【正文】 r 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。 zones), it is referred to as a bipolar device, or bipolar transistor. A transistor thus has three elements with three leads connecting to these elements. To operate in a working circuit it is connected with two external voltage or polarities. One external voltage is working effectively as a diode. A transistor will, in fact, work as a diode by using just this connection and fetting about the top half. An example is the substitution of a transistor for a diode as the detector in a simple radio. It will work just as well as a diode as it is working as a diode in this case. The diode circuit can be given forward or reverse bias. Connected with forward bias, as in , drawn for a PNP transistor, current will flow from P to the bottom N. If a second voltage is applied to the top and bottom sections of the transistor, with the same polarity applied to the bottom, the electrons already flowing through the bottom N section will promote a flow of current through the transistor bottomtotop. By controlling the degree of doping in the different layers of the transistor during manufacture, this ability to conduct current through the second circuit through a resistor can be very marked. Effectively, when the bottom half is forward biased, the bottom section acts as a generous source of free electrons (and because it emits electrons it is called the emitter). These are collected readily by the top half, which is consequently called the collector, but the actual amount of current which flows through this particular circuit is controlled by the bias applied at the center layer, which is called the base. Effectively, therefore, there are two separate 39。 and 39。 or positive charges (P type). A single crystal of germanium or silicon treated with both Ntype dope and Ptype dope forms a semiconductor diode, with the working characteristics described. Transistors are formed in a similar way but like two diodes backtoback with a mon middle layer doped in the opposite way to the two end layers, thus the middle layer is much thinner than the two end layers or zones. Two configurations are obviously possible, PNP or NPN (Fig. 12Bl). These descriptions are used to describe the two basic types of transistors. Because a transistor contains elements with two different polarities (., 39。 or 39。doped39。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 si