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Unformatted text preview: TL/H/8745 TheMonolithicOperationalAmplifier:ATutorialStudyAN-A National Semiconductor Appendix A A December 1974 The Monolithic Operational Amplifier: A Tutorial Study Invited Paper IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6 Abstract A study is made of the integrated circuit opera- tional 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 relation- ships 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 some approaches to speed and bandwidth improvement are de- veloped. Brief sections are included on new JFETbipolar circuitry and die area reduction techniques using transcon- ductance reduction. 1.0 INTRODUCTION The integrated circuit operational amplifier (IC op amp) is the most widely used of all linear circuits in production to- day. 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 per- formance characteristics of the op amp are poorly under- stood. It is the intent of this study to develop an understanding for op amp behavior in as direct and intuitive a manner as pos- sible. 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 representa- tive op amp configuration shown in Figures 1 and 2 . The rationale for starting with the particular circuit of Figure 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 volt- age gain stages, an input differential 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 LM101 [ 1 ] , m A 741 [ 2 ] , and the newer dual and quad op amps [ 3 ] , so the results of our study relate directly to these devices. Even for TL/H/87451 FIGURE 1. Basic two-stage IC op amp used for study. Minimal modifications used in actual IC are shown in Figure 2. BI-FET TM is a trademark of National Semiconductor Corp....
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