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.
Since active devices usually supply signal energy to an electronic circuit, and since energy can only be transformed and not created, a source of energy is needed when active devices are present. This energy is usually obtained from batteries or through rectification of sinusoidal voltages supplied by power companies. When inserted into an electronic circuit, such a source of energy fixes the quiescent operation of the circuit; that is, it allows the circuit to be biased to a given operating point with no signal applied, so that when a signal is present it will be processed properly. To be useful, an electronic circuit produces one or more outputs; often inputs are applied to produce the outputs. These inputs and outputs are called the signals and, consequently, generally differ from the bias quantities, though often it is hard to separate signal and bias variables. Biasing of electronic circuits is an important, non- trivial, and often overlooked aspect of their operation.
Schematic diagrams define the components of the article that constitute a complete set and the interconnections between components; they usually give a detailed representation of the working principle of the article and may serve as a basis for developing other design documents, such as electrical installation blueprints and specification sheets.
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.