PhysicsMediumClass 12

Logic Gates — AND, OR, NOT, NAND, NOR, XOR

Semiconductor

JEE Qs

Hard

min

Master truth tables and Boolean algebra, especially De Morgan's theorems, to simplify complex circuits and understand universal gates.

🧮 Key Formulas

NOT gate: Y = A'

AND gate: Y = A ⋅ B

OR gate: Y = A + B

NAND gate: Y = (A ⋅ B)'

NOR gate: Y = (A + B)'

XOR gate: Y = A ⊕ B = A'B + AB'

XNOR gate: Y = A ⊙ B = AB + A'B'

De Morgan's Theorem 1: (A + B)' = A' ⋅ B'

De Morgan's Theorem 2: (A ⋅ B)' = A' + B'

✅ Key Points for JEE

1Master the truth table, logic symbol, and Boolean expression for each of the seven basic gates (AND, OR, NOT, NAND, NOR, XOR, XNOR).
2NAND and NOR gates are 'universal gates' because any other logic gate (AND, OR, NOT) can be constructed using only NAND gates or only NOR gates. This is a frequently tested concept.
3Boolean algebra identities and De Morgan's theorems are crucial for simplifying complex logic expressions and converting between different gate implementations.
4Understand the 'bubble' convention in logic symbols (e.g., AND gate followed by a bubble forms a NAND gate) and its implication for inversion.
5XOR gate acts as an inequality detector (output 1 if inputs are different) and an odd-parity checker for multiple inputs; XNOR acts as an equality detector and even-parity checker.

⚠️ Common Mistakes

✕Confusing the truth tables or Boolean expressions of NAND/NOR with AND/OR, especially for multiple input gates.
✕Incorrectly applying De Morgan's theorems, often forgetting to invert individual variables after breaking the bar.
✕Errors in simplifying Boolean expressions, leading to incorrect final logic circuits.
✕Misinterpreting the output of XOR/XNOR gates, particularly when asked about their function (e.g., comparator, parity checker).

📝 Practice Questions

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Q27.Consider the following statements: A. The junction area of solar cell is made very narrow compared to a photo diode. B. Solar cells are not connected with any external bias. C. LED is made of lightly doped p-n junction. D. Increase of forward current results in continuous increase of LED light intensity. E. LEDs have to be connected in forward bias for emission of light. Choose the correct answer from the options given below: (1) B, E Only (2) B, D, E Only (3) A, C Only (4) A, C, E Only

2025·MCQMedium

Q43.Which of the following circuits has the same outpur as that of the given circuit? (1) (2) (3) (4)

2025·MCQMedium

Q33.Which of the following circuits represents a forward biased diode? Choose the correct answer from the options given below : (1) (A) and (D) only (2) (B), (D) and (E) only (3) (C) and (E) only (4) (B), (C) and (E) only

2025·ConceptualEasy

Q26. A B Y 0 0 1 0 1 1 1 0 0 1 1 1 To obtain the given truth table, following logic gate should be placed at G: 2025 (22 Jan Shift 2) JEE Main Previous Year Paper (1) OR Gate (2) AND Gate (3) NOR Gate (4) NAND Gate

2025·MCQEasy

Q28.The output of the circuit is low (zero) for : (A) X = 0, Y = 0 (B) X = 0, Y = 1 (C) X = 1, Y = 0 (D) X = 1, Y = 1 Choose the correct answer from the options given below : (1) (B), (C) and (D) only (2) (A), (B) and (C) only (3) (A), (C) and (D) only (4) (A), (B) and (D) only

2025·MCQEasy

Q45. In the circuit shown here, assuming threshold voltage of diode is negligibly small, then voltage VAB is correctly represented by : (1) VAB would be zero at all times (2) (3) (4)

2025·MCQMedium

NCERT Chapters

Class 12 Physics Ch 14: Semiconductor Electronics: Materials, Devices and Simple Circuits