9 - Active filters

Abstract

This chapter focuses on active filters employing the operational amplifiers (op-amps). The active filters replace emitters by op-amps in the form of integrated circuits. The frequency-sensitive filtering networks are either placed before the op-amp inputs or in the feedback circuits. First-order low-pass filters are often used to perform the running average of a signal having high-frequency fluctuations superimposed on a relatively slow mean variation. For this purpose, it is simply necessary to make the filter time constant CR much greater than the period of the high-frequency fluctuations. A practical point to remember is that all op-amp active high-pass filters show a band-pass characteristic. This is because their response eventually fails at frequencies that exceed the closed-loop bandwidth of the op-amp. The Sallen–Key second-order active filters are relatively simple to construct, in a way that they operate as expected. Although they are simple to be constructed, the various component values must have a definite relationship that is a function of the circuit Q-factor. Moreover, frequency-dependent negative resistance (FDNR) circuits can be used to make an active filter based on a passive ladder filter. The advantage of using the FDNR lies with the fact that the passive ladder circuits have low sensitivity to component tolerance.