Recall that an operational amplifier is a direct-coupled amplifier with two (differential) inputs and a single output. In Chapter 12,we listed I11ne«es.rablc character! tics of an operational amplifier; we wish now to focus on three of those. We will an ideal operational amplifier to be one that has the following attributes:
L It has infinite gain.
2. It has infinite input impedance.
3. It has zero output impedance.
Although no real amplitier* can satisfy any of these requirements, we will see that most modern amplifiers have such large gains and input impedances, and such small output impedances, that a negligibly small error results from assuming ideal characteristics. A detailed study of the ideal amplifier will therefore be beneficial in terms of unders anding a tical amplifiers are used as well as in buildingsome important theoretical concepts that have broad implicat ions in many areas of electronics.
The standard symbol for an operational amplifier. Note that the two inputs arc labeled “+” and “-” and the input signals are correspondingly designated vt and Vj-. In relation to our previous discussion of differential amplifiers, these inputs correspond to Vjl and Vj2> respectively, when the single-ended output is V,,2 (see Figure 12-2). In other words, if the inputs are out-of-phase signals, the amplifier output will be in phase with u; and out of phase with Uj-‘ For this reason, the + input is called the noninuerting input and the – input is called the inverting inp IIt. In many applications, one of the amplifier inputs is grounded, so v” is in phase with the input if the signal is connected to the non inverting terminal, and v” is out of phase· vith the input if the signal is connected to the inverting input. These ideas are summarized in the table accompanying
At this point, a legitimate question that may have already occurred to the reader is this: If the gain is infinite, how can the output be anything other than a severely clipped waveform? Theoretically, if the amplifier has infinite gain, an infinite input voltage must result in an infinitely large output voltage. The answer, of course, is that the gain is not truly infinite, just very large. Nevertheless, it is true that a very small input voltage will cause the amplifier output to be driven all the way to its extreme positive or negative voltage limit. The practical answer is thai an operational umpl.licr is seldom used in such a way that the full gain is applied to an input. Instead. external resistors are connected to and around the amplifier in such a way that the signal undergoes vastly smaller amplification. The resistors cause gain reduction through signal [ccdback, which we will soon study in considerable detail.