A gate is defined as a digital circuit which
follows some logical relationship between the input and output voltages. It is a digital circuit which either allows a signal to pass
through as stop, it is called a gate.
The Logic Gates are building blocks at
digital electronics. They are used in digital electronics to change on voltage level (input voltage) into another (output voltage)
according to some logical statement relating them.
A logic gate may have one or more inputs, but it
has only one output. The relationship between the possible values of input and output voltage is expressed in the form of a table called
truth table or table of combinations.
Truth table of a
Logic Gates is a table that shows all the input and output possibilities for the logic gate.
George Boole in 1980 invented a different kind of algebra based on
binary nature at the logic, this algebra of logic called BOOLEAN ALGEBRA. A logical statement can have only two values, such as HIGH/LOW,
ON/OFF, CLOSED/OPEN, YES/NO, TRUE/FALSE, CONDUCTING/NON-CONDUCTING etc. The two values of logic statements one denoted by the binary number
1 and 0. The binary number 1 is used to denote the HIGH value. The logical statements that logic gates follow are called Boolean
Types of Gates
There are three types of basic logic gates
which follows Boolean expression.
i) OR gate
ii) AND gate
The OR gate is a two inputs and
one output logic gate. It combing the input A and B with the output Y following the Boolean expression.
Y = A + B
The Boolean algebra, the addition
symbol (+) is called OR (i.e. OR operation OR operator).
The various possible combinations of the
input and output of the OR gate can be easily understand with the help of the electrical circuit. In this electric circuit, a
parallel combination of two switches A and B is connected to a battery and a lump L.
The following interference can be easily drawn
from the working of electrical circuit is :
a) If switch A & B are open lamp do not
glow (A=0, B=0)
b) If Switch A open B closed then (A=0,
B=1) Lamp glow.
c) If switch A closed B open then (A=1,
B=0) Lamp glow.
d) If switch A & B are closed then (A=1,
B=1) Lamp glow.
As we see truth table we found same as it
The AND gate is also a two
inputs and one output logic gate. It combines the input A and B with the output Y following the Boolean expression.
Y = A . B
The Boolean algebra, the multiplication symbol
(. dot or x Gross) is taken to mean AND.
Y = A . B have Y is
equal to A AND B.
The various possible combination of the input
and outputs of the AND gate can be easily found with the help of the electrical circuit. Here a series combination of the switch A and B is
connected to a battery and a lump L.
The following conclusions can be easily drawn
from the working of electrical circuit :
a) If both switches A&B are open (A=0,
B=0) then lamp will not glow. (y=0)
b) If Switch A closed & B open (A=1,
B=0) then Lamp will not glow. (y=0)
c) If switch A open & B closed (A=0,
B=1) then Lamp will not glow. (y=0)
d) If switch A & B both closed (A=1,
B=1) then Lamp will glow. (y=1)
As we see truth table we found same as it is
The NOT gate is a one
inputs and one output logic gate. It combines the input A with the output following the Boolean expression.
Y = A
i.e. Y not equal A. The way, the NOT gate gives
the output it is also called inverter. It is represented by the symbol.
The Boolean algebra, the
negative sign (-) is called NOT. The equation Y= A called Boolean expression.
The possible input and output combination
of a NOT gate can be easily discussed with the help of electrical circuit. Here, the switch is connected in parallel to the lump of
the battery. The following conclusion can be easily drawn from the working of the electrical circuit.
a) If switch A is open (i.e. A=0), the
lump will glow (i.e. Y=1)
b) If Switch A is closed (i.e. A=1), the
lump will not glow (Y=0).
It follows that in the given electrical circuit,
the lump glows (or output is obtained), when the switch A is not closed. Far this reason, the electrical circuit is called not gate. The
two possible input-output combinations can be written in the form of the table. It is called truth table of NOT gate.
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