/ˌkɑːmbɪˈneɪʃənəl ˈsɜːrkɪt/
noun … “Logic circuit with output determined only by current inputs.”
Combinational Circuit is a type of digital logic circuit whose output depends solely on the present values of its inputs, with no reliance on past states or stored memory. Unlike sequential circuits, combinational circuits have no internal state, clock, or feedback loops. They are built entirely from logic gates and implement Boolean expressions directly.
In a Combinational Circuit, any change in input propagates through the circuit and produces a corresponding change in output after a finite propagation delay. This makes combinational logic predictable, fast, and foundational to digital computation.
Key characteristics of Combinational Circuit include:
- Stateless behavior: no memory of previous inputs.
- Deterministic output: same inputs always produce the same output.
- No clock signal: operates purely on signal propagation.
- Boolean foundation: described using Boolean logic equations.
- Hardware simplicity: constructed from interconnected logic gates.
Common examples of Combinational Circuits include adders, subtractors, multiplexers, demultiplexers, encoders, decoders, and comparators. These circuits form the arithmetic and decision-making core of CPUs and digital systems.
Workflow example: Simple combinational logic (half adder):
a = 1
b = 1
sum = a XOR b -- sum = 0
carry = a AND b -- carry = 1
Here, the outputs sum and carry depend only on the current values of a and b, with no stored state involved.
Conceptually, a Combinational Circuit is like a calculator keypress: the result depends only on what you press right now, not on what you pressed before.
See Logic Gates, Boolean Logic, Digital, CPU, Sequential Circuit.