Basic Logic Gates

1. BASIC LOGIC GATES

2. Lesson Objectives
At the end of discussion, the students will
be able to:
● Identify and explain the function of the basic
logic gates;
● Draw the symbols for the basic logic gates;
and
● Develop truth tables for the basic logic gates.

3. Introduction
● All digital equipment, whether simple or complex, is
constructed of only a few basic circuits. These circuits,
referred to as logic elements, perform some logic
function on binary data.
● There are two basic types of logic circuits: decision-
making and memory.
● Decision-making logic circuits monitor binary inputs
and produce an output based on the status of the
inputs and the characteristics of the logic circuit.
● Memory circuits are used to store binary data.

4. AND Gate
● It is a logic circuit that has two or more
inputs and a single output.
● It produces an output of 1, only when all
its inputs are 1. If any of the inputs are 0,
the output is 0.

5. Logic Symbol for an AND Gate

6. AND Gate
● It performs the basic operation of
multiplication.
● The output of an AND Gate is represented
by the equation Y=A.B or Y=AB
● It is represented by the dot (.) between
the two variables A and B.

7. AND Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for a two-input AND Gate
0
0
0
1

8. AND Gate
● Truth Table shows the output for each possible input. The inputs are
designated A and B, and the output is designated Y.
● The total number of possible combination in the truth table is determined
by the following formula:
N= 2𝑛
where: N= the total number of possible combinations
n= the total number of input variables
Example:
For two input variables, N= 22 = 4
For three input variables, N= 23
= 8
For four input variables, N= 24 = 16
For eight input variables, N= 28 = 256

9. OR Gate
● It produces a 1 output if any of its inputs are
1. The output is 0 if all the inputs are 0.
● It performs the basic operation of addition.
The algebraic expression for the output of an
OR gate is Y=A+B.
● The plus (+) sign designates the OR function.

10. Logic Symbol for an OR Gate

11. OR Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for a two-input OR Gate
0
1
1
1

12. NOT Gate
● The simplest logic circuit is the NOT gate.
● It performs the function called inversion, or
complementation, and is commonly referred to as
an inverter.
● The purpose of the inverter is to make the output
state the opposite of the input state.
● If a 1 (high) is applied to the input of an inverter, a
0 (low) appears on its output, and vice versa.

13. Logic Symbol for a NOT Gate

14. NOT Gate
INPUTS OUTPUT
A Y
0
1
Truth Table for a NOT Gate
1
0

17. NAND Gate
● It is the combination of an inverter and an
AND gate.
● It is called a NAND gate from the NOT-AND
function it performs.
● It is the most used logic function because it
can be used to construct an AND gate, OR
gate, inverter, or any combination of these
functions.

18. Logic Symbol for a NAND Gate

19. NAND Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for a two-input NAND Gate
1
1
1
0

20. NAND Gate
Using the NAND gate to generate other logic functions

21. NOR Gate
● It is the combination of an inverter and an OR
gate.
● It is called a NOR gate from the NOT-OR
function it performs.
● It is the most used logic function because it
can be used to construct an AND gate, OR
gate, inverter, or any combination of these
functions.

22. Logic Symbol for a NOR Gate

23. NOR Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for a two-input NOR Gate
1
0
0
0

24. Exclusive OR and NOR Gate
● A less common but still important gate is called an
exclusive OR gate, abbreviated as XOR.
● An XOR gate has only two inputs, unlike the OR
gate, which may have several inputs.
● However, the XOR is like the OR gate in that it
generates a 1 output if either input is a 1.
● The exclusive OR is different when both inputs are
1 or 0. In that case, the output is a 0.

25. Logic Symbol for an exclusive OR Gate

26. Exclusive OR Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for an exclusive OR Gate
0
1
1
0

27. Logic Symbol for an exclusive NOR Gate

28. Exclusive NOR Gate
INPUTS OUTPUT
A B Y
0 0
1 0
0 1
1 1
Truth Table for an exclusive NOR Gate
1
0
0
1

29. Pin Configuration
Quad 2-input AND Gate (7408)

30. Pin Configuration
Quad 2-input OR Gate (7432)

31. Pin Configuration
Hex Inverter (7404)

32. Pin Configuration
Quad 2-input NAND Gate (7400)

33. Pin Configuration
Quad 2-input NOR Gate (7402)

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