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NCERT Class XII Chapter
Semiconductor Electronics
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Question : 19 of 19
Marks: +1, -0
Write the truth table for the circuits given in figure consisting of NOR gates only. Identify the logic operations (OR, AND, NOT) performed by the two circuits.
Solution:  
Boolean expression for logic circuit (a)
Here the given NOR gate with short circuit input is acting as NOT gate.
Its truth table is
A Y
0 1
1 0
Boolean expression for logic circuit (b)
So, output by De-Morgan’s theorem, Y = (A‾+B‾)‾\overline{(\overline{A}+\overline{B})} = A‾‾⋅B‾‾\overline{\overline{\mathrm{A}}} \cdot \overline{\overline{\mathrm{B}}} = AB
Hence, the logic circuit acts like AND gate.
Its truth table is
A B Y
00 0
01 0
1 0 0
1 1 1
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