Question

The output of a 2-input multiplexer is connected back to one of its inputs as shown in the figure. Match the functional equivalence of this circuit to one of the following options. 

• A. D Flip-flop
• B. D Latch
• C. Half-adder
• D. Demultiplexer

Hint:

A multiplexer with feedback can easily implement latches.
Remember: MUX + feedback $\Rightarrow$ Latch.
Latches are level-sensitive, while flip-flops are edge-triggered.

Answer 1

The Correct Option is B

Step 1: Understanding the circuit.
A 2-to-1 multiplexer has two data inputs (labelled 0 and 1), one select input (S), and one output (Q).
Here, the output $Q$ is fed back to input '0', while an external data input is applied at input '1'.
Step 2: Behavior of the MUX.
When $S=0$: The multiplexer output $Q$ is connected to input '0'. But since '0' is already $Q$, this creates a feedback loop and the output holds its previous value.
When $S=1$: The multiplexer output $Q$ follows the external data input (D).
Step 3: Functional equivalence.
This is exactly how a D latch works:
When Enable (S=1) $\Rightarrow$ Output follows input D.
When Enable (S=0) $\Rightarrow$ Output holds its previous value.
Step 4: Eliminating wrong options.
(A) D Flip-flop: Requires clock edge triggering, which is not present here. So False.
(C) Half-adder: Performs arithmetic sum of two bits, not related here. False.
(D) Demultiplexer: Performs data distribution to multiple outputs, not applicable here. False.
\[ \boxed{\text{The circuit is functionally equivalent to a D Latch.}} \]