3
Within Chapter 1, 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 amplifi er types are distinguished more
by the nature of their internal circuit topologies than anything else. The voltage feedback op amp topol-
ogy is the classic structure, having been used since the earliest vacuum tube based op amps of the 1940s
and 1950s, through the fi rst IC versions of the 1960s, and includes most op amp models produced today.
The more recent IC variation of the current feedback amplifi er has come into popularity in the mid-to-late
1980s, 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 on 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 fi t 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 infi nite set of analog gray scales; they don’t
always fi t 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 infl uence 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 this chapter various op amp performance specifi cations are also discussed, along with those specifi ca-
tion 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 specifi cations are also application
drivers; in fact, they are the most important since they will determine system performance. We choose the
best op amp to fi t the application, based on the required bias current, bandwidth, distortion, and so forth.
CHAPTER 1
Op Amp Basics
James Bryant, Walt Jung, Walt Kester