Logic Gates - Be Engineer

Logic Gates

Logic gates are electronic circuits that operate on one or more input signals to produce an output signal. Electrical signals such as voltages or currents exist as analog signals having values over a given continuous range, say, 0 to 3 V, but in a digital system these voltages are interpreted to be either of two recognizable values, 0 or 1.

Voltage‐operated logic circuits respond to two separate voltage levels that represent a binary variable equal to logic 1 or logic 0. For example, a particular digital system may define logic 0 as a signal equal to 0 V and logic 1 as a signal equal to 3 V. In practice, each voltage level has an acceptable range, as shown in Fig. 1.3 . The input terminals of digital circuits accept binary signals within the allowable range and respond at the output terminals with binary signals that fall within the specified range. The intermediate region between the allowed regions is crossed only during a state transition. Any desired information for computing or control can be operated on by passing binary signals through various combinations of logic gates, with each signal representing a particular binary variable. When the physical signal is in a particular range it is interpreted to be either a 0 or a 1.

Signal levels for binary logic values

Symbols for digital logic circuits

The gates are blocks of hardware that produce the equivalent of logic‐1 or logic‐0 output signals if input logic requirements are satisfied. The input signals x and y in the AND and OR gates may exist in one of four possible states: 00, 10, 11, or 01.

The timing diagrams illustrate the idealized response of each gate to the four input signal combinations. The horizontal axis of the timing diagram represents the time, and the vertical axis shows the signal as it changes between the two possible voltage levels. In reality, the transitions between logic values occur quickly, but not instantaneously. The low level represents logic 0, the high level logic 1. The AND gate responds with a logic 1 output signal when both input signals are logic 1. The OR gate responds with a logic 1 output signal if any input signal is logic 1. The NOT gate is commonly referred to as an inverter. The reason for this name is apparent from the signal response in the timing diagram, which shows that the output signal inverts the logic sense of the input signal.