Analog active filters

This paper highlights major contributions, during the past half century, in the design of analog active networks with emphasis on active filters in the audiofrequency range. It is not possible to include the contributions of everyone in the field, the author, therefore, acknowledges and apologizes for the omission of the many noteworthy contributions not listed in this paper. Comprehensive lists of references are, however, available in the two IEEE edited books on active filters [1], [2]. John Linvill has summarized [3] the three decades of active networks, from the 1930\´s to the 1950\´s as follows: "The decade of the 1930\´s was one of tremendous strides in the synthesis of passive networks. The procedures established in that time are the backbone of the attack on many of today\´s designs of active networks, which frequently reduce to the appropriate synthesis of the passive parts of the system. The 1940\´s saw the full-scale development of the feedback system and the assessment of the limitations inherent in the parasitic elements that always accompany active components. The problem of realization of low-drift structures in the presence of drifting elements was attacked. The first encounter of the network theorist with elements that refuse to behave like a simple model brought new ideas and approaches. The 1950\´s were the decade of the transistor. The transistor, less capable of simple representation than its predecessors, offered new capabilities to reward the network designer. The physicist and the network theorist, each being more dependent on the other, have been drawn together. Many of the old problems have been resolved, without the earlier simplifying assumptions." The decade of the Sixties saw the emergence of the operational amplifier (op amp) as an off-the-shelf active element. The monolithic op amp introduced commercially in the late 1960\´s revived interest and stimulated much research work on active filters. The op amp is undoubtedly the most important linear circuit element to date. Sensitivity considerations also became an important research topic in its own right. During this decade a number of research papers also appeared in the area of distributed networks, resulting in active- filters (publications are listed in [4], 151). In the early to mid-1970\´s, the active- filter became a practical product with widespread use. In the late-1970\´s the switched-capacitor element came on the scene as a practical replacement for resistors in analog MOS integrated circuits. The partnership of MOS op amps and switched-capacitor networks accelerated work in active filters. The rapid development in the field and its compatibility with large scale integration (LSI) has made switched-capacitor filters one of the dominant technologies in active filters in the early-1980\´s and are expected to do so well into the 1980\´s.