Question

Which of the following is not a combinational circuit?
A. Encoders
B. Decoders
C. Registers
D. Multiplexers

Answer 1

Hint: The combinational circuit is a circuit in which the different logic gates are combined to perform the assigned function logically by the use of Boolean expression of the logic gates. To answer the question, explain all the options and check whether the above circuits are constructed by the use of logic gates.

Complete answer:
Let us define a combinational circuit. The combinational circuit is a circuit in which the different logic gates are combined to perform the assigned function logically by the use of Boolean expression of the logic gates. To answer this question, let us briefly explain each of the options as follows,

Encoders: An encoder is a combinational logic circuit which is constructed by combination of NOT gate and AND gate. It has n number of inputs and m number of outputs depending on the input states. Since the encoder is constructed by logic gates, it is a combinational circuit.

Decoder: The decoder performs the inverse operation of the encoder. The decoder also works logically by the use of Boolean expression. Therefore, we can say that the decoder is a combinational circuit.

Multiplexers: In multiplexers, the input is also in the form of binary numbers. The multiplexer selects one of the inputs and routes it into the single output. As we know, the multiplexer is constructed by combining NOT, OR and AND gates. Since the multiplexer is constructed by the use of logic gates, it is a combinational circuit.

Registers: The register is a set of data holding units which is the part of a computer processor. It is not constructed by the use of logic gates and does not perform any operation. Thus, we can say that the register is not a combinational circuit.

Hence, the correct answer is option C.

Note: Students should remember that there are two main types of digital circuits: combinational circuits and sequential circuits. The combinational circuits also have three types: arithmetic and logical circuits, data handling combinational circuits and code converting combinational circuits. Encoders, decoders, multiplexers and demultiplexers fall in these categories.