The path taken by an electric current in flowing through a conductor through one complete run of a set of wires from a power source, such as a panelboard, to various electrical devices and back to the same power source. The wires used for various circuits are prescribed by codes, such as the National Electrical Code.
The digital computer is based on digital electronic circuits. Although some of the circuits are quite sophisticated, such as the microprocessors integrated on a single chip, the concepts behind most of the circuits involved in digital computers are quite simple compared to the circuits used for analog signal processing. The most basic circuit is the inverter; a simple realization based upon the MOS transistor is shown in Fig. 3a. The upper (depletion-mode) transistor acts as a load “resistor” for the lower (enhancement-mode) transistor, which acts as a switch, turning on (into its resistive region) when the voltage at point A is above threshold to lower the voltage at point B. Adding the output currents of several of these together into the same load resistor gives a NOR gate, a two-input version of which is shown in Fig. 3b; that is, the output is high, with voltage at VDD, if and only if the two inputs are low. Placing the drains of several of the enhancement-mode switches in series yields the NAND gate, a two-input version of which is shown in Fig. 3c; that is, the output is low if and only if both inputs are high. From the circuits of Fig. 3, the most commonly used digital logic circuits can be constructed. Because these circuits are so simple, digital circuits and digital computers are usually designed on the basis of negation logic, that is, with NOR and NAND rather than OR and AND circuits.
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.
An interconnection of electronic devices, an electronic device being an entity having terminals which is described at its terminals by electromagnetic laws. Most commonly these are voltage-current laws, but others, such as photovoltaic relationships, may occur.