ABSTRACT
All active elements are useful in the realization of filter functions, although some of them are more useful than others. Among these, the operational amplifier (opamp) is the most versatile active element in use today up to frequencies of the order of 100 kHz. This chapter describes first- and second-order stages using the opamp(s) as their active element(s). Alongside its use in the integrator circuit, the opamp can be also used in the realization of other first-order transfer functions, which are useful in filter design. Such functions are lowpass, highpass, and allpass. Although the lowpass and highpass functions can be realized using RC circuits only, the presence of the opamp in the circuit can provide it with gain and isolation from the circuit that follows it. An abundance of circuits realizing, in particular, second-order functions have been proposed over the past 30 to 35 years, some of which are more “suitable” than others.
