Author: M Abo Bakar Aslam

Logic Gates

Logic gates are basic building blocks of combinational-logic-circuit. A combinational-logic-circuit is a digital circuit whose output depends only on the present inputs. It does not store any past information and does not use memory elements.

1. Types of Logic Gates

There are following seven logic gates.

  1. AND Gate
  2. OR Gate
  3. NOT Gate
  4. NAND Gate
  5. NOR Gate
  6. XOR Gate
  7. XNOR Gate

2. Basic Logic Gates

There are following basic logic gates (first three). These are so called basic-logic-gates because we can construct rest of the gates using these basic gates.

  1. AND Gate
  2. OR Gate
  3. NOT Gate


3. AND Gate

The AND gate will generate outputs as HIGH only when all inputs are HIGH. It performs logical multiplication in digital electronics. See more detail

AND Gate with 2 Inputs
Figure 1: AND Gate with 2 Inputs

4. OR Gate

The OR gate produces a HIGH output when any input is HIGH. It performs logical addition in combinational logic circuits. See more detail

OR Gate with 2 Inputs
Figure 2: OR Gate with 2 Inputs

5. NOT Gate

Input is just inverted to generate output. The NOT gate, also called an inverter It output will be opposite to its input. It converts HIGH to LOW and LOW to HIGH. See more detail

NOT Gate
Figure 3: NOT Gate

6. NAND Gate

The NAND gate is the inverse of the AND gate and outputs LOW only when all inputs are HIGH. See more detail

NAND Gate with 2 Inputs
Figure 4: NAND Gate with 2 Inputs

7. NOR Gate

The NOR gate is the inverse of the OR gate and produces HIGH only when all inputs are LOW. See more detail

NOR Gate with 2 Inputs
Figure 5: NOR Gate with 2 Inputs

8. XOR Gate

Output will be 1, if number of 1's in inputs are ODD. This is also called ODD Function. See more detail

XOR Gate with 2 Inputs
Figure 6: XOR Gate with 2 Inputs

9. XNOR Gate

Output of XOR gate is just inverted. See more detail

XNOR Gate with 2 Inputs
Figure 7: XNOR Gate with 2 Inputs