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314 IEEE JOURNAL OF SOLID-STATECIRCUITS, VOL. sC-9, NO. 6, DECEMBER 1974 The Monolithic Op Amp: A Tutorial Study JAMES E. SOLOMON, MEMBER, IEEE Invited Paper Absfracf—A study is made of the integrated circuit operational amplifier (IC op amp) to explain details of its behavior in a simplified and understandable manner. Included are analyses of thermal feedback effects on gain, basic relationships for bandwidth and slew rate, and a discussion of pole-splitting frequency compensation. Sources of second-order bandlimiting in the amplifier are also identified and som(s approaches to speed and bandwidth improve- ment are developed. Brief sections are included on new JFET- bipolar circuitry and die area reduction techniques using transcon- ductance reduction, I. INTRODUCTION T HE integrated circuit operational amplifier (IC op amp) is the most widely used of all linear circuits in production today. Over one hundred million of the devices will be sold in 1974 alone, and production costs are falling low enough so that op amps find applications in virtually every analog area. Despite this wide usage, however, many of the basic performance characteristics c)f the op amp are poorly understood. It is the intent of this study to develop an under- standing for op amp behavior in as direct and intuitive a manner as possible. This is done by using a variety of simplified circuit models which can be analyzed in some cases by inspection, or in others by writing just a few equations. These simplified models are generally developed from the single representative op amp con- figuration shown in Figs. 1 and 2. The rationale for starting with the particular circuit of Fig. 1 is based on the following: this circuit contains, in simplified form, all of the important elements of the most commonly used integrated op amps. It consists essentially of two voltage gain stages, an input differ- ential amp and a common emitter second stage, followed by a class-AB output emitter follower which provides low impedance drive to the load. The two interstages are frequency compensated by a single small “pole-splitting” capacitor (see below) which is usually included on the op amp chip. In most respects this circuit is directly equivalent to the general purpose LM 101 [1], VA 741 [2], and the newer dual and quad op amps [3],, so the results of our study relate directly to these devices. Even for more exotic designs, such as wide-band amps using feedforward [4], [5], or the new FET input cir- cuits [6], the basic analysis approaches still apply, and performance details can be accurately preclicted. It has also been founcl that a good understanding of the limita- Manuscript received July 1, 1974; revised August 8, 1974. The outhor is with the National Semicondl~ctor Corporation, Santa Clara, Calif. 95051.
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