---
title: "Define combinational logic circuit"
subject: "Digital Logic"
chapter: 3
key: "semester_3/digital_logic:combinational-circuit-define"
source: https://www.bctengineer.com/q/semester_3/digital_logic/combinational-circuit-define
site: BCT Engineering
---

# Define combinational logic circuit

Digital Logic · Chapter 3: Combinational Logic Circuits

- Asked in: 2075 Chaitra, 2.5 marks
- Video: [Combinational Logic Circuits](https://www.youtube.com/watch?v=dePcRQr0ITs) — EngineeringSguruji
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/digital_logic/combinational-circuit-define
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-3

## Question

Define combinational logic circuit.

## Short answer

A combinational logic circuit is a digital circuit whose outputs at any instant depend only on the present inputs. It contains no memory elements or feedback, and is built solely from logic gates.

## Answer

A **combinational logic circuit** is a digital circuit whose outputs at any instant depend only on the present combination of its inputs. It has no memory and no feedback, so past inputs do not affect the output.

```text
           +--------------------+
 x1 ──────>|                    |──────> z1
 x2 ──────>|   Combinational    |──────> z2
  :        |   logic (gates)    |   :
 xn ──────>|                    |──────> zm
           +--------------------+
     n inputs                 m outputs
```

**Key features**

- Built only from logic gates (AND, OR, NOT, NAND, NOR, XOR).
- Output = f(present inputs); it can be fully described by a truth table or Boolean expressions, one per output.
- No clock and no storage elements (no flip-flops).
- An n-input circuit has $2^n$ input combinations.

**Examples:** half adder, full adder, subtractor, multiplexer, demultiplexer, encoder, decoder, magnitude comparator, code converters.

By contrast, a **sequential circuit** contains memory (flip-flops), so its output depends on both present inputs and past history (the present state).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Output depends on:
   A. Only present inputs
   B. Past inputs
   C. Clock
   D. Memory
   Answer: A. No memory. (Key point: Definition)

2. Which is NOT combinational?
   A. Decoder
   B. Flip-flop
   C. Full adder
   D. Multiplexer
   Answer: B. Has memory. (Key point: Not combinational)

3. n-input circuit has how many combos?
   A. n^2
   B. 2n
   C. 2^n
   D. n
   Answer: C. 2^n. (Key point: Combos)

## Related in Digital Logic

- [Full subtractor using decoder; combinational circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afull-subtractor-decoder-combinational) — Digital Logic, chapter 4
- [K-map Σ(0,2,3,4,7,8,10,13)+d, logic circuit](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Akmap-logic-circuit-0234) — Digital Logic, chapter 3
- [Combinational vs sequential; BCD-to-decimal decoder](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-to-decimal-decoder) — Digital Logic, chapter 4
- [Combinational vs sequential; latch as bounce eliminator](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Alatch-bounce-eliminator) — Digital Logic, chapter 6
- [Priority encoder; 4-to-2 priority encoder logic](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apriority-encoder-4to2-expressions) — Digital Logic, chapter 4
- [Circuit giving 2's complement of a 4-bit number](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Atwos-complement-generator) — Digital Logic, chapter 5

## Related in other subjects

- [Combinational vs sequential; JK flip-flop](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomb-vs-seq-jk) — Basic Electronics Engineering, chapter 6