Besides biasing of the junctions for linear operation, any state of the two junctions can occur. For example, both junctions might be forward-biased, in which case the transistor is said to be in saturation and acts nearly as a short circuit between E-C, while if the junctions are simultaneously back-biased the transistor is said to be cut off and acts as an open circuit between all terminals. The transistor can be controlled between saturation and cutoff to make it act as an electronically controlled switch. This mode of operation is especially useful for binary arithmetic, as used by almost all digital computers, where 0 and 1 logic levels are represented by the saturation and cutoff transistor states.
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.
A board whose surfaces have printed current conductors with contact areas, which are used to connect components mounted on the boards according to the circuit diagram of a functional subassembly for electric or radio apparatus, and also have plated circuit holes and nonplated mounting holes. There are about 200 methods for making printed circuit boards; among the most important are the photochemical, photoelectrochemical, and offset-electrochemical methods. The methods differ in the means of producing the conductive coating or the form in which the pattern of the printed conductors is realized.