A logic gate is an elementary building block of a digital circuit.
Most logic gates have two inputs
and one output. At any given moment, every terminal is in one of the
two binaryconditions low (0) or high (1), represented by different voltage levels. The logic state
of a terminal can, and generally does, change often, as the circuit processes data. In most logic
gates, the low state is approximately zero volts (0 V), while the high state is approximately five
volts positive (+5 V). There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR,
and XNOR.
The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in
the same way as the logical "and" operator. The following illustration and table show the circuit
symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left
and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise,
the output is "false."
AND gate
Input 1 Input 2 Output
1
1
1 1 1
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive
"or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then
the output is "false."
OR gate
Input 1 Input 2 Output
1 1
1 1
1 1 1
The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The output is
"true" if either, but not both, of the inputs are "true." The output is "false" if both inputs are
"false" or if both inputs are "true." Another way of looking at this circuit is to observe that the
output is 1 if the inputs are different, but 0 if the inputs are the same.
XOR gate
Input 1 Input 2 Output
1 1
1 1
1 1
A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic
inverter devices, has only one input. It reverses the logic state.
Inverter or NOT gate
Input Output
1
1
The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the
logical operation "and" followed by negation. The output is "false" if both inputs are "true."
Otherwise, the output is "true."
NAND gate
Input 1 Input 2 Output
1
1 1
1 1
1 1
The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if both
inputs are "false." Otherwise, the output is "false."
NOR gate
Input 1 Input 2 Output
1
1
1
1 1
The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output
is "true" if the inputs are the same, and"false" if the inputs are different.
XNOR gate
Input 1 Input 2 Output
1
1
1
1 1 1
Using combinations of logic gates, complex operations can be performed. In theory, there is no
limit to the number of gates that can be arrayed together in a single device. But in practice, there
is a limit to the number of gates that can be packed into a given physical space. Arrays of logic
gates are found in digital integrated circuits (ICs). As IC technology advances, the required
physical volume for each individual logic gate decreases and digital devices of the same or
smaller size become capable of performing ever-more-complicated operations at ever-increasing
speeds.
See an introductory tutorial on logic gates:
