Logic gates are the devices used as basic building blocks of all the digital circuits. The basic logic gates are NOT, AND and OR along with NOR, NAND, EX-OR etc. We have to use different laws, rules and theorems to analyze the digital circuits. By connecting the gates mentioned earlier, in different ways, we can build circuits that can perform arithmetic and other functions associated with human brain. Because they simulate mental processes, gates are often called as logic circuits.
A logic Gate is an electronic circuit having one or more than one inputs and only one output. The relationship between the input and the output is based on a "certain logic". Based on this logic gates are named as NOT gate, AND gate, OR, NAND, NOR etc.
CLASSIFICATION OF LOGIC GATES:
LOGIC GATES | ||
Basic Gates | Universal Gates | Derived Gates |
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NOT GATE | INVERTER
The NOT gate or Inverter is a logic gate having one input (A) and one output (Y). It is called "Inverter" because its output is the inverted version or "complement of its input. This is shown in the truth table of NOT gate.Truth Table
Input A | Output Y |
0 | 1 |
1 | 0 |
Symbol
 |
| Equivalent Circuit |
A is input while Y is the output.
The bubble (o) in the symbol of a NOT gate indicates inversion. The operation of a NOT gate for various input combinations is as shown in above figure.
AND GATE
AND is one of the logic operators. It performs the logical multiplication on its inputs. The output is high (Y=1) if and only if all the inputs to the gate are high (1). The output is low (0), if atleast one of the inputs is low (0). AND gate can have two or more inputs and only one output.
Symbol
 |
| Logical Symbol |
Truth Table
Inputs | Outputs | |
A | B | Y |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
Boolean Expression
The expression relating the inputs and output of a gate is called as the Boolean expression. The Boolean expression. The Boolean expression for an AND gate is;
Y = A.B
where the "dot" between A and B represents multiplication.
By substituting different values of A and B into the Boolean expression we can get the corresponding state of output. This is how we can verify the truth table of the gate.
OR GATE
An OR gate performs the logical addition on its inputs therefore its output will be high (1) if any one or both the inputs are high (1). Its output will be low (0) if and only if both the inputs are simultaneously low (0).Logic Symbol
 |
| Logical Symbol |
Truth Table
Inputs | Outputs | |
A | B | Y |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
Boolean Expression
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