Often, though, they are used for other purposes, such as to route signals in logic circuits. Transistors can be considered the workhorses of modern electronic circuits, and consequently many types of transistors have been developed, among which the most widely used are the bipolar junction transistor (BJT), the junction field-effect transistor (JFET), and the metal oxide silicon field-effect transistor (MOSFET).
Because some circuits can be very complicated, and since even the simplest circuits may have complicated behavior, the area of computer-aided design (CAD) of electronic circuits has been extensively developed. A number of circuit simulation programs are available, some of which can be run on personal computers with good results. These programs rely heavily upon good mathematical models of the electronic devices. Fortunately, the area of modeling of electronic devices is well developed, and for many devices there are models that are adequate for most purposes. But new devices are constantly being conceived and fabricated, and in some cases no adequate models for them exist. Thus, many of the commerical programs allow the designer to read in experimentally obtained data for a device from which curve fitting techniques are used to allow an engineer to proceed with the design of circuits incorporating the device. Reproducibility and acceptability of parts with tolerances are required for the commerical use of electronic circuits. Consequently, theories of the reliability of electronic circuits have been developed, and most of the computer-aided design programs allow the designer to specify component tolerances to check out designs over wide ranges of values of the elements. Finally, when electronic circuits are manufactured they can be automatically tested with computer-controlled test equipment. Indeed, an area that will be of increasing importance is design for testability, in which decisions on what to test are made by a computer using knowledge-based routines, including expert systems. Such tests can be carried out automatically with computer-controlled data-acquisition and display systems.
Fortunately, most of these transistors occur in pairs, such as the npn and the pnp bipolar junction transistors, or the n-channel and the p-channel MOSFETs, allowing designers to work symmetrically with positive and negative signals and sources. This statement may be clarified by noting that transistors can be characterized by graphs of output current i versus output voltage v that are parametrized by an input current (in the case of the bipolar junction transistor) or input voltage (in the MOSFET and JFET cases). Typically, the curves for an npn bipolar junction transistor or an n-channel field-effect transistor are used in the first quadrant of the output i-v plane, while for a pnp bipolar junction transistor or a p-channel field-effect transistor the same curves show up in the third quadrant. Mathematically, if i = f(v) for an npn bipolar junction transistor or n-channel field-effect device, then i = -f(-v) for a pnp bipolar junction transistor or p-channel field-effect device when the controlling parameters are also changed in sign.
Electronic circuits are also classified as analog or digital. Analog circuits work with signals that span a full range of values of voltages and currents, while digital circuits work with signals that are at prescribed levels to represent numerical digits. Analog signals generally are used for continuous-time processes, while digital ones most frequently occur where transistions are synchronized via a clock. However, there are situations where it is desirable to transfer between these two classes of signals, that is, where analog signals are needed to excite a digital circuit or where a digital signal is needed to excite an analog circuit. For example, it may be desired to feed a biomedically recorded signal, such as an electrocardiogram into a digital computer, or it may be desired to feed a digital computer output into an analog circuit, such as a temperature controller. For such cases, there are special electronic circuits, called analog-to-digital and digital-to-analog converters.