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.
The active elements are the sources of electric energy for the circuit; they may be batteries, direct-current generators, or alternating-current generators. The passive elements are resistors, inductors, and capacitors. The electric circuit is described by a circuit diagram or map showing the active and passive elements and their connecting conducting paths.
In design documentation, a document showing in conventional notation the components of an article or installation and the connections or links between components. Diagrams are usually drawn without regard to scale or to the actual spatial arrangement of the components. They may be classified according to the type of components and connections between components as electrical, pneumatic, hydraulic, kinematic, or combination diagrams. They may also be classified according to the purpose for which they will be used.
Circuit diagrams show connectors between components and the locations of connectors and lead-ins; such diagrams also illustrate methods of laying out, mounting, and fastening conductors, cables, and piping. Others show external connections to other articles; such diagrams are used for the installation and operation of complex units. Diagrams showing the principal parts of a complex and the interconnections between subassemblies when the complex is installed and operated are designed primarily to give a general representation of the complex. Layout diagrams show the relative spatial arrangement of components.