2. Logic Gates
• A binary signal can have two states; logic '0'
and logic '1'.
• Logic states are represented two voltage or
current levels.
• Positive logic System
o Higher voltage or current level represents logic '1'
o Lower voltage or current level represents logic '0'; system.
• Negative logic system
o Higher voltage or current level represents logic '0'
o Lower voltage or current level represents logic '1'
4. Logic Gates
• Basic logic circuits with one or more inputs and one
output are known as gates
• Gates are used as the building blocks in the design of
more complex digital logic circuits
5. AND Gate
• An AND gate is a logic circuit whose output remains at
logic '1' when all the inputs are at logic '1'.
• Conversely, if any of the input is at logic '0' the output will
be at logic '0'. This gate has one output and many inputs.
6. OR Gate
• An OR gate is a logic circuit whose output remains at
logic '1' when any one or all of the inputs are at logic '1'.
• On the other hand, if all the inputs are at logic '0' the
output will be at logic '0'. This gate has one output and
many inputs.
7. NOT Gate
• A NOT gate is a logic circuit whose output remains at
logic '1' when the input is at logic '0' andvice versa.
• It is also known as inverter. This gate has one input and
one output.
8. BASIC GATES
• AND, OR and NOT gates are known as basic gates as
they implement the three basic logic operations.
9. XOR Gate
• The exclusive- OR (XOR) is a logic circuit whose output
is at logic '1'when odd numbers of inputs are at logic'1'.
On the contrary, the output is at logic '0' when either all
the inputs are at logic '0' or even number of inputs are at
logic'1'. It has two inputs and one output
10. XNOR Gate
• The exclusive- NOR (XNOR) is a logic circuit whose
output is at logic '0' when odd numbers of inputs are at
logic'1'. On the contrary, the output is at logic '1' when
either all the inputs are at logic '0' or even number of
inputs are at logic'1'.
• It is obtained by connecting a NOT gate at the output of
an XOR gate. It also has two inputs and one output.
11. NAND Gate:
• NAND gate can also be obtained by connecting a NOT
gate at the output of an AND gate. It is defined as a logic
circuit whose output remains at logic '1' if any or all the
inputs are at logic '0'.
• The output remains at logic '0' if all the inputs are at logic
'1'. This gate has one output and many inputs.
12. NOR Gate
• NOR gate can also be obtained by connecting a NOT gate
at the output of an OR gate. It is defined as a logic circuit
whose output remains at logic '1' if all the inputs are at
logic '0'.
• The output remains at logic '0' if any or all the inputs are at
logic '1'. This gate has one output and many inputs.
13. Universal Gates
• NAND and NOR gates are also known as universal
gates as all other gates can be derived from them.
