Combinational Circuits | Combinational Logic


In digital Electronics, there are two type of digital circuits 1) Combinational Circuits 2) Sequential Circuits.

Combinational circuit is a circuit whose output depend upon the combination of input variables. Its output is depend only on the present input combination, doesn’t depend on any previous input or output. Combination circuit is made up of logic gate.

Now, some characteristics of combinational circuits are:-

  1. At any time instant, the output of the combinational circuit is depend on the present input combination.
  2. These circuits are developed using AND, OR, NOT, NAND and NOR logic gate. These logic gate are the basics building block of combinational circuit.
  3. Combinational circuit doesn’t use any memory element or any feedback. So, it’s the output depend only on the present input combination, doesn’t depend on any previous input or output.
  4. It can have n number of inputs and m number of outputs.
Block Diagram of Combinational Circuit

Application of combinational circuit

A combinational circuit is an digital logic circuit whose output depends on the present combination of inputs. These type of circuits are used in digital electronics, such as computers ALU to perform various arithmetic operations like addition, subtraction, multiplication etc.

Some other operation perform by combinational circuit like multiplexing is perform by multiplexer, decoding is perform by decoder , code conversion is perform by code converter etc.

Examples of Combinational Circuits

There are many different type of combinational circuit :

  1. Half Adder: If we want to add two single bit binary number we use half adder. The half adder having two inputs and two outputs. This circuit has two input A and B, two outputs carry (c) and sum (s).
Block Diagram of Half Adder

2. Full Adder: If we want to add three single bit binary number we use full adder. full adder add two one-bit numbers A and B, and previous carry c. It having three inputs and two outputs carry (c) and sum (s).

Block Diagram of Full Adder

3. Half Subtractor: Half Subtractor is a combinational circuit that subtract two binary bit at the input and produces two output i.e. difference (D) and borrow (B).

Block Diagram of Half Subtractor

4.Full Subtractor: The drawback of an half subtractor can be overcome by full Subtractor. It is a combinational circuit having three input, two one bit number A, B and previous borrow Bin and two output Difference (D) and Borrow (Bo).

Block Diagram of Full Subtractor

5.Decoder: The basics function of decoder is to detects or decode a particular code. It is a combinational circuit that has ‘n’ input lines and maximum 2n output lines. Decoder is identical to a demultiplexer without any data input. It performs operations which are exactly opposite to those of an encoder.

Block Diagram of Decoder

Q1: What is a combinational circuit?

A combinational circuit is a digital circuit whose outputs depend solely on the current input values. It does not have any memory elements, and the outputs are determined by the logical combination of the input signals using various logic gates.

Q2: What are the basic building blocks of combinational circuits?

The basic building blocks of combinational circuits are logic gates such as AND, OR, NOT (inverter), XOR, NAND, and NOR gates. These gates can be combined in different ways to create more complex combinational circuits.

Q3: What are the design considerations for combinational circuits?

When designing combinational circuits, several factors need to be considered, including the desired functionality, the number of inputs and outputs, signal propagation delay, power consumption, complexity, fan-out, and noise immunity.

Q4: How are combinational circuits different from sequential circuits?

Combinational circuits produce outputs based only on the current input values, while sequential circuits have outputs that depend on both the current inputs and the circuit’s internal state (stored in memory elements like flip-flops). Sequential circuits have memory and can store information, enabling them to perform more complex functions.

Q5: What are the applications of combinational circuits?

Combinational circuits are used in various digital systems and electronic devices. Some common applications include arithmetic logic units (ALUs), multiplexers, demultiplexers, encoders, decoders, adders, subtractors, comparators, and various control circuits in computers, calculators, digital signal processors, and communication systems.

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