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The output f of the 4-to-1 Mux shown in the given figure is

A 4-to-1 multiplexer with data inputs 3, 2 and 1 tied to Vcc, data input 0 tied to ground, select lines x and y at the bottom and output f

  • A.\(\overline{xy} + x\)
  • B.\(x + y\)
  • C.\(\overline{x} + \overline{y}\)
  • D.\(x\overline{y}\)

Answer: B

The multiplexer output is \(x + y\) (option B). Data inputs 1, 2 and 3 are tied to \(V_{CC}\) (logic 1) and input 0 is grounded (logic 0), so the output is 0 only when the select lines choose input 0, that is x = 0 and y = 0, and it is 1 for every other combination. That is the OR function.

\[ f = \overline{x}\,\overline{y}\cdot 0 + \overline{x}y\cdot 1 \]

\[ \quad + x\overline{y}\cdot 1 + xy\cdot 1 = x + y \]

Options A and C are 1 for most or all inputs, and option D, \(x\overline{y}\), is 1 for only one combination, so none of them matches the OR behaviour.

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