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Difference Between Combinational And Sequential Circuit Explained!

Combinational and sequential circuits are types of digital circuits, that primarily differ from each other in terms of output generation techniques. Read ahead about their characteristics & differences.

Shreeya Thakur

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Difference Between Combinational And Sequential Circuit Explained!

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Table of content:

Combinational Circuit
Sequential circuit
Combinational circuit VS Sequential circuit
Frequently Asked Questions
Summing Up

Combinational and sequential circuits are fundamental types of digital circuits widely used in digital electronics. A combinational circuit generates outputs based solely on its current inputs, without considering previous inputs. In contrast, a sequential circuit generates outputs based on both current inputs and the previous state of the circuit, incorporating memory elements.

Hence, the primary distinction between combinational and sequential circuits lies in their output generation: combinational circuits do not have memory, while sequential circuits rely on memory to determine their output.

To understand the difference between combinational and sequential circuits, let's examine the characteristics of each in detail below.

Combinational Circuit

Block diagram

Combinational vs sequential circuit

Characteristics of a Combinational Circuit

Output generation depends upon only the current input.
This type of circuit is a time-independent circuit i.e. independent of a clock signal.
Its elementary building block is a logic gate.
It does not have any memory elements.
It does not have any feedback path between input and output.
It is used for arithmetic circuits and boolean circuits.
It is fast in speed.
It is easy to design and also easy to use.

Example:

Multiplexer, demultiplexer, encoder, decoder, half-adder, full-adder, etc.

Sequential circuit

Block diagram

Combinational vs sequential circuits

Characteristics of a Sequential Circuit

Output generation depends upon the previous output and current input signals.
It depends upon its previous feedback or previous output. It uses the current state (previous output) as control inputs for the next operations.
This type of circuit depends upon time instant, i.e. depends upon clock cycles.
This can be two types- Synchronous circuit (uses clock pulse) and Asynchronous circuit (does not use clock pulse).
Its basic building block is a flip-flop (binary storage device).
It has memory elements.
It is mostly used to build memory devices.
It is slow in speed.
Its design and use are complex in comparison with combinational circuits.

Example:

Flip-flops like SR flip-flop, JK flip-flop, T flip-flop, registers, counters, etc.

Combinational circuit vs. Sequential circuit

The difference between combinational and sequential circuits is given in the table below:

Basis of Comparison	Combinational circuit	Sequential Circuit
Definition	These are the types of circuits that give output depending only on the present input.	These are the types of circuits that give output depending upon present input and previous output.
Feedback	It does not use any feedback.	It has a feedback path from output to input. This feedback path is used to get previous output and provide present output.
Time dependency	It is not time-dependent.	It uses a clock pulse i.e. it is time-dependent.
Performance	Its performance is fast in speed compared to sequential circuits because it only uses present input to give present output.	Its performance is slow in speed compared to combinational circuits because it uses present input as well as previous output to give output.
Complexity	It is easy to build and easy to use and handle.	It is complex to build and also complex to use compared to combinational circuits.
Elementary building units	Its elementary building blocks are logic gates i.e. AND, OR, NOT, NAND, NOR.	Its elementary building blocks are flip-flops i.e. memory units.
Memory elements	It does not have any memory elements, so it cannot store data.	It has flip-flops as memory elements, so it can store the previous output in it.
Operations	It is used for arithmetic and boolean operations.	It is used as a memory device to store data in digital circuits.
Example	Some examples are, Multiplexer, De-multiplexer, Encoder, Decoder, Half-adder, Full-adder, etc. which are used for arithmetic operation and boolean operation.	Some examples are Flip-flops, Registers, Counters, etc. that are used to store data.

Summing up...

Combinational and sequential circuits are two fundamental categories in digital electronics. The key difference between combinational and sequential circuits is that combinational circuits produce output based solely on the current input, whereas sequential circuits generate output based on both the current input and the current state (which reflects the previous output). Further, combinational circuits are best suited for immediate computations, while sequential circuits are essential for operations that require memory and time-based control, making them both crucial in digital system design.

Combinational circuits are time-independent, meaning they do not rely on clock pulses, have no feedback, and lack memory units. Examples include arithmetic and Boolean circuits like multiplexers (MUX) and demultiplexers (De-MUX). On the other hand, sequential circuits are time-dependent, utilizing clock pulses and feedback, which means they incorporate memory elements. Examples include registers, flip-flops, and counters that can store data.

Understanding the distinct roles and characteristics of combinational and sequential circuits is fundamental, as they form the backbone of all digital systems, enabling everything from simple logic operations to complex data processing and storage.

Hope this article helped you grasp the basics and understand the major difference between combinational and sequential circuits. Happy learning!

Frequently Asked Questions

Q1. How do combinational circuits and sequential circuits differ in terms of output?

The output in combinational circuits is only dependent on their current inputs. Thus, unless the input changes, the output of a combinational circuit will not change.

The output in sequential circuits is dependent on both, their current inputs and previous state. Thus, if a sequential circuit's input does not change, but its previous state changes, the output will change.

Q2. What are the major types of combinational circuits?

The major types of combinational circuits are arithmetic circuits, multiplexers and demultiplexers, encoders and decoders, and logic gates.

Q3. What are the major types of sequential circuits?

The major types of sequential circuits are registers, counters, synchronous and asynchronous sequential circuits, and finite-state machines.

Q4. How are combinational and sequential circuits implemented?

Logic gates (AND, OR, NOT, NAND, NOR) are used to implement combinational circuits. Flip-flops (memory units) are used to implement sequential circuits.

Q5. What are the two key differences between combinational and sequential circuits?

The key differences between sequential and combinational circuits lie in their output generation and memory capability. In a combinational circuit, the output depends solely on the current inputs, with no regard for past inputs. As a result, combinational circuits lack memory and cannot store data. In contrast, sequential circuits generate output based on both current inputs and past states, thanks to their memory elements, such as flip-flops, which allow them to store and recall previous outputs.

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Edited by

Shreeya Thakur

Sr. Associate Content Writer at Unstop

I am a biotechnologist-turned-content writer and try to add an element of science in my writings wherever possible. Apart from writing, I like to cook, read and travel.

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Computer Science