【正文】 or output circuit. (Note: this description applies only when the emitter connection is mon to both circuits ~ known as mon emitter configuration.) This is the most widely used way of connecting transistors, but there are, of course, two other alternative configurations mon base and mon emitter. But, the same principles apply in the working of the transistor in each case. The particular advantage offered by this circuit is that a relatively small base current can control and instigate a very much larger collector current (or, more correctly, a small input power is capable of producing a much larger output power). In other words, the transistor works as an amplifier. With this mode of working the baseemitter circuit is the input side。 and 39。 or 39。UNIT 2 A: The Operational Amplifier One problem with electronic devices corresponding to the generalized amplifiers is that the gains, Au or A~, depend upon internal properties of the twoport system (p, fl, R~, Ro, etc.)?~ This makes design difficult since these parameters usually vary from device to device, as well as with temperature. The operational amplifier, or OpAmp, is designed to minimize this dependence and to maximize the ease of design. An OpAmp is an integrated circuit that has many ponent part such as resistors and transistors built into the device. At this point we will make no attempt to describe these inner workings. A totally general analysis of the OpAmp is beyond the scope of some texts. We will instead study one example in detail, then present the two OpAmp laws and show how they can be used for analysis in many practical circuit applications. These two principles allow one to design many circuits without a detailed understanding of the device physics. Hence, OpAmps are quite useful for researchers in a variety of technical fields who need to build simple amplifiers but do not want to design at the transistor level. In the texts of electrical circuits and electronics they will also show how to build simple filter circuits using OpAmps. The transistor amplifiers, which are the building blocks from which OpAmp integrated circuits are constructed, will be discussed. The symbol used for an ideal OpAmp is shown in Fig. 12A1. Only three connections are shown: the positive and negative inputs, and the output. Not shown are other connections necessary to run the OpAmp such as its attachments to power supplies and to ground potential. The latter connections are necessary to use the OpAmp in a practical circuit but are not necessary when considering the ideal 0pAmp applications we study in this chapter. The voltages at the two inputs and the output will be represented by the symbols U+, U, and Uo. Each is measured with respect t~ ground potential. Operational amplifiers are differential devices. By this we mean that the output voltage with respect to ground is given by the expression Uo =A(U+ U) (12Al) where A is the gain of the OpAmp and U+ and U the voltages at inputs. In other words, the output voltage is A times the difference in potential between the two inputs. Integrated circuit technology allows construction of many amplifier circuits on a single posite chip of semiconductor material. One key to the success of an operational amplifier is the cascading of a number of transistor amplifiers to create a very large total gain. That is, the number A in Eq. (12A1) can be on the order of 100,000 or more. (For example, cascading of five transistor amplifiers, each with a gain of 10, would yield this value for A.) A second important factor is that these circuits can be built in such a