Logic Circuit I (Standard Single Logic Gate)

Class: SSS Two

Definitions of Logic Gate

1. A logic gate is an elementary building block of a digital circuit.
2. It is also defined as an electrical circuit that performs one or more logical operations on one or more input signals.
3. Logic gates are devices that implement Boolean functions, i.e. they perform a logic operation on one or more bits of input and give a bit as an output.

Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V).

Types of Standard Single Logic Gate

There are three types of standard single logic gates. They are:

1. AND gate
2. OR gate
3. NOT gate

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AND Gate

An AND gate will give a high output (1) only if all of the inputs are high (1). For example, in a simple lighting circuit with two switches in series, the lamp will light only if both switches are pressed.

Logic Symbol for AND Gate

Truth Table of the AND Gate

INPUT A	INPUT B	OUTPUT C
0	0	0
0	1	0
1	0	0
1	1	1

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OR Gate

An OR gate is a digital logic gate that gives an output of 1 when any of its inputs are 1, otherwise 0.

Logic Symbol for OR Gate

Truth Table of the OR Gate

INPUT A	INPUT B	OUTPUT C
0	0	0
0	1	1
1	0	1
1	1	1

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NOT Gate

A NOT gate is often called an inverter. It is slightly different because it has just one input. It will give a high output (1) if the input is low (0) and vice versa.

Logic Symbol for NOT Gate

Truth Table of the NOT Gate

INPUT A	OUTPUT C
0	1
1	0

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Logic Equations (Boolean Algebra)

The function of a logic gate can also be represented using Boolean algebra equations.

• AND gate Equation: $X = A \cdot B$ The expression reads “X equals A AND B”.
• OR gate Equation: $X = A + B$ The expression reads “X equals A OR B”.
• NOT gate Equation: $X = \bar{A}$ The expression reads “X equals NOT A”. The $\bar{A}$ (complement bar) is used to represent the NOT operation.

Test Your Knowledge

Select the correct answer for each question and click 'Submit' to see your score.

1. What is the fundamental purpose of a logic gate?

A) To store large amounts of data B) To perform logical operations on input signals C) To generate random numbers D) To display information on a screen

2. A logic gate that has a high output only if all of its inputs are high is a(n):

A) OR gate B) NOT gate C) XOR gate D) AND gate

3. What is the symbol for the OR gate operation in a Boolean algebra equation?

A) . (a center dot) B) * (an asterisk) C) + (a plus sign) D) \(\bar{A}\) (a complement bar)

4. According to the truth table, what is the output of an OR gate when one input is 0 and the other is 1?

A) 0 B) 1 C) It depends on the gate type D) Undefined

5. The NOT gate is often called a(n):

A) Inverter B) Multiplier C) Converter D) Amplifier