pole low-pass filters, and shows how other kinds of filters can be traced back to an equivalent low-pass filter calculation. For an all-pole filter, one needs some circuits (cells) able to synthesize: a couple of complex-conjugate poles; and a real negative pole. For a couple of complex-conjugate poles, a widespread solution is the Sallen-Key filter cell. The frequency transform is a combination of a low-pass and a high-pass filter response. Frequency transforms yield to approximation of functions having behaviors different from low-pass. In this case, different kinds of cells must be employed for the synthesis of the circuit.