Definition

• Logic circuits are fundamental components in digital electronics, used to perform logical operations on binary inputs.
• there are two types of logic circuits
  1. combinational circuits
  2. sequential circuits

Combinational circuits

• made from the basic and universal gates
• The output is defined by the logic and it is depend only on the present input states y the previous states.

Types of Logic Circuits

Adder

• circuits which performs the addition of binary bits.
• The logic circuits which performs the addition of two bit is called half adder and the three bit is called full adder

Half Adder

• the two inputs of the half adder are augend and added, the outputs are sum and carry.

Full Adder

• The three inputs of the full adder are augend, added and the carry inputs from the previous addition, the outputs are sum and carry
• The Full Adder can be implemented using two Half Adders and OR Gate

Subtractor

• Logic circuit which is used to subtract two binary numbers and provides difference and borrow output.

Half Subtractor

• Half Subtractor is used for subtracting one single bit binary from another bit binary

Full Subtractor

• Logic circuit used to subtract three single bit binary digits

Parallel Adder-Subtractor

1. Four bit Parallel binary Adder

• In practical situations it’s required to add two data each containing more than one bit.
• Two binary numbers each of n bits can be added by means of a full adder circuit.

2. Four Bit Parallel Binary Subtractor

• We can design four bit parallel subtractor by connecting three full subtractor and one half adder

3. Parallel Binary Adder-Subtractor

• The addition and subtraction operations can be perform using a common adder circuit, where a EX-OR gate is connected in the second input along with the mode selection bit M.

4. Carry look ahead Adder

• In parallel adder the carry input depends in the carry output if the previous stage
• This processes leads to time delay in addition. This delay is called propagation delay.
• The process can be speeding up by eliminating the inter stage carry delay called look ahead carry addition.
• In uses two functions carry generate and carry propagate

BCD Adder

• In digital system, the decimal number is represented in the form of binary coded decimal (BCD).
• The ten digit (0-9) decimal numbers are represented by the binary digits
• The circuit which add the two BCD number is called BCD adder. The BCD cannot be greater than 9.
• There are three different cases in BCD Addition i)Sum is less than or equal to 9 with carry 0 ii)Sum is greater than 9 with carry 0 iii)Sum equals 9 or less with carry 1