Information Technology - Logic Gates

Online Certificate Courses Information Technology - Logic Gates Tutorial. 

What is Logic Gates

Logic Gates are the building blocks of any electronic device or digital system which implements boolean algebraic equations having one or more input and only one output. 

The basic building blocks of digital electronics are logic gates that perform simple binary logic functions (AND, OR, NOT, etc.). From these devices, more complex circuits can be constructed to do arithmetic, which acts as memory elements. 

Logic Gates are named as:  AND gate, OR gate, NOT gate etc. 

Features Of Logic Gates

1. The flow of digital signals is controlled by transistors in various configurations depending on the logic family. For most purposes, we can imagine that the logic gates are composed of ideal switches with just two states: OPEN and CLOSED. The state of a switch is controlled by a digital signal. The switch remains closed so long as a logical (1) signal is applied. A logical (0) control signal keeps it open.

2. Logic signals interact by means of gates. The three fundamental gates AND, OR, and NOT, are named after. The three fundamental operations of logic that they carry out. The AND and OR gates each have two inputs and one output. The output state is determined by the states of the two inputs.

3. The function of each gate is defined by a truth table, which specifies the output state for each possible combination of input states. The output values of the truth tables can be understood in terms of two switches. If the switches are in series, you get the AND function. Parallel switches perform the OR operation. A bubble after a gate or at an input indicates NOT.

4. When several gates are combined to perform a complex logical operation, a good design uses as few as possible. Boolean algebra, the mathematics of two-valued variables, is the theoretical tool used to simplify complex logical expressions.

AND Gate

The AND gate has two or more inputs and a single output. The output of an AND gate is in 1 state if all inputs are in the 1 state or the output will be zero if any of the inputs is zero.

OR Gate

The OR gate has two or more inputs and a single output. The output of an OR gate is in 1 state if any of the inputs is in the 1 state.

NOT Gate

The NOT gate has a single input and a single output and its symbol and truth table (truth table contains a table of all possible input values and their corresponding outputs values). It performs the operation of inversion, i.e., the output of a NOT gate takes a 1  (high) state if the input takes the 0 (low) state and vice-versa. A circuit, which performs a logic negation, is called a NOT circuit or inverter since it inverts the output with respect to the input.

Truth Tables

Truth Tables are used to show logic gate functions. The NOT gate has only one input, but all the others have two inputs. When constructing a truth table, the binary values 1 and 0 are used. Every possible combination, depending on the number of inputs, is produced. Basically, the number of possible combinations of 1s and 0s is 2n where n= number of inputs. For example, 2 inputs have 22 combinations (i.e. 4), 3 inputs have 23 combinations (i.e. 8) and so on.

Not Gate: Example

Not Gate Image

Input A Output X
0 1
1 0

                               Truth Table

The output (X) is true (i.e. 1 or ON) if: INPUT A is NOT TRUE (i.e. 0 or OFF) Truth table for: X = NOT A.

 

AND Gate: Example

AND Gate Image

Input A Input B Output X
0 0 0
0 1 0
1 0 0
1 1 1

                                  Truth Table

The output (X) is true (i.e. 1 or ON) if: INPUT AAND INPUT B are BOTH TRUE (i.e. 1 or ON) Truth table for: X = A AND B.

 

OR Gate: Example

OR Gate Image

Input A Input B Output X
0 0 0
0 1 1
1 0 1
1 1 1

                                      Truth Table

The output (X) is true (i.e. 1 or ON) if: INPUT A OR INPUT B is TRUE (i.e. 1 or ON) Truth table for X = A OR B.


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