Fifteen Eighty Four

‘A transducer for separating waves on the basis of the frequencies’ or ‘electric filters’ is an interesting but wide study area on account of their ever-increasing areas of applications. Traditionally, filters are classified on the basis of type of passive or active components used which decided their useful frequency range of operation. For example, resonant cavity filters at microwave level, piezoelectric crystal unit and ceramic filters for MHz range and usually in the intermediate frequency range of radio reception. Lumped inductor and capacitor filter were primarily used for the frequency spectrum of 20 kHz to 100 kHz, but extendable to lower side up to few Hz or up to few MHz on higher side.

With the introduction of active devices, a new era of compact filtering started with a number of advantages while overcoming major limitations of passive filters, especially for the lower and sub-audio frequency band. With the advent of Silicon monolithic operational amplifier (OA), low cost mass production of active filters became possible in mid 1960s. So, a new subject of active RC filter design started. Since then with every stage in technology advancements and development of new design techniques progress in active analog filter continued and still continues. Active RC filter design methodology which was basically voltage-mode (VM) and OA based became almost synonymous with active filter design. Though refinements do continue, basics of design techniques are now well established for the continuous-time active OA RC filters. At the same, time other parallel techniques and approaches evolved, such as switched capacitor (SC) and current-mode (CM) filtering employing either OAs or CM devices.

With improved design techniques and fast paced advancements in fabrication technologies gave a big boost to active filter manufacturing and increased options in applications. One of the major areas of application of analog filters since beginning is tele-communication systems. Their earlier use included such applications as with PCM-CODEC chips for the digitization of telephones and dual-tone multi frequency signalling for touch-tone dialling.

Another major application of active filters is in the area of instrumentation, for example noise reduction in digital voltmeters, harmonic filtering in transformer vibrations.

Medical electronics became one another important area of applications where the frequency range of operation lies from very low to sub-audio frequency range. For example, processing of heart signal or Electro-cardiograph (ECG), signal from brain or Electro-encephalograph and Surface Electromyographic signal from muscles etcetera.

Other important areas of application include process control systems and entertainment electronics involving audio and video filtering and wave shaping. Pre- and post-filtering of analog to digital converters, spectrum analysers and seismology research as well employ active analog filters as almost essential part.

Many good books and vast literature are available on the design of VM and CM filters and SC filter approach basics, as well as, at advanced level. In addition, good books are also available where focus is on specialized field of study having examples of the usage of analog filters as a part of that specialized subject. However, in general, most of the books on filter design do not provide specific practical examples of the designed filters. Whereas, later type of books does not appear directly in the under graduate curricula. Cambridge University Press not provided an opportunity, but supported at every stage to present a text book forming a middle path in the form of ‘Continuous Time Active Analog Filters’. It tries to include updated basic filter design techniques and large number of simple practical filter applications. It is expected that inclusion of the examples will be exciting to readers, and even may become a motivating factor for advanced studies on analog filter design and that of allied fields like SC and CM filter design.

Muzaffer A. Siddiqi retired as Professor and Chairman from the Department of Electronics Engineering at Aligarh Muslim University, India. He has over 43 years of teaching, research and academic experience at undergraduate and postgraduate levels.