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The Art of Electronics
is considered by many to be the 'Bible' of electronic design. The book
is a very practical, well written guide to electronics. The authors
emphasize the methods actually used by circuit designers--a combination
of basic laws, rules of thumb, and a large bag of tricks. It is
written in a very readable style, and with just a touch of humor.
The Art of Electronics
 Excerpt:
Chapter 7: Precision Circuits and Low-Noise Techniques
In the preceding chapters we have dealt with many aspects of analog circuit design,
including the circuit properties of passive devices, transistors, FETs, and op-amps, the
subject of feedback, and numerous applications of these devices and circuit methods. In
all our discussions, however, we have not yet addressed the question of the best that can
be done, for example, in minimizing amplifier errors (nonlinearities, drifts, etc.) and in
amplifying weak signals with minimum degradation by amplifier "noise." In many
applications these are the most important issues, and they form an important part of the
art of electronics. In this chapter, therefore, we will look at methods of precision
circuit design and the issue of noise in amplifiers. With the exception of the
introduction to noise in Section 7.11, this chapter can be skipped over in a first
reading. This material is not essential for an understanding of later chapters. |
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Precision Op-Amp Design Techniques
In the field of measurement and control there is often a need for circuits of high
precision. Control circuits should be accurate, stable with time and temperature, and
predictable. The usefulness of measuring instruments likewise depends on their accuracy
and stability. In almost all electronic subspecialties we always have the desire to do
things more accurately -- you might call it the joy of perfection. Even if you don't
always actually need the highest precision, you can still delight in the joy of fully
understanding what's going on.
7.01: Precision versus dynamic range
It is easy to get confused between the concepts of precision and dynamic range, especially
since some of the same techniques are used to achieve both. Perhaps the difference can
best be clarified by some examples: A 5-digit multimeter has high precision; voltage
measurements are accurate to 0.01% or better. Such a device also has wide dynamic range;
it can measure millivolts and volts on the same scale. A precision decade amplifier (one
with selectable gains of 1, 10, and 100, say) and a precision voltage reference may have
plenty of precision, but not necessarily much dynamic range. An example of a device with
wide dynamic range but only moderate accuracy might be a 6-decade logarithmic amplifier
(log amp) built with carefully trimmed op-amps but with components of only 5% accuracy;
even with accurate components a log amp might have limited accuracy because of lack of log
conformity (at the extremes of current) of the transistor junction used for the
conversion. Another example of a wide-dynamic-range instrument (greater than 10,000:1
range of input currents) with only moderate accuracy (1%) is the coulomb meter described
in Section 9.26. It was originally designed to keep track of the total charge put through
an electrochemical cell, a quantity that needs to be known only to approximately 5% but
that may be the cumulative result of a current that varies over a wide range. It is a
general characteristic of wide-dynamic-range design that input offsets must be carefully
trimmed in order to maintain good proportionality for signal levels near zero; this is
also necessary in precision design, but, in addition, precise components, stable
references, and careful attention to every possible source of error must be used to keep
the sum total of all errors within the so-called error budget.
7.02: Error budget
A few words on error budgets. There is a tendency for the beginner to fall into the trap
of thinking that a few strategically placed precision components will result in a device
with precision performance. On rare occasions this will be true. But even a circuit
peppered with 0.01% resistors and expensive op-amps won't perform to expectations if
somewhere in the circuit there is an input offset current multiplied by a source
resistance that gives a voltage error of 10mV, say. With almost any circuit there will be
errors arising all over the place, and it is essential to tally them up, if for no other
reason than to locate problem areas where better devices or a circuit change might be
needed. Such an error budget results in rational design, in many cases revealing where an
inexpensive component will suffice, and eventually permitting a careful estimate of
performance.
7.03: Example circuit: precision amplifier with automatic null offset
In order to motivate the discussion of precision circuits, we have designed an extremely
precise decade amplifier with automatic offset. This gadget lets you "freeze"
the value of the input signal, amplifying any subsequent changes from that level by gains
of exactly 10, 100, or 1000. This might come in particularly handy in an experiment in
which you wish to measure a small change in some quantity (e.g., light transmission or
radio frequency absorption) as some condition of the experiment is varied. It is
ordinarily difficult to get accurate measurements of small changes in a large dc signal,
owing to drifts and instabilities in the amplifier. In such a situation a circuit of
extreme precision and stability is required. We will describe the design choices and
errors of this particular circuit in the framework of precision design in general, thus
rendering painless what could otherwise become a tedious exercise. A note at the outset:
Digital techniques offer an attractive alternative to the purely analog circuitry used
here. Look forward to exciting revelations in chapters to come!
Circuit description: The basic circuit is a follower (U1) driving an
inverting amplifier of selectable gain (U2), the latter offsettable by a signal applied to
its noninverting input. Q1 and Q2 are FETs, used in this application as simple analog
switches; Q3 - Q5 generate suitable levels, from a logic-level input, to activate the
switches. Q1 through Q5 and their associated circuitry could all be replaced by a relay,
or even a switch, if desired, For now, just think of it as a simple SPST switch.
When the logic input is HIGH ("autozero"), the switch is closed, and U3 charges
the analog "memory" capacitor (C1) as necessary to maintain zero output. No
attempt is made to follow rapidly changing signals, since in the sort of application for
which this was designed the signals are essentially dc, and some averaging is a desirable
feature. When the switch is opened, the voltage on the capacitor remains stable, resulting
in an output signal proportional to the wanderings of the input thereafter.
There are a few additional features that should be described before going on to explain in
detail the principles of precision design as applied here: (a) U4 participates in a
first-order leakage-current compensation scheme, whereby the tendency of C1 to discharge
slowly through its own leakage (100,000M, minimum, corresponding to a time constant of 2
weeks!) is compensated by a small charging current through R15 proportional to the voltage
across C1. (b) Instead of a single FET switch, two are used in series in a "guarded
leakage-cancellation" arrangement. The small leakage current through Q2, when
switched OFF, flows to ground through R23, keeping all terminals of Q1 within millivolts
of ground. Without any appreciable voltage drops, Q1 hasn't any appreciable leakage! (See
Section 4.15 and Fig. 4.50 for similar circuit tricks.) (c) The offsetting voltage
generated at the output of U3 is attenuated by R11 - R14, according to the gain setting.
This is done to avoid problems with dynamic range and accuracy in U3, since drifts or
errors in the offset holding circuitry are not amplified by U2 (more on this later)...
Table of Contents:
Preface
Preface to first edition
Chapter 1: Foundations
Chapter 2: Transistors
Chapter 3: Field-effect transistors
Chapter 4: Feedback and operational amplifiers
Chapter 5: Active filters and oscillators
Chapter 6: Voltage regulators and power circuits
Chapter 7: Precision circuits and low-noise techniques
Chapter 8: Digital electronics
Chapter 9: Digital meets analog
Chapter 10: Microcomputers
Chapter 11: Microprocessors
Chapter 12: Electronic construction techniques
Chapter 13: High-frequency and high-speed techniques
Chapter 14: Low-power design
Chapter 15: Measurements and signal processing
Appendix A: The oscilloscope
Appendix B: Math review
Appendix C: The 5% resistor color code
Appendix D: 1% Precision resistors
Appendix E: How to draw schematic diagrams
Appendix F: Load lines
Appendix G: Transistor saturation
Appendix H: LC Butterworth filters
Appendix I: Electronics magazines and journals
Appendix J: IC prefixes
Appendix K: Data sheets
Bibliography
Index
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