Intuitive Analog Circuit Design. A Problem-Solving Approach using Design Case Studies

480 Pages·2006·10.13 MB·English

by  Marc T. Thompson

Preface
What? Another textbook on analog circuit design? Well, this is not a textbook per se; rather, it
is designed to be more of a design handbook for practicing engineers and students interested
in learning more real-world techniques for designing and analyzing analog circuits using tran-
sistors, diodes, and operational amplifiers. The hope is that the reader will find a good mixture
of theoretical techniques and also real-world design examples and test results.
This author is a practicing electrical engineer in the analog and power electronics realm who
also has had the opportunity to teach at Worcester Polytechnic Institute.
The careful reader, upon review of the chapter references, will note that I have a fondness for
using older references. This is due in part to the fact that the authors of these older texts and
papers did not have computers available to them for circuit simulations and mathematical
number-crunching. These references, in many cases, give very useful approximations, intui-
tive insights, and different ways of looking at difficult circuit analysis problems.
Intended Audience
This text is loosely based on a set of course notes designed for my graduate-level analog cir-
cuit design seminar offered at Worcester Polytechnic Institute. Students who take my course
have already taken undergraduate-level courses covering transistors, signal processing, Bode
plots and the like. Furthermore, it is the author's hope that the techniques shown in the book
will be useful for practicing analog (and perhaps even digital!) design engineers.
Text Outline
Chapters 1 and 2 offer introductory material. Chapter 1 serves as an introduction and motiva-
tion to analog circuit design in general, with selected history thrown into the mix. Chapter 2
covers important signal-processing concepts that are used in later chapters, so that the reader
will be on the "same page" as the author.
Chapters 3 to 8 cover the bipolar device physics, the bipolar junction transistor (BJT),
transistor amplifiers, and approximation techniques for bandwidth estimation and switching
speed analysis.
Chapter 9 covers the basics of CMOS and CMOS amplifiers. The bandwidth estimation tech-
niques developed in earlier chapters for amplifier design work well for CMOS devices as well.
xiii
Preface
Chapter 10 covers transistor switching. How do you get a transistor to turn ON and OFF
quickly, and how do you estimate that speed?
Chapter 11 is a review of feedback systems and of the Bode plot/phase margin method of
designing stable feedback systems.
Chapters 12-13 cover the design, use and limitations of real-world operational amplifiers,
including voltage-feedback and current-feedback op-amps.
Chapter 14 covers the basics of analog low-pass filter design, including ladder and active
implementations of Butterworth, Chebyshev, elliptic and Bessel filters.
Cha 9ter 51 covers real-world design issues such as PC board layout rules-of-thumb and the
use and limitations of passive components.
Cha 9ter 61 is a potpourri of useful design techniques and tricks that don't fit into the other
chapters.
Throughout the text, some illustrative analysis problems and MATLAB| and PSPICE design
examples are haphazardly sprinkled.
stnemgdelwonkcA
The author would like to acknowledge the learned professors and teaching assistants at the
Massachusetts Institute of Technology who taught him many of the techniques shown in
this book. They include Prof. Jim Roberge, Prof. Bill Siebert, Prof. Bill Peake, Prof. Marty
Schlecht, Leo Casey, Tom Lee, Prof. Harry Lee, Prof. Campbell Searle, Prof. Amar Bose, and
Prof. Dick Thornton.
This text has evolved from courses the author taught over the past years at Worcester Poly-
technic Institute. Therefore, further thanks go to W.P.I. and its students and faculty who
directly and indirectly contributed to this text. The author also acknowledges the indirect
contributions of his W.P.I. students who, through their probing questions and careful read-
ing of the course notes, have identified numerous typographical errors and half-truths, which
hopefully have been fully expunged from this edition.
The author gratefully acknowledges the patience and technical support provided by the staff
at Elsevier. The author also wishes to thank my friend and colleague Dr. Alexander Kusko,
who reviewed the manuscript and offered many useful suggestions. Also thanks to my friend
Dr. Jeff Roblee, who read the manuscript from a mechanical engineering point-of-view and
offered many useful suggestions.
Plots were created using MATLAB and the Microsim Student Version of PSPICE, version 8.0.
Marc .T Thompson
Halward Labs
Halward, Massachusetts
,rebotcO 2005
xiv
In tro du ctio n d n a Mo tiva tio n
ehT Need for Analog srengiseD
There is an inexorable trend in recent years to "go digital"~in other words, to do more and
more signal processing in the digital domain due to a purported design flexibility. However,
the world is an analog place and the use of analog processing allows electronic circuits to
interact with the physical world. Not discounting the importance of digital signal processing
(DSP) and other digital techniques, there are many analog building blocks such as operational
amplifiers, transistor amplifiers, comparators, A/D and D/A converters, phase-locked loops
and voltage references (to name just a few) that are still used and will be used far into the
future. Therefore, there is a continuing need for course development and education covering
basic and advanced principles of analog circuit design.
One reason why analog electronic circuit design is so interesting is the fact that it encom-
passes so many different disciplines. Here's a partial "shopping list," in no particular order, of
disciplines encompassed by the broad field of analog circuit design:
Analog filters: Discrete or ladder filters, active filters, switched capacitor filters,
crystal filters.
Audio amplifiers: Power op-amps, output (speaker driver) stages.
Oscillators: Including LC, crystal, relaxation and feedback oscillators, phase-locked
loops, video demodulation.
Device fabrication and device physics: Metal oxide semiconductor field effect
transistors (MOSFETs), bipolar transistors, diodes, insulated gate bipolar transistors
(IGBTs), silicon-controlled rectifiers (SCRs), MOS-controlled thyristors (MCTs), etc.
ICfabrication: Operational amplifiers, comparators, voltage references, PLLs, etc.
Analog-to-digital interface: A/D and D/A, voltage references.
Since we do live in a world where more and more digital processing is taking place, analog
designers must also become comfortable with digital-processing concepts so that we can all
work together. In the digital world, some subsystem designs are based on analog counterparts.
When designing a digital filter, one often first designs an analog prototype and then through
an analog-to-digital transformation the filter is converted to the digital domain. For example,
a bilinear transformation may be used where a filter designed in the s-domain (analog, using
inductors, capacitors, and/or active elements) is transformed to a filter in the z-domain (digi-
tal, with gain elements and delays).
This technique stems in part from the fact that designers are in general more comfortable
working in the analog domain when it comes to filtering. It's very easy to design a second-
order analog Butterworth filter (you can find the design in any number of textbooks or analog
filter cookbooks) but the implementation in the digital domain requires additional steps or
other simulation tools.
Also, at sufficiently high frequencies, a digital transmission line or a high-speed signal trace
on a PC board must be treated as a distributed analog system with traveling waves of voltage
and current. Increasing density of digital integrated circuits and faster switching speeds are
adding to the challenges of good PC board design due to extra power requirements and other
issues such as ground bounce.
The bottom line is... it behooves even digital designers to know something about analog design.
Some Early History of Technological Advances ni Analog
Integrated stiucriC
The era of semiconductor devices can arguably be traced back as far as Dr. Julius Lilienfeld,
who has several U.S. patents giving various MOS structures (Figure 1-1). In three patents,
Dr. Lilienfeld gave structures of the MOSFET, MESFET and other MOS devices.
Introduction and Motivation
We entered the bipolar transistor semiconductor era over 50 years ago with early work in
solid-state physics and the invention of the bipolar transistor, and significant technological
advances in analog circuit design and device fabrication are still being made. In 1947-1948,
Bardeen, Brattain and Shockley demonstrated the first bipolar transistors (Figure 1-2). ~
~dxaM ,7 .3391 ,J ,E!LU ~LEV 810,009,i
z,
l~ /~r -i ......
............... 0:-:
.... ~ C~ ~ ~ Jal
Figure I-I 6xcerot from ~'lclofoeili_l 0.5. t~ret~O 1,900,018 ~ (1933).
~1~ ~, !591 w. YELKCOHS 2,569,347
C~R~IT I~L~T :UTZLIZING $~CON~C?IT~ ~7~kL
riled e~a~J 26, 1~ 5 te~o%,-~teehS I
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!
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_j~ :.'n,nnn 'Z :__~ ' ,~l,1--.--.J
FtG. 3
s6:
orugH :2-1 s from 5hockloy's .5.U patont 2,569,.?47 (195/).
See U.S. Patent #2,569,347, "Circuit Element Utilizing Semiconductive Materials," issued
September 25, 1951 to William Shockley. Bardeen, Brattain and Shockley shared the 1956
Nobel prize in physics for their discoveries related to the transistor. An excellent description of
semiconductor transistor physics si given in Shockley's Nobel lecture "Transistor Technology
Evokes New Physics," dated December ,11 1956.
Lilienfeld had three patents in succession covering basic MOS transistor structures.
2
Chapter I
The first integrated circuits were produced around 1959 by teams at Fairchild Semiconduc-
tor and Texas Instruments (Figure 1-3). TI claims invention of the integrated circuit, with
J. S. Kilby's U.S. patent "Miniaturized Electronic Circuits" #3,138,743, filed Feb. 6, 1959.
Workers at Fairchild filed for a patent on the first planar IC (arguably more easily manufac-
tured than the TI invention) shortly after; see R. N. Noyce, "Semiconductor Device-and-Lead
Structure," U.S. patent # 2,981,877, filed July 30, .95911
These ICs had minimum feature sizes of around 125 micrometers. Since then, device
geometries have gotten smaller and smaller with the invention and rapid improvements in the
integrated circuit (IC). Moore's Law, named for Fairchild and Intel founder Gordon Moore,
predicts that the density of transistor packaging in integrated circuits doubles approximately
every 81 months, a trend that has proven to be remarkably accurate over the past 30 years.
At the time of this writing, 3 IC manufacturers are using 90-nanometer CMOS manufacturing
processes, and smaller transistor sizes are anticipated. The smaller size allows the packaging
of more and more complicated structures in a given die area. Researchers 4 are also actively
working on three-dimensional integrated circuit structures in an attempt to pack more and
more functionality into a given die volume.
After the invention of the integrated circuit around 1958-1959 by workers at Texas Instru-
ments and Fairchild, the first integrated-circuit operational amplifiers were introduced in the
J_
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" j~.~
~,~,;.~ ~~' __,,~.~
t tPrr~~ect~-T (j-~~o. p~.s I
L-F'UPNI
p,e.4
(b)
Figure 1-3: Diagrams from competing CI patents 5 from saxeT Instruments )a( and Fairchild .)b(
Fall, 2003; note that in 1983 a typical minimum linewidth in ICs was 5.1 micrometers (1500
3
nanometers). Manufacturers are aiming for 56 and even 50 nanometer gate lengths. See Gordon
Moore's paper "The Role of Fairchild in Silicon Technology in the Early Days of 'Silicon Valley' "
where the history of Fairchild IC development is recounted.
See, e.g., Matrix Semiconductor, Inc. and A" Vertical Leap for Microchips" by Thomas .H Lee.
4
5 Full text and images of patents are available from the U.S. Patent office, .vog.otpsu.www//:ptth
Introduction and Motivation
early to mid-1960s. The first commercially successful op-amps were the Fairchild laA709
(1965) and the National LM101 (1967), designed by the legendary analog wizard Bob
Widlar.6 These devices had a voltage offset of a few millivolts and a unity-gain bandwidth
of around a megahertz and required external components for frequency compensation. Soon
after (1968), the ubiquitous Fairchild laA741, the industry's first internally compensated
op-amp, was introduced and became a bestseller. In the 741, a 30-picofarad compensating ca-
pacitor was integrated onto the chip using metal-oxide technology. It was somewhat easier to
use than the LM101 because this compensating capacitor was added internal to the IC. 7 The
corresponding price reductions and specification improvements of the monolithic IC op-amps
as compared to the earlier discrete designs (put forth, for instance by Philbrick) 8 made these
IC op-amps instant successes.
Since that time, op-amps have been designed and introduced with significantly better voltage
offset and bandwidth specifications, as well as improvements in other specifications such as
input current, common mode range, and the like. FET input op-amps became available in the
1970s with lower input current than their bipolar counterparts. Novel topologies such as the
current-feedback op-amp have been introduced with success, for high-speed applications. 9
Typical high-speed op-amps today have bandwidths of hundreds of megahertz. ~~ Power
The earlier laA702 op-amp was designed by Widlar and introduced in 1963 by Fairchild but never
6
achieved much commercial success. Widlar went back to the drawing board and came up with the
709 around 1965; it was the first op-amp to cost less than $10. After a salary dispute with Fairchild,
Widlar moved to National Semiconductor where he designed the LM101 and later improved the
design resulting in the LM101A (1968). Details and history of the LM101 and 709 are given in the
Widlar paper "Design Techniques for Monolithic Operational Amplifiers" with citation given at the
end of this chapter.
The 147 does not need an external compensation capacitor as did previous op-amps such as the
7
LM101 and the 709. The "plug and play" ease of use of the 741 apparently offsets the fact that under
most applications with closed-loop gains greater than 1 the device is over-compensated. More details
on op-amp topologies are given in a later chapter in this book. Details and history of the 709, LM 101
and 147 op-amps are also given in Walt Jung's IC Op-Amp Cookbook, 3rd edition, pp. 75-98.
8 For instance, the Philbrick K2-W op-amp, made with discrete components (vacuum tubes!), and
sold from 1951 to 1971. It had a small signal bandwidth of around 300 kHz and an open-loop gain
of 10,000 or so. The units were priced at around $22. See the article by Bob Pease, "What's all
this K2-W Stuff, Anyway?" Philbrick also made the P2, a low input current discrete operational
amplifier built with a handful of transistors and other discrete components, and priced at around
$200. See, "The Story of the P2uThe First Successful Solid-State Operational Amplifier with
Picoampere Input Currents" by Bob Pease, found in Analog Circuit Design Art Science and
,seitilanosreP edited by Jim Williams.
The current-feedback op-amp does not have constant gain-bandwidth product as does the standard
9
voltage feedback op-amp.
0l See, e.g., National's LM6165 with a gain-bandwidth product (GBP) of 725 MHz, the Linear
Technology LT1818 with GBP -- 400 MHz, or the Analog Devices AD8001 with GBP = 600 MHz.
Chapter I
op-amps 11 exist that can drive speakers or other heavy resistive or inductive loads with several
amperes of load current. Low-power op-amps with sub-milliwatt standby power dissipation
are now commonplace. Rail-to-rail op-amps are now available.
These advances have opened new applications and product markets for devices based on ana-
log and digital signal processing. Currently, cellular telephone, cable television, and wireless
internet technologies are driving the business in RF analog circuit design and miniature hand-
held power electronics. Low-power devices enable the design of battery-powered devices
with long battery life.
Digital .sv Analog Implementation" Designer's Choice
In many instances, functions that might be implemented in the digital domain would be
difficult, costly and power-hungry to implement as compared to a relatively simple analog
counterpart. For instance, consider the design of a logarithmic amplifier. One can exploit the
well-known logarithmic/exponential voltage-current relationship 21 of a bipolar transistor oper-
ated in the forward-active region, as given by:
EsVq
)cI(
Ic = Is e kr
1-1
VB E kT
-- q In -~- s
This relationship holds over many orders of magnitude of transistor collector current. There-
fore, one can use a transistor PN junction to implement a low-cost logarithmic amplifier
erugiF( 1-4). The input-output transfer function of this circuit, assuming an ideal transistor
and op-amp, is:
v ~ - -~ln 1-2
q
This circuit provides an output voltage that is proportional to the natural logarithm of the
input voltage. An implementation in the digital domain would be considerably more involved.
The same principles can be applied to do analog multiplication. 3~
The logarithmic relationship between voltage and current in a bipolar transistor can be used
to implement analog multipliers, dividers and square root circuits. Let's look at the circuit of
l~ One example is the National LM12, also designed by Bob Widlar. An excellent IEEE paper
discussing the design of the LM12 is A" Monolithic Power Op Amp" with citation at the end of
this chapter.
One can also exploit this exponential relationship to design analog multipliers, such as the "Gilbert
21
Cell," and analog dividers and square and cube root circuits.
31 See, for instance, the translinear principle.
Introduction and Motivation
R
1.
o +
i-
Figure 1-4: Simple logarithmic amplifier.
erugiF 1-5. By using the translinear principle (discussed later in this book) we can find the
relationship between the various transistor collector currents"
3-1
Ic1Ic2 - Ic3Ic4
This means we can express the output current oI "sa
I o ~/IlI 2 1-4
-
ccV
o1~
I
0 I
I
I
I
I
I
/
/
Figure 1-5.A translinear circuit, where the output current si
equal to the square root of the product of the two inputs.
Now, consider the design of a fifth-order elliptic low-pass filter with a cutoff frequency of
5 megahertz. This is a typical specification for a video low-pass filter. This filter could be
implemented in the digital domain with discrete hardware or a digital signal processor.
You can implement this sharp-cutoff filter 4~ with just a handful of discrete components
erugiF( 1-6). Note that in this filter the source and termination resistances are each 75~,
Elliptic filters are commonly used in analog video filtering for anti-aliasing where a very sharp
cutoff transition band is required. Elliptic and other ladder filter designs are in tabulated form in
Anatol Zverev's Handbook of Filter Synthesis. And, yes, you can build practical analog filters using
inductors.
Chapter I
corresponding to the characteristic impedance of a typical video BNC cable. Again, an
implementation in the digital domain would be significantly more complicated, especially if a
high-frequency cutoff is required.
~L
L4
Vout
1V C3 -
92 F 1.9 Fp 326 pF~- 42 Fp 282 Fp T L
RL
~ --- _ _ 75
(a)
0 . ......................iltai:i'iOe;r ...........o.;.411. ..........;.i,;e,,,pt,ciii..i:'ii.i .................................................................... '.............................
,
ioo,_ i ............................. i ............. ! .....
-150~ i .............. r ................ , ............... , .................................................
zHKOOI zHKI(03 zHM0.1 3.0MHz zHMOI 30MHz zHMOOI
vdb(vout) ycneuqerF
(b)
Figure 1-6: Fifth order elliptic ladder filter with 5 MHz cutoff frequency.
)a( Circuit. )b( Frequency response.
,oS Why oD We emoceB Analog ?srengiseD
One possible answer to this hypothetical question is to note that in any given analog design
problem there is not one absolute, unique and correct answer, or "perfect" design. As a matter
of fact, if you think that you have arrived at the unique, perfect solution in the analog domain
you are undoubtedly mistaken. In the analog design space, there are infinities of possibilities
in how to implement a given function. The challenge, and eventually (hopefully )! the reward
to the analog designer, is to meet these requirements in a given design space meeting cost,
size, and/or performance constraints.