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From the circuit of the following logic gates the basic logic gate obtained is ................

Logic circuit: inputs A and B each pass through a NOT gate; the two NOT outputs feed a NOR gate; the NOR output and B itself feed a NAND gate with output Y

  • A.NOT gate
  • B.AND gate
  • C.OR gate
  • D.NAND gate

Answer: D

The circuit is a NAND gate.

Follow the signals through. The NOT gates give \(\bar{A}\) and \(\bar{B}\). The NOR gate then gives, by De Morgan's theorem,

\[ \overline{\bar{A} + \bar{B}} = A\cdot B \]

The final NAND gate takes \(AB\) and \(B\):

\[ Y = \overline{(AB)\cdot B} = \overline{AB} \]

since \(AB\cdot B = AB\). So \(Y\) is A NAND B.

The output depends on both inputs, so it is not a NOT gate, and it is the complement of AND, so it is neither AND nor OR: for \(A = B = 0\) it gives 1, which an AND or OR gate would not.

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