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Logic Gates

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Logic gates
LOGIC GATES

A gate is a circuit which can have any number of inputs but only one output.

These gates are the fundamental of flip flops which is used basically in computers. Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false.


KINDS OF LOGIC GATES

  • AND GATE

  • NOT GATE

  • OR GATE

  • NAND GATE / NOT AND GATE

  • NOR GATE / NOT OR GATE


And gate
AND GATE

  • This kind of gate requires 1 as input to all its inputs in order to produce an output of 1.

Where:

A and B are inputs

Q is output

Symbol of AND Gate


And gate truth table
AND GATE TRUTH TABLE

  • The output Q is true if input A AND input B are both true: Q = A AND BAn AND gate can have two or more inputs, its output is true if all inputs are true.


Not gate

A Q

input output

NOT GATE

  • This is a very special type of gate which is also called inverter.

  • Whatever may be the input the output will be just the opposite.

Where:

A is input

Q is output

Symbol of NOT Gate


Not truth table

A Q

input output

NOT TRUTH TABLE

The output Q is true when the input A is NOT true, the output is the inverse of the input: Q = NOT AA NOT gate can only have one input.


Or gate

A

Q

inputs output

B

OR GATE

  • This gate functions as the addition of the two Binary Inputs.

  • An OR gate can have two or more inputs, its output is true if at least one input is true.

Where:

A and B are inputs

Q is output

Symbol of OR Gate


Or gate truth table

A

Q

inputs output

B

OR GATE TRUTH TABLE

  • The output Q is true if input A OR input B is true (or both of them are true): Q = A OR BAn OR gate can have two or more inputs, its output is true if at least one input is true.


Nand gate

A

Q

inputs output

B

NAND GATE

  • NAND gate (NAND = Not AND)

  • The NAND GATE functions by multiplying the binary inputs but the output is the opposite of the result.

  • This is an AND gate with the output inverted, as shown by the 'o' on the output. 

Where:

A and B are inputs

Q is output

Symbol of NAND Gate


Nand gate truth table

A

Q

inputs output

B

NAND GATE TRUTH TABLE

  • The output is true if input A AND input B are NOT both true: Q = NOT (A AND B)A NAND gate can have two or more inputs, its output is true if NOT all inputs are true.


Nor gate

A

Q

inputs output

B

NOR GATE

This gate is just the opposite of OR Gate. The outputs of this gate are just the opposites of the outputs OR Gate.

This is an OR gate with the output inverted, as shown by the 'o' on the output. 

Where:

A and B are inputs

Q is output

Symbol of NOR Gate


Nor truth table

A

Q

inputs output

B

NOR TRUTH TABLE

  • The output Q is true if NOT inputs A OR B are true: Q = NOT (A OR B)A NOR gate can have two or more inputs, its output is true if no inputs are true.



Exercise 1
EXERCISE #1:

Complete the truth table of the logic gate below


Exercise 2
EXERCISE #2:

Make a truth table of the following logic gate.


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