27 Operational amplifiers E J. Kennedy University of Tennessee 27.1 Ideal and Practical Models Op Amp. Practical Op Amps. SPICE Computer Models John V Wait University of Arizona(Retired) Noninverting Circuits 27.1 Ideal and Practical models E Kennedy The concept of the operational amplifier(usually referred to as an op amp)originated at the beginning of the Second World War with the use of vacuum tubes in dc amplifier designs developed by the George A Philbrick Co [some of the early history of operational amplifiers is found in williams, 1991]. The op amp was the basic building block for early electronic servomechanisms, for synthesizers, and in particular for analog computers used to solve differential equations. with the advent of the first monolithic integrated-circuit (IC)op amp in 1965(the HA709, designed by the late Bob widlar, then with Fairchild Semiconductor), the availability of op amps was no longer a factor, while within a few years the cost of these devices(which had been as high as $200 each) rapidly plummeted to close to that of individual discrete transistors Although the digital computer has now largely supplanted the analog computer in mathematically intensive applications, the use of inexpensive operational amplifiers in instrumentation applications, in pulse shaping, in filtering, and in signal processing applications in general has continued to grow. There are currently many commercial manufacturers whose main products are high-quality op amps. This competitiveness has ensured a marketplace featuring a wide range of relatively inexpensive devices suitable for use by electronic engineer physicists, chemists, biologists, and almost any discipline that requires obtaining quantitative analog data from instrumented experiments. Most operational amplifier circuits can be analyzed, at least for first-order calculations, by considering the op amp to be an"ideal"device. For more quantitative information, however, and particularly when frequency response and dc offsets are important, one must refer to a more "practical"model that includes the internal limitations of the device. If the op amp is characterized by a really complete model, the resulting circuit ma be quite complex, leading to rather laborious calculations. Fortunately, however, computer analysis using the program SPICE significantly reduces the problem to one of a simple input specification to the computer. Today, nearly all the op amp manufacturers provide SPICe models for their line of devices, with excellent correlation obtained between the computer simulation and the actual measured results. The Ideal Op amp An ideal operational amplifier is a dc-coupled amplifier having two inputs and normally one output(although in a few infrequent cases there may be a differential output). The inputs are designated as noninverting (designated or NI)and inverting(designated -or Inv ) The amplified signal is the differential signal, between the two inputs, so that the output voltage as indicated in Fig. 27.1 is c 2000 by CRC Press LLC© 2000 by CRC Press LLC 27 Operational Amplifiers 27.1 Ideal and Practical Models The Ideal Op Amp • Practical Op Amps • SPICE Computer Models 27.2 Applications Noninverting Circuits 27.1 Ideal and Practical Models E.J. Kennedy The concept of the operational amplifier (usually referred to as an op amp) originated at the beginning of the Second World War with the use of vacuum tubes in dc amplifier designs developed by the George A. Philbrick Co. [some of the early history of operational amplifiers is found in Williams, 1991]. The op amp was the basic building block for early electronic servomechanisms, for synthesizers, and in particular for analog computers used to solve differential equations. With the advent of the first monolithic integrated-circuit (IC) op amp in 1965 (the mA709, designed by the late Bob Widlar, then with Fairchild Semiconductor), the availability of op amps was no longer a factor, while within a few years the cost of these devices (which had been as high as $200 each) rapidly plummeted to close to that of individual discrete transistors. Although the digital computer has now largely supplanted the analog computer in mathematically intensive applications, the use of inexpensive operational amplifiers in instrumentation applications, in pulse shaping, in filtering, and in signal processing applications in general has continued to grow. There are currently many commercial manufacturers whose main products are high-quality op amps. This competitiveness has ensured a marketplace featuring a wide range of relatively inexpensive devices suitable for use by electronic engineers, physicists, chemists, biologists, and almost any discipline that requires obtaining quantitative analog data from instrumented experiments. Most operational amplifier circuits can be analyzed, at least for first-order calculations, by considering the op amp to be an “ideal” device. For more quantitative information, however, and particularly when frequency response and dc offsets are important, one must refer to a more “practical” model that includes the internal limitations of the device. If the op amp is characterized by a really complete model, the resulting circuit may be quite complex, leading to rather laborious calculations. Fortunately, however, computer analysis using the program SPICE significantly reduces the problem to one of a simple input specification to the computer. Today, nearly all the op amp manufacturers provide SPICE models for their line of devices, with excellent correlation obtained between the computer simulation and the actual measured results. The Ideal Op Amp An ideal operational amplifieris a dc-coupled amplifier having two inputs and normally one output (although in a few infrequent cases there may be a differential output). The inputs are designated as noninverting (designated + or NI) and inverting (designated – or Inv.). The amplified signal is the differential signal, ve, between the two inputs, so that the output voltage as indicated in Fig. 27.1 is E.J. Kennedy University of Tennessee John V. Wait University of Arizona (Retired)