Logic Gates and Truth Table
Logic gate The digital circuit that can be analyzed with the help of Boolean algebra is called logic gate or logic circuit. A logic gate has two or more inputs but only one output.
There are primarily three logic gates namely the OR gate, the AND gate and the NOT gate.
Truth table The operation of a logic gate or circuit can be represented in a table which contains all possible inputs and their corresponding outputs is called the truth table. To write the truth table, we use binary digits 1 and 0.
OR gate
The OR gate has two inputs (A and B) and only one output (Y) (Fig. 43).
Fig. 43
Boolean expression is Y = A + B and is read as “Y equals A OR B.”
Realization of OR gate
Fig. 44
 A = 0, B = 0
 A = 0, B = 1
 A = 1, B = 0
 A = 1, B = 1
Truth table for OR gate
A

B

Y = A+ B

0

0

0

0

1

1

1

0

1

1

1

1

AND gate
The AND gate has two inputs (A and B) and only one output (Y). Boolean expression Y = A · B is read as Y equals A AND B.
Fig. 45
Realization of AND gate
Fig. 46
 A = 0, B = 0
 A = 0, B = 1
 A = 1, B = 0
 A = 1, B = 1
Truth table for AND gate
A

B

Y = A. B

0

0

0

0

1

0

1

0

0

1

1

1

NOT Gate
The NOT gate has only one input and only one output. Boolean expression is and is read as “y equals NOT A.”
Fig. 47
Realization of NOT gate The transistor is so biased that the collector voltage V_{CC} = V (voltage corresponding to 1 state)
The resistors R and R_{1} are so chosen that if the input is low, i.e., O, the transistor is in the cut off and hence the voltage appearing at the output will be the same as applied V. Hence Y = V (or state 1).
Fig. 48
If the input is high, the transistor current is in saturation and the net voltage at the output Y is 0 (in state 0).
Truth table for NOT gate
A


0

1

1

0
