With so many excellent texts about analog integrated circuit design now available, the
need for yet another compilation of contributions may be questioned. Nevertheless, this
book fills a notable void, in addressing a topic that, while a common aspect of a product
designer's life, is only occasionally addressed in engineering texts. It is about Trade-
Offs: What they are; the circumstances in which they arise; why they are needed; how
they are managed, and the many ingenious ways in which their conflicting demands
can be resolved. We call it a Designer's Companion, since it is more in the nature of
a reference work, to dip into when and where some new perspectives on the topic are
needed, rather than a text to be read in isolation and absorbed as a whole. However, it
is an aspect of a trade-off that it is peculiar to each situation and there are no recipes for
their instant resolution. That being true, their treatment here is frequently by example,
suggestive rather than definitive. The personal insights, intuitions and inventiveness of
the designer remain vital to the pursuit of a well-balanced solution, but which is even
then only one of many, so its selection requires a relative-value judgment.
Understanding how to cope with trade-offs is an indispensable and inextricable
part of all engineering. In electronics, and particularly in analog design, the dilemmas
arise in the choice of basic cell topology, its biasing, the specific element values and
in making performance compromises. For example, wireless communication systems
are becoming increasingly sophisticated: they must operate at ever higher carrier frequencies,
while using increasingly complex modulation modes, and posing extremely
stringent performance demands. Meeting these requirements is only made more dif-
ficult as the dimensions of transistors and passive elements in modern IC processes
continue to shrink, and as time-to-market and cost pressures mount. Similar trends are
found throughout the field of electronics: in power management, fiber-optics, clock
generation for CPUs, high-precision instrumentation for signal generation and metrology,
and in analytical equipment of numerous kinds in science, industry, medicine and
more recently in forensics and security.
Simply stated, the need for a trade-off is generated by the dilemma of being faced
with a multiplicity of paths forward in the design process, each providing a different
set of benefits or posing different risks, and which can only be resolved by giving up
certain benefits in exchange for others of comparable value. The trade-off invariably
generates a constellation of considerations which are specific to each situation, within
a particular design context and set of circumstances that will often have never occurred
before, and whose resolution will have little general applicability.
It is these latter features that make writing about trade-offs so difficult: they are not
easy to anticipate in a systematic treatment, and they don't teach lessons of universal
applicability. Furthermore, a trade-off calls for creativity: it requires us to provide what
isn't there, in the data. Trade-offs cannot be made by tossing a coin; they are rarely of
an either-or character to begin with. The longer one mulls over the unique particulars,
the more likely it is that a panoply of solutions will present themselves, to be added to
one's bulging list of options. At some point, of course, ingenuity has to be curbed, and
a decision has to be made.
Edward de Bono has noted that "In the end, all [human] decisions are emotional".
In resolving a trade-off, our intervention as laterally thinking, resourceful individuals
is not required if the facts unequivocally speak for themselves, that is, if the resolution
of a transient dilemma can be achieved algorithmically. It involves selecting one from
several similarly attractive choices. We invariably try to apply all sorts of wisdom and
logic to our choice of which car or house to buy; but when logic fails to force the answer,
as it so often does, we fall back on emotion. The essential role of emotion as an intrinsic
part of rational intelligence and an ally to creative thought has recently been illuminated
by a fewpioneering psychologists. Intriguingly, in the index to Antonio Damasio's 1994
book Descartes' Error, one finds the entry "Decision making: see Emotion". Coping
with trade-offs also requires the inquisitive anticipation of the circumstances in which
they may arise, and a good deal of practice in playing out What If? scenarios. Joel
Arthur Barker1 makes this observation, in which we may want to substitute "the next
IC development" in place of "the new worlds coming":
Some anticipation can be scientific, but the most important aspect of anticipation
is artistic. And, just like the artist, practice and persistence will dramatically
improve your abilities. Your improved ability will, in turn, increase your ability
in dealing with the new worlds coming. [Emphases added]
Although often referred to as "an art" in casual conversation, circuit design is more
correctly viewed as a craft. The central emphasis in formal treatments of integrated
circuit design is generally on acquiring a thorough knowledge of the underlying electronic
principles, and of semiconductor processes and devices, aided by a fluency in
mathematics, familiarity with the particular domain of specialization under consideration,
and a basic ability for applying various pre-packaged concepts, techniques and
algorithms. But this hides the importance of developing the knack of making all the
right judgments in practicing this craft, and the value of cultivating a personal flair in
coping with the realities beyond the covers of the textbook.
Contrarily, from the layman's perspective, design is perceived as a linear intellectual
process, which proceeds something like this: One is faced with a set of objectives, and
then calls on experience to assemble all the pieces in a methodical, step-by-step fashion,
making fact-driven decisions along the way. As each part of the product is considered,
logic prevails at every juncture, and the whole gradually takes on a shape that is as
optimal as it is inevitable, to become another testament to the power of the underlying
rules and theories.
As a seasoned product designer, you will know that from the outset this will be far
from the reality. Inspired guesses (more charitably labeled "engineering judgments")
1 Joel Arthur Barker, Paradigms: The Business of Discovering the Future, 1994. This highly
recommended work was previously published in 1992 under the title Future Edge. By that
time anything with the word "Future" in its title was already becoming passé, so perhaps
it enjoyed only lackluster sales. By contrast, "Paradigms" was a very marketable word in
1994.
Foreword xxv
are scattered all along the path, from start to finish. To begin with, those Objectives,
that are supposed to inform every step of the proceedings and give the development
a sure sense of direction, are either insufferably detailed and give one a feeling of
being imprisoned in a straightjacket, or they are so comically sketchy and perhaps
mutually inconsistent, that anything approaching a focused, optimal solution is out of
the question. Regrettably, as your own experience may testify, both of these extremes
are all-too common, as well as every flavour in between. Each in its own way is
mischievously setting the stage for the first trade-off to be needed.
In the over-constrained scenario, one designer may be inclined to take a stab at
satisfying the provider of the objectives with the desired results, no less, but no more,
either: a just-right solution. This could be unwise, however, since the writer of these
specifications might be viewing the development in a way that is strongly influenced by
a prior discrete-element solution, and could be unaware of the special advantages that
can be provided by a monolithic implementation. On the other, this tactic might be the
right one if the product needs to meet only this one customer's need, and development
time is severely limited, and die cost must be minimized. Another designer might
adopt the opposite rationale: Sure, the product will meet all those fussy requirements,
but it could be capable of doing a lot more, too. By skillful design, many additional
applications and features can be anticipated, and the versatility extended to embrace
these, for little extra design effort or manufacturing cost. Thus, each of these two
designers is making a trade-off, right at the start, about how to interpret and react to
the challenge implicit in the specifications.
Similarly, when faced with scant information about what is needed of this new
product, one designer's approach might be to opt for caution, and painstakingly solicit
more detailed information from the provider of the objectives. This only generates
another trade-off, since the provider/user may in fact be no more informed than the
designer; but, perhaps to hide his ignorance, he will nonetheless generate more numbers
based on estimates and prior practice, in otherwords, more guesses. If these are received
and acted on with unmerited respect, the outcome could be a disaster. Alternatively,
if they are treated with disdain, and another set of guesses is substituted, the outcome
could be equally undesirable.
Meanwhile, a second designer may lean on her specialized experience with similar
products, and assume that the missing information can be adequately interpolated,
without the need for any further consultation. That tactic could work out well, or it
could be just the beginning of a monstrous headache for both the potential user and
the designer. In all these scenarios, it is painfully evident that the tools needed for
resolution of this particular dilemma will be found in no text book (including this one!)
and they each in their own way call for a trade-off to be made. And this before the
design has even begun.
These sketches also make us aware of the arbitrariness of the trade-off. It's an
idiosyncratic response to a dilemma. The more practiced the engineer, the more likely
it is that the majority of the hundreds of trade-offs that eventually will have to be made,
during the course of developing even a relatively straightforward analog circuit, will be
based on good judgment, and a balanced consideration of all the alternatives that came
to mind. But we cannot say that these decisions will be entirely rational, or optimal.
There are no algorithms for success.
This book covers ten subject areas: Design Methodology; Technology; General
Performance; Filters; Switched Circuits; Oscillators; Data Converters; Transceivers;
Neural Processing; and Analog CAD. It addresses a diversity of trade-offs ranging from
such well-known couplets as frequency versus dynamic range, or gain-bandwidth vs
power consumption, or settling-time vs phase-noise in PLLs, to some of the more
subtle trade-offs that arise in design for robustness in manufacture and in the "polygon
world" of IC layout. During its several years in development, it has transcended its
original scope, becoming a designer's desktop companion while also having value as a
graduate textbook, inasmuch as numerous fundamental relationships leading to design
conflicts are explained, in many cases with practical examples.
Its thirty-three chapters come from a variety of sources, including some of the
world's most eminent analog circuits and systems designers, to provide, for the first
time, a timely and comprehensive text devoted to this important aspect of analog circuit
design. Those authors who are professional designers are faced every day with difficult
decisions on which the success of their products depend, and not always with all
the analytic horsepower that may be demanded by some of the situations. Taken in
aggregate, the trade-offs that they choose eventually shape the competitive stature and
reputation of the companies for whom they work. Other authors allow themselves to
take a more academic view of the nature of a trade-off, and as a group are more inclined
to have greater optimism about the amenability of challenging circumstances to yield
to formal approaches, and even a degree automation.
The first section on Design Methodology opens with a discussion by Toumazou
about the nature and value of qualitative reasoning, in contrast to the usual emphasis in
engineering on the towering importance of quantitative analysis. The underlying need
for intuition, playful inventiveness and emotion in the pursuit of an engineering life is
picked up by Gilbert, in Chapters 2 and 33, although the more serious focus here is
nonetheless on making decisions within the context of commercial product development.
In all these chapters, the sheer breadth of the field allows only an introduction
to the subject matter.
The next three chapters, in the Technology section, range from the "Big Picture"
of VLSI, and in particular, some of the trade-offs in CMOS circuit development, as
explored by Mezhiba and Friedman, to the specific and detailed topic of bandgap
voltage references, as perceived by Staveren, Kouwenhoven, Serdijn and Verhoeven
(Chapter 5). Perched between these two chapters is a presentation of the less-familiar
floating-gate devices and circuits that have a unique, although limited, scope of applications
and might also comfortably fit into the later (and short) section on Neural
Processing, in Chapter 30 of which Hanggi, Dogaru and Chua discuss specialized
trade-offs in integrated neural networks.
In some cases, the emphasis is on the tension between two dominant aspects of
performance. This approach is particularly evident in the five chapters about General
Performance issues. A very basic trade-off is that which arises between amplifier
bandwidth and gain; this is discussed by Toumazou and Payne in Chapter 7, and from
a different perspective by Meyer in Chapter 8. Aspects of frequency compensation
in integrated amplifiers is explored in Chapter 9, by Staveren, Kouwenhoven, Serdijn
and Verhoeven. In amplifier design, one cannot increase bandwidth without regard for
noise, and this in turn is strongly influenced by the power consumption that one can
afford to assign to the amplifier. Noise and bandwidth are likewise linked by device
geometry. Attempts to push bandwidth may impact DC offsets or gain accuracy in
certain cases, or distortion and intermodulation in others. Thus, trade-offs are usually
multi-faceted, and in a very real way, nearly all the key specifications that will appear
in a product data sheet will be linked to a considerable extent. Vittoz and Tsividis face
up to these harsh realities in Chapter 10.
In the section on Filters, the many conflicts and compromises that surround
continuous-time active-filter design are addressed by Moschytz in Chapter 11, and
by Fox in Chapter 12. The particular way in which trade-offs arise in Log-Domain
(Translinear) Filters is discussed by Drakakis and Burdett in Chapter 13. The next
section is about Switched Circuits in general, and includes four differing perspectives.
The optimization of comparators is the focus of Chapter 14, by Rodríguez-Vázquez,
Delgado-Restituto, Domínguez-Castro and de la Rosa, while a general overview of
switched-capacitor circuits is presented by Baschirotto in Chapter 15, followed by a
review of the compatibility of such circuits with advanced digital technologies, provided
by Leelavattananon. This section closes with Chapter 17, which offers some
thoughts by Hughes and Worapishet about the differences and trade-offs that arise
between the standard switched-capacitor circuits that are now well established and the
less well-known switched-current forms that are sometimes viewed as equally useful,
in certain situations.
Communications circuits are a minefield of trade-offs, and the very stringent performance
required of Oscillators are examined in the Chapters 18 and 19 of this section.
In the first, by Ham, some of the special problems of maintaining low phase-noise
using the relatively poor on-chip components (principally low-Q inductors and lossy
varactors of limited range) are put under scrutiny. A different perspective on the same
subject is provided by Hajimiri. The next three chapters, in the section on Data Converters,
provide insights from the foremost exponents of these extremely important
gateways between the analog and digital domains. The first, which sets forth principles
for the systematic design of high-performance data converters, is authored by an
impressive team composed of Gielen, Vandenbussche, Van de Plas, Daems, den Bosch,
Steyaert and Sansen. The following Chapter 21 is more specialized in its approach:
Gielen and Lauwers discuss particular issues of power modeling for data converters
and filters. Chapter 22, authored by Schreier, Steensgaard and Temes, provides a
definitive account of the fundamental trade-off between speed and dynamic range in
over-sampled converters.
The focus next shifts to Transceivers, in several very different arenas. In Chapter
23, Abidi shares his considerable experience in the design of wireless circuits, and
the systems of which they are an integral part, where power conservation is a dominant
concern. This is followed by a review by Forbes of the design trade-offs that arise in
optical receivers. Finally, Chapter 25 closes this section with some considerations for
analog front-ends in digital subscriber-line systems. In all these cases, the overarching
challenge is the attainment of a very high dynamic range, entailing the simultaneous
provision of low distortion, of various disparate types, with a near-fundamental noise
floor. The endless search for low noise is also featured in Chapter 26, as illuminated by
Kouwenhoven, Staveren, Serdijn and Verhoeven, and again, noise and intermodulation
are the central challenges in mixer design, the topic of the next chapter by Kathiresan
and Toumazou. Phase detectors once bore a passing resemblance to mixers, and their
close cousin, the analog multiplier; but in today's phase-locked loops, there is a more
pressing need to capture both phase and frequency information. Some special techniques
are presented by Li and Ismail. The closing chapter of this section, authored by
Chan, Tu and Toumazou, looks at the trade-offs that arise in the design of various sorts
of power amplifiers.
The final section is concerned with CAD for analog design. Chandra and Roberts
present an overview of a design methodology for analog circuits using Matlab and
Simulink, while in Chapter 32, Gielen adds a concluding word about the possibilities
for using symbolic analysis tools for analog circuits.
Clearly, no book on the topic of trade-offs can ever be truly representative of the
entire field of analog design, nor exhaustive in its treatment of those subjects which do
get included. The primary function of any engineering text is to inform, and provide
accurate and authoritative guidance of both a general and specific sort. However, as
earlier suggested in this Foreword, and as these chapters testify, it is unlikely that very
many general recommendations can be made regarding trade-offs, and the specialized
case histories have a strictly limited scope of application. But another function of any
good text is to enthuse, to inspire, to illuminate the less-explored corners of the domain,
and to point the way to new perspectives on each topic. It is hoped that the material
assembled here serves that objective.
Barrie Gilbert
11 March 2002
