Active Simulation of Passive Ladder Filters

Abstract

Doubly resistively terminated LC ladder filters exhibit minimal element passband sensitivity, a property which can be maintained by active filter circuits that simulate this type of passive filters. The stopband behavior for such structures is also good due to the fact that the zeros are produced by appropriate resonant circuits. The major active simulation methods are presented in this Chapter. An active filter can be derived from a reference doubly terminated LC ladder filter by simply replacing the inductors with simulated inductors. This obvious simulation method preserves the ladder structure and the low sensitivity property. A second method is to use frequency dependent impedance scaling which introduces a new actively realizable circuit element, the frequency dependent negative resistance or supercapacitor. A third family of active simulation methods is derived from the general LTA (Linear Transformation Active) method which is presented in detail in this Chapter. It is also shown how some other simulation methods can be derived as special cases of the LTA method.