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Within previous articles about op amps, discussions are focused on the basic aspects of op amps. After a brief introductory section, this begins with the fundamental topology differences between the two broadest classes of op amps, those using voltage feedback and current feedback. These two amplifier types are distinguished more by the nature of their internal circuit topologies than anything else. The voltage feedback op amp topology is the classic structure, having been used since the earliest vacuum tube based op amps of the 1940 and 1950’s, through the first IC versions of the 1960’s, and includes most op amp models produced today. The more recent IC variation of the current feedback amplifier has come into popularity in the mid-to-late 1980’s, when higher speed IC op amps were developed. Factors distinguishing these two op amp types are discussed at some length.
Details of op amp input and output structures are also covered in this chapter, with emphasis how such factors potentially impact application performance. In some senses, it is logical to categorize op amp types into performance and/or application classes, a process that works to some degree, but not altogether.
In practice, once past those obvious application distinctions such as "high speed" versus "precision," or "single" versus "dual supply," neat categorization breaks down. This is simply the way the analog world works. There is much crossover between various classes, i.e., a high speed op amp can be either single or dual-supply, or it may even fit as a precision type. A low power op amp may be precision, but it need not necessarily be single-supply, and so on. Other distinction categories could include the input stage type, such as FET input (further divided into JFET or MOS, which in turn are further divided into NFET or PFET and PMOS and NMOS, respectively), or bipolar (further divided into NPN or PNP). Then, all of these categories could be further described in terms of the type of input (or output) stage used.
So, it should be obvious that categories of op amps are like an infinite set of analog gray scales; they don’t always fit neatly into pigeonholes, and we shouldn’t expect them to. Nevertheless, it is still very useful to appreciate many of the aspects of op amp design that go into the various structures, as these differences directly influence the optimum op amp choice for an application. Thus structure differences are application drivers, since we choose an op amp to suit the nature of the application, for example single-supply.
In the next articles various op amp performance specifications are also discussed, along with those specification differences that occur between the broad distinctions of voltage or current feedback topologies, as well as the more detailed context of individual structures. Obviously, op amp specifications are also application drivers, in fact they are the most important, since they will determine system performance. We choose the best op amp to fit the application, based on for the required bias current, bandwidth, distortion, etc.
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