One means of doing analog-to-digital conversion is to use a clocked counter that feeds a digital-to-analog converter, whose output is compared with the analog signal to stop the count when the digital-to-analog output exceeds the analog signal. The counter output is then the analog-to-digital output. The comparator for such an analog-to-digital converter is similar to an open-loop operational amplifier (which changes saturation level when one of the differential input levels crosses the other). Other types of analog-to-digital converters, called flash converters, can do the conversion in a shorter time by use of parallel operations, but they are more expensive.
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.
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.
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.