Introduction to CMOS OP-AMPs and Comparators - Hardcover

Gregorian, Roubik

Hardcover

ISBN 10: 0471317780 ISBN 13: 9780471317784

Publisher: Wiley-Interscience, 1999

Synopsis

A step-by-step guide to the design and analysis of CMOS operational amplifiers and comparators

This volume is a comprehensive text that offers a detailed treatment of the analysis and design principles of two of the most important components of analog metal oxide semiconductor (MOS) circuits, namely operational amplifiers (op-amps) and comparators. The book covers the physical operation of these components, their design procedures, and applications to analog MOS circuits-particularly those involving switched-capacitor circuits, and analog-to-digital (A/D) and digital-to-analog (D/A) converters. Roubik Gregorian, a leading authority in the field, gives circuit designers the technical knowledge they need to design high-performance op-amps and comparators suitable for most analog circuit applications. In this self-contained treatment, which is loosely based on his well-received 1986 book, Analog MOS Integrated Circuits for Signal Processing (coauthored with Gabor C. Temes), Gregorian reviews the required basics before advancing to state-of-the-art topics and problem-solving techniques. This valuable guide:
* Clearly explains configuration and performance limitation issues affecting the operation of CMOS op-amps and comparators
* Details advanced design procedures to improve performance
* Provides practical design examples suitable for a broad range of analog circuit applications
* Incorporates hundreds of illustrations into the text
* Concludes each chapter with problems and references to advanced topics, useful in textbook adoptions

Introduction to CMOS Op-Amps and Comparators is invaluable for analog and mixed-signal designers, for senior and graduate students in electrical engineering, and for anyone who would like to keep up with this essential technology.

About this title

About the Author

ROUBIK GREGORIAN is Senior Vice President, Chief Technology Officer, and General Manager for the Communications Division at Exar Corporation in Fremont, California. He is a well-known industry expert who has made many original contributions to analog MOS and switched-capacitor circuits. Dr. Gregorian earned his PhD and MSc in electrical engineering from the University of California at Los Angeles and his BSc in electrical engineering from Tehran University. He has published numerous papers, is the coauthor of Analog MOS Integrated Circuits for Signal Processing, and holds eighteen patents in the area of switched-capacitors and analog MOS circuits.

Excerpt. © Reprinted by permission. All rights reserved.

INTRODUCTION TO CMOS OP-AMPS AND COMPARATORS

By ROUBIK GREGORIAN

JOHN WILEY & SONS, INC.

Copyright © 1999 John Wiley & Sons
All right reserved.
ISBN: 978-0-471-31778-4

Contents

PREFACE...................................................................................................ix1 INTRODUCTION............................................................................................11.1 Classification of Signal Processing Techniques........................................................11.2 Examples of Applications of Op-Amps and Comparators in Analog MOS Circuits............................6Problems..................................................................................................16References................................................................................................162 MOS DEVICES AS CIRCUIT ELEMENTS.........................................................................172.1 Semiconductors........................................................................................172.2 MOS Transistors.......................................................................................212.3 MOS Transistor Types: Body Effect.....................................................................272.4 Small-Signal Operation and Equivalent Circuit of MOSFET Transistors...................................302.5 Weak Inversion........................................................................................392.6 Impact Ionization.....................................................................................402.7 Noise in MOSFETS......................................................................................412.8 CMOS Process..........................................................................................44Problems..................................................................................................45References................................................................................................473 BASIC ANALOG CMOS SUBCIRCUITS...........................................................................483.1 Bias Circuits in MOS Technology.......................................................................483.2 MOS Current Mirrors and Current Sources...............................................................553.3 MOS Gain Stages.......................................................................................633.4 MOS Source Followers..................................................................................743.5 MOS Differential Amplifiers...........................................................................773.6 Frequency Response of MOS Amplifier Stages............................................................84Problems..................................................................................................92References................................................................................................944 CMOS OPERATIONAL AMPLIFIERS.............................................................................954.1 Operational Amplifiers................................................................................954.2 Single-Stage Operational Amplifiers...................................................................1004.3 Two-Stage Operational Amplifiers......................................................................1064.4 Stability and Compensation of CMOS Amplifiers.........................................................1124.5 Dynamic Range of CMOS Op-Amps.........................................................................1264.6 Frequency Response, Transient Response, and Slew Rate of Compensated CMOS Op-Amps.....................1324.7 Noise Performance of CMOS Op-Amps.....................................................................1374.8 Fully Differential Op-Amps............................................................................1404.9 CMOS Output Stages....................................................................................1494.10 Op-Amps with Rail-to-Rail Input Common-Mode Range....................................................164Problems..................................................................................................170References................................................................................................1735 COMPARATORS.............................................................................................1755.1 Circuit Modeling of a Comparator......................................................................1755.2 Single-Ended Auto-Zeroing Comparators.................................................................1775.3 Differential Comparators..............................................................................1825.4 Regenerative Comparators (Schmitt Triggers)...........................................................1925.5 Fully Differential Comparators........................................................................1985.6 Latches...............................................................................................205Problems..................................................................................................212References................................................................................................2136 DIGITAL-TO-ANALOG CONVERTERS............................................................................2146.1 Digital-to-Analog Conversion: Basic Principles........................................................2146.2 Voltage-Mode D/A Converter Stages.....................................................................2186.3 Charge-Mode D/A Converter Stages......................................................................2316.4 Hybrid D/A Converter Stages...........................................................................2346.5 Current-Mode D/A Converter Stages.....................................................................2386.6 Segmented Current-Mode D/A Converter Stages...........................................................244Problems..................................................................................................252References................................................................................................2547 ANALOG-TO-DIGITAL CONVERTERS............................................................................2557.1 Analog-to-Digital Conversion: Basic Principles........................................................2557.2 Flash A/D Converters..................................................................................2637.3 Interpolating Flash A/D Converters....................................................................2707.4 Two-Step A/D Converters...............................................................................2737.5 Successive-Approximation A/D Converters...............................................................2827.6 Counting and Tracking A/D Converters..................................................................2947.7 Integrating A/D Converters............................................................................295Problems..................................................................................................300References................................................................................................3018 PRACTICAL CONSIDERATIONS AND DESIGN EXAMPLES............................................................3038.1 Practical Considerations in CMOS Op-Amp Design........................................................3038.2 Op-Amp Design Techniques and Examples.................................................................3168.3 Comparator Design Techniques and Examples.............................................................349Problems..................................................................................................355References................................................................................................355INDEX.....................................................................................................357

Chapter One

INTRODUCTION

Operational amplifiers (op-amps) and comparators are two of the most important building blocks for analog signal processing. Op-amps and a few passive components can be used to realize such important functions as summing and inverting amplifiers, integrators, and buffers. The combination of these functions and comparators can result in many complex functions, such as high-order filters, signal amplifiers, analog-to-digital (A/D) and digital-to-analog (D/A) converters, input and output signal buffers, and many more. Making the op-amp and comparator faster has always been one of the goals of analog designers. In this chapter the basic concept of digital and analog signal processing is introduced. Then a third category of signal processing, the sampled-data analog technique, which is in between the two main classifications, is described. Finally, a few representative examples are given of circuits and systems utilizing CMOS op-amps and comparators, to illustrate the great potential of these components as part of an MOS-LSI chip.

1.1 CLASSIFICATION OF SIGNAL PROCESSING TECHNIQUES

Electrical signal processors are usually divided into two categories: analog and digital systems. An analog system carries signals in the form of voltages, currents, charges, and so on, which are continuous functions of the continuous-time variable. Some typical examples of analog signal processors are audio amplifiers, passive- or active-RC filters, and so on. By contrast, in a digital system each signal is represented by a sequence of numbers. Since these numbers can contain only a finite number of digits (typically, coded in the form of binary digits, or bits) they can only take on discrete values. Also, these numbers are the sampled values of the signal, taken at discrete time instances. Thus both the dependent and independent variables of a digital signal are discrete. Since the processing of the digital bits is usually performed synchronously, a timing or clock circuit is an important part of the digital system. The timing provides one or more clock signals, each containing accurately timed pulses that operate or synchronize the operation of the components of the system. Typical examples of digital systems are a general-purpose digital computer or a special-purpose digital signal processor dedicated to (say) calculating the Fourier transform of a signal via the fast Fourier transform (FFT), or a digital filter used in speech analysis, and so on.

By contrast, analog signal processing circuits utilize op-amps, comparators, resistors, capacitors, and switches to perform such functions as filters, amplifiers, rectifiers, and many more. To understand the basic concepts of the most commonly used configurations of an analog circuit, consider the simple analog transfer function

[V.sub.out](s) / [V.sub.in](s) 4 b / [s.sup.2] ' as ' b (1.1)

It is easy to verify that the RLC circuit shown in Fig. 1.1a can realize this function (Problem 1.1). Although this circuit is easy to design, build, and test, the presence of the inductor in the circuit makes its fabrication in integrated form impractical. In fact, for low-frequency applications, this circuit may well require a very large valued, and hence bulky, inductor and capacitor. To overcome this problem, the designer may decide to realize the desired transfer function using an active-RC circuit. It can readily be shown that the circuit in Fig. 1.1b, which utilizes three operational amplifiers, is capable of providing the transfer function specified in Eq. (1.1). This circuit needs no inductors and may be realized with small discrete components for a wide variety of specifications (Problem 1.2). It turns out, however, that while integration of this circuit on a bipolar chip is, in principle, feasible (since the amplifiers, resistors, and capacitors needed can all be integrated), there are some major practical obstacles to integration. These include the very large chip area needed by the RC components, as well as the stringent accuracy and stability requirements for these elements. These requirements cannot readily be satisfied by integrated components, since neither the fabricated values nor the temperature-induced variations of the resistive and capacitive elements track each other. The resulting pole-zero variations are too large for most applications.

Prior to mid-1970s, analog circuits such as the one shown in Fig. 1.1 were implemented using integrated bipolar op-amps and discrete passive components. In the 1970s two developments made it possible to fully integrate analog circuits in metal-oxide semiconductor (MOS) technology. The first development was the emergence of a technique called switched-capacitor (SC) circuits, which is an effective strategy for solving both the area and the matching problems by replacing each resistor in the circuit by the combination of a capacitor and a few switches. Consider the branches shown in Fig. 1.2. Here, the four switches [S.sub.1], [S.sub.2], [S.sub.3], and [S.sub.4] open and close periodically, at a rate which is much faster than that of the variations of the terminal voltage [v.sub.A] and [v.sub.B]. Switches [S.sub.1] and [S.sub.4] operate synchronously with each other but in opposite phase with [S.sub.2] and [S.sub.3]. Thus when [S.sub.2] and [S.sub.3] are closed, [S.sub.1] and [S.sub.4] are open, and vice versa. Now when [S.sub.2] and [S.sub.3] close, ITLITL is discharged. When [S.sub.2] and [S.sub.3] open, [S.sub.1] and [S.sub.4] close, and ITLITL is recharged to the voltage [v.sub.C] 4 [v.sub.A] 1 [v.sub.B]. This causes a charge q 4 ITLITL([v.sub.A] 1 [v.sub.B]) to flow through the branch of Fig. 1.2. Next, ITLITL is again discharged by [S.sub.2] and [S.sub.3], and so on. If this cycle is repeated every T seconds (where T is the switching period or clock period), the average current through the branch is then

[i.sub.av] 4 q/T 4 C/T ([v.sub.A] 1 [v.sub.B]). (1.2)

Thus [i.sub.av] is proportional to the branch voltage [v.sub.A] 1 [v.sub.B]. Similarly, for a branch containing a resistor R, the branch current is i 4 (1/R)([v.sub.A] 1 [v.sub.B]). Thus the average current flowing in these two branches are the same if the relation R 4 T/C holds. Physically, the switches transform the capacitor ITLITL, a nondissipative memoried element, into a dissipative memoryless (i.e., resistive) one.

It is plausible therefore that the branch of Fig. 1.2 can be used to replace all resistors in the circuit of Fig. 1.1b. The resulting stage is shown in Fig. 1.3. In this circuit, switches that belong to different "resistors" but perform identical tasks have been combined. Furthermore, the second operational amplifier (op-amp) in Fig. 1.1b, which acted merely as a phase inverter, has been eliminated. This was possible since by simply changing the phasing of two of the switches associated with capacitor [ITLITL.sub.3], the required phase inversion could be accomplished without an op-amp.

As Fig. 1.3 illustrates, the transformed circuit contains only capacitors, switches, and op-amps. A major advantage of this new arrangement is that now all time constants, previously determined by the poorly controlled RC products, will be given by expressions of the form (T/ITL.sub.1])[ITLITL.sub.2] 4 T([C.sub.2]/[C.sub.1]). Here the clock period T is usually determined by a quartz-crystal-controlled clock circuit and hence is very accurate and stable. The other factor of the time constant is [ITLITL.sub.2]/[ITLITL.sub.1], that is, the ratio of two on-chip MOS capacitances. Using some simple rules in the layout of these elements, it is possible to obtain an accuracy and stability on the order of 0.1% for this ratio. The resulting overall accuracy is at least 100 times better than what can be achieved with an on-chip resistor and capacitor for the RC time constant.

A dramatic improvement is also achievable for the area required by the passive elements. To achieve a time constant in the audio-frequency range (say 10 krad/s), even with a large (10-pF) capacitor, a resistance of 10-MV is required. Such a resistor will occupy an area of about [10.sup.6] [mm.sup.2], which is prohibitively large; it is nearly 10% of the area of an average chip. By contrast, for a typical clock period of 10 ms, the capacitance of the switched capacitor realizing a 10-MV resistor is ITLITL 4 T/R 4 [10.sup.25]/[10.sup.7] 4 [10.sup.112] F 4 1 pF. The area required realizing this capacitance is about 2500 [mm.sup.2], only 0.25% of that needed by the resistor that it replaces.

The second development that made the realization of the fully integrated analog MOS circuits possible was the design of the MOS op-amp. Perhaps the most generally useful analog circuit function is that of the operational amplifier. Prior to about 1977, there existed a clear separation of the bipolar and MOS technologies, according to the function required. MOS technology, with its superior device density, was used mostly for digital logic and memory applications, while all required analog functions (such as amplification, filtering, and data conversion) were performed using bipolar integrated circuits, such as bipolar op-amps. Since that time, however, rapid progress made in MOS fabrication techniques made it possible to manufacture much more complex and flexible chips. In addition, new developments occurred in communication technology (such as digital telephony, data transmission via telephone lines, adaptive communication channels, etc.) which required analog and digital signal processing circuitry in the same functional blocks. The analog functions most often needed are filtering (for antialiasing, smoothing, band separation, etc.), amplification, sample-and-hold operations, voltage comparison, and the generation as well as precise scaling of voltages and currents for data conversion. The separation of these analog functions from the digital ones merely because of the different fabrication technologies used is undesirable, since it increases both the packaging costs and the space requirements and also, due to the additional interconnections required, degrades the performance. Hence there was strong motivation to develop novel MOS circuits, which can perform these analog functions and which can also share the area on the same chip with the digital circuitry.

Compared with bipolar technology, MOS technology has both advantages and disadvantages. MOS device has extremely high impedance at its input (gate) terminal, which enables it to sense the voltage across a capacitor without discharging it. Also, there is no inherent offset voltage across the MOS device when it is used as a charge switch. Furthermore, high-quality capacitors can be fabricated reliably on an MOS chip. These features make the realization of such circuits as precision sample-and-hold stages feasible on an MOS chip. This is usually not possible in bipolar technology.

On the negative side, the transconductance of MOS transistors is inherently lower than that of bipolar transistors. A typical transconductance value for a moderate-sized MOS device is around 2.5 mA/V; for a bipolar transistor, it may be about 50 times larger. This leads to a higher offset voltage for an MOS amplifier than for a bipolar amplifier. (At the same time, however, the input capacitance of the MOS transistor is typically much smaller than that of a bipolar transistor.) Also, the noise generated in an MOS device is much higher, especially at low frequencies, than in a bipolar transistor. The conclusion is that the behavior of an amplifier realized on an MOS chip tends to be inferior to an equivalent bipolar realization in terms of offset voltage, noise, and dynamic range. However, it can have much higher input impedance than that of its bipolar counterpart.

As a result of these properties, the largest use of the MOS op-amp is expected to be as part of an MOS-LSI (large-scale integration) chip. Here the design of the op-amp can take advantage of the important performance specifications that are needed. The loading of the op-amp is often very light and usually only a small-valued capacitor has to be driven by these op-amps. Switched-capacitor circuits fall especially into this category, where element-value accuracy is important but the signal frequency is not too high and the dynamic range required is not excessive. Voice- and audio-frequency filtering and data conversion are in this category and represent the bulk of the past applications.

In addition to frequency-selective switched-capacitor filtering introduced in Fig. 1.3, which has been the most common application of MOS op-amps, there are many other functions for which op-amps and comparators can be used. These include analog-to-digital (A/D) and digital-to-analog (D/A) data conversion, programmable-gain amplification for AGC and other applications, peak-detection, rectification, zero-crossing detection, and so on. They have also been used extensively in large mixed-signal analog/digital systems such as voice codecs, high-speed data communication moderns, audio codecs, and speech processors. This range will expand continuously as the quality (bandwidth, dynamic range, power consumption, etc.) of the components, especially op-amps and comparators, improves.

1.2. EXAMPLES OF APPLICATIONS OF OP-AMPS AND COMPARATORS IN ANALOG MOS CIRCUITS

In this section, a few selected examples of practical analog MOS circuits are given where CMOS op-amps and comparators are used extensively. Of course, the reader should not expect to understand the details of these systems at this stage. However, the diagrams may give an idea of the potentials of these components in analog signal processing.

As mentioned earlier, one of the most important applications of CMOS op-amps is in switched-capacitor filters. Figure 1.4a shows the circuit diagram of a seventh-order switched-capacitor filter. Its measured frequency response is shown in Fig. 1.4b. The measured passband variation for the device is less than 0.06 dB. This represents a superior performance, which could not have been achieved without extensive trimming using any other filter technology.

An obvious application of a CMOS op-amp is the realization of charge-mode digital-to-analog converters (DAC). It can be obtained by combining a programmable capacitor array and an offset-free switched-capacitor gain stage. An example of an N-bit charge-mode DAC is shown in Fig. 1.5, where [V.sub.ref] is a temperature-stabilized constant reference voltage. The output of the DAC is the product of the reference voltage and the binary-coded digital signal ([b.sub.1], [b.sub.2], [b.sub.3], ..., [b.sub.N]). In Chapter 6 the design of such circuits is discussed in some detail.

Modulators, rectifiers, and peak detectors belong to an important class of nonlinear circuits, which can be implemented with a combination of op-amps and comparators. In an amplitude modulator the amplitude of a signal x(t) (usually called the carrier) is varied (modulated) by m(t), the modulating signal. Hence the output signal y(t) is the product of x(t) and m(t), or y(t) 4 x(t)m(t). A periodic carrier signal, which is readily generated from a stable clock source, is a square wave alternating between two equal values 5V. An easy way to perform modulation with a square-wave carrier is to switch the polarity of the input signal m(t) periodically. A stray-insensitive switched-capacitor modulator circuit which performs according to this principle is shown in Fig. 1.6. The clock phases [f.sub.1] and [f.sub.2] are operated at the fast clock rate [v.sub.c], while the phase [f.sub.a] changes at the slow carrier-frequency rate [v.sub.ca]. Normally, [v.sub.c] is much larger (by a factor of 30 or more) than [v.sub.ca].

(Continues...)

Excerpted from INTRODUCTION TO CMOS OP-AMPS AND COMPARATORSby ROUBIK GREGORIAN Copyright © 1999 by John Wiley & Sons. Excerpted by permission.
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Edition details

• Publisher: Wiley-Interscience
• Publication date: 1999
• Language: English
• ISBN 10: 0471317780
• ISBN 13: 9780471317784
• Binding: Hardcover
• Edition number: 1
• Number of pages: 360