# Physics Class 12 NCERT Solutions: Chapter 14 Semiconductor Electronics Materials Devices and Simple Circuits Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 18. Write the table for circuit given in Figure below consisting of NOR gates and identify the logic operation which this circuit is performing

(Hint: then A and B inputs of second NOR gate will be and hence . Similarly work out the values of Y for other combinations of A and B. Compare with the truth table of gates and find the correct one.)

Answer:

A and B are the inputs of the given circuit. The output of the first NOR gate is . It can be observed from the following figure that the inputs of the second gate become the output of the first one.

Hence, the output of the combination is given as:

The truth table for this operation is given as:

A | B | |

0 | 0 | 0 |

0 | 1 | 1 |

1 | 0 | 1 |

1 | 1 | 1 |

This is the truth table of an OR gate. Hence, this circuit functions as an OR gate.

Q: 19. 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.

Answer:

(A) A acts as the two inputs of the NOR gate and Y is the output, as shown in the following figure. Hence, the output of the circuit is

Output,

The truth table for the same is given as:

A | |

This is the truth table of a NOT gate. Hence, this circuit functions as a NOT gate.

(B) A and B are the inputs and Y is the output of the given circuit. By using the result obtained in solution (A) , we can infer that the outputs of the first two NOR gates are , as shown in the following figure.

are the inputs for the last NOR gate. Hence, the output for the circuit can be written as:

The truth table for the same can be written as:

A | B | |

0 | 0 | 0 |

0 | 1 | 0 |

1 | 0 | 0 |

1 | 1 | 1 |