Q. What is Combinational circuits?

Combinational circuits are interconnected circuits of gates according to definite rules to generate an output relying on its input value. A well-formed combinational circuit must not have feedback loops. A combinational circuit can be signified as a network of gates and consequently can be expressed by a boolean expression or a truth table.

Output of combinational circuit is related to its input by a combinational function that is independent of time. Consequently for an ideal combinational circuit output must change instantly according to changes in input however in actual case there is a minor delay. The delay is generally proportional to depth or number of levels it implies that maximum numbers of gates on any path from input to output. For illustration depth of the combinational circuit in figure below is 2.

Figure: A two level AND-OR combinational circuit

The basic design issue concerning combinational circuits is: Minimization of number of gates. Normal circuit constraints for combinational circuit designs are:

• Depth of circuit must not exceed a specific level
• Number of input lines to a gate (fan in) and to how many gates its output can be fed (fan out) are restraint by circuit power constraints.