Continuous-Time Active Filter Design -KINGDWARF.pdf

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Deliyannis, Theodore L. et al "Frontmatter"
Continuous-Time Active Filter Design
Boca Raton: CRC Press LLC,1999
Theodore L. Deliyannis
University of Patras, Greece
Yichuang Sun
University of Hertfordshire, U.K.
J. Kel Fidler
University of York, U.K.
Continuous-Time Active Filter Design
©1999 CRC Press LLC
Preface
“In this digital age, who needs continuous-time filters?” Such an obvious question, and
one which deserves an immediate response. True, we do live in a digital age—digital com-
puters, digital communications, digital broadcasting. But, much though digital technology
may bring us advantages over analog systems, at the end of the day a digital system must
interface with the real world—the analog world. For example, to gain the advantages that
digital signal processing can offer, that processing must take place on
bandlimited
signals, if
unwanted aliasing effects are not to be introduced. After the processing, the signals are
returned to the real analog world after passing through a
reconstruction
filter. Both band-
limiting and reconstruction filters are analog filters, operating in
continuous time.
This is but
one example—but any system that interfaces with the real world will find use for continu-
ous-time filters.
The term
continuous-time
perhaps needs some explanation. There was the time when ana-
log filters were just that—they processed analog signals in real time, in contrast to digital
filters which performed filtering operations on digital representations of samples of sig-
nals, often not in real time. Then in the 1970s, along came sampled data filters. Sampled
data filters did not work with digital representations of the sampled signal, but operated
on the samples themselves. Perhaps the best known example of such an approach is that of
switched-capacitor filters,
which as the name suggests, use switches (usually transistor
switches) together with capacitors and active devices to provide filter functions. Note that
these filters are
discontinuous
in time as a result of the switching which takes place within
the circuits; indeed continuous bandlimiting and reconstruction filters are needed as a
result. Much research took place in the 1970s and 1980s on switched capacitor filters as a
result of the advantages for integrated circuit realization that they promised. There was so
much stress on research in this area that development of the more conventional analog fil-
ters received relatively little attention. However, when switched capacitor filters failed to
provide all the solutions, attention once again turned to the more traditional approaches,
and the name
continuous-time filters
was coined to differentiate them from their digital and
sampled data counterparts.
This book is about continuous-time filters. The classic LCR filters built with inductors,
capacitors, and resistors are such filters, of course, and indeed are still much in use. How-
ever, these filters are unsuitable for implementation in the ubiquitous integrated circuit,
since no satisfactory way of making inductors on chip has been found. That is why so much
attention has been paid to active continuous-time filters over the years. Active filters offer
the opportunity to integrate complex filters on-chip, and do not have the problems that the
relatively bulky, lossy, and expensive inductors bring—in particular their stray magnetic
fields that can provide unwanted coupling in a circuit or system. It is therefore active con-
tinuous-time filters on which we shall concentrate here.
As just mentioned, active filters have been around for some time as a means of overcom-
ing the disadvantages associated with passive filters (of which the use of inductors is one).
It is a sobering realization that the Sallen and Key circuit (which uses a voltage amplifier,
resistors, and capacitors, and is one of the most popular and enduring active-RC filter
“architectures”) has been around for about 40 years, yet research into active filters still pro-
ceeds apace after all that time. Tens of thousands of journal articles and conference papers
must have been published and presented over the years. The reasons are manifold, but two
©1999 CRC Press LLC

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