ACTIVE R-FILTERS - REVIEW OF THEORY AND PRACTICE

Active filters that derive their frequency response from internal amplifier dynamics, but use no external capacitors in their implementation, are referred to as 'active R' filters. Because of their potential advantages in terms of miniaturization (i. c. fabrication), ease of design and high-frequency performance, these circuits are receiving increasing attention in the technical literature. The paper reviews the published synthesis procedures for 1st-, 2nd- and higher-order active R filters; it is shown that the 1st- and 2nd-order circuits developed to date are special cases of a general active R topology. The paper discusses nonideal aspects and limitations of active R filter performance, which are usually neglected in the literature; in particular, the potentially severe effect of temperature-induced drift in the amplifier's dynamic response is considered, and methods of drift compensation are discussed. Experimental results are presented to verify filter performance and the theoretical derivations.