If a system has two or more input variables and two or more output variables, simultaneous equations for the output variables can be written. In general, when the number of inputs and outputs is large, the simultaneous equations are written in matrix form.
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.
MOSFETs have three regions of operation: cutoff, saturated, and resistive. The MOSFET also has three terminals, the gate G, the drain D, and the source S. A key parameter characterizing the MOSFET is a threshold voltage Vth. When the G-S voltage is below the threshold voltage, no drain current flows and the transistor is cut off.
In basic diagrams, the standard symbols for the circuit components are arranged in correspondence with the operational principle of the device, as well as to facilitate the reading of the diagram. In design diagrams, all or some of the components are represented as equivalent circuits; in such diagrams, sources of electromotive force, current, resistance, inductance, capacitance, and so on are assumed to be components with lumped parameters.