What are the logic gates and their truth tables?

Basic logic gates: AND: output 1 only when ALL inputs are 1. OR: output 1 when at least one input is 1. NOT: output is complement of input. NAND: AND followed by NOT. NOR: OR followed by NOT. XOR: output 1 when inputs are different. NAND and NOR are universal gates (can implement any logic function).

Samacheer Kalvi 12th Basic Electronics Engineering Exercise Solutions

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Basic Electronics Engineering — Exercise Solutions

Key Concepts Quick Recap

Device	Symbol	Function
Diode	→	One-way current flow; rectification
Zener diode	⬧	Voltage regulation; reverse breakdown
Transistor (BJT)	NPN/PNP	Amplification and switching
LED	→ with arrows	Converts electrical energy to light
MOSFET	G-D-S	Voltage-controlled switch/amplifier
AND gate	D shape	Output HIGH only if all inputs HIGH
OR gate	Shield shape	Output HIGH if any input HIGH
NOT gate (inverter)	Triangle + circle	Output is complement of input
NAND gate	AND + circle	Universal gate: output LOW only if all HIGH

I. Choose the Best Answer (MCQs)

1. Silicon is a: (a) Conductor (b) Insulator (c) Semiconductor (d) Superconductor

Answer(c) Semiconductor Silicon: 4 valence electrons; not a conductor or insulator at normal conditions; conductivity increases with temperature and doping. Most commonly used semiconductor in electronics.

2. In a p-type semiconductor, the majority carriers are: (a) Electrons (b) Holes (c) Protons (d) Neutrons

Answer(b) Holes p-type: doped with trivalent impurity (boron, aluminium, gallium) → one less valence electron → creates a "hole" (positive charge carrier). Majority: holes. Minority: electrons.

3. A diode in forward bias condition: (a) Has very high resistance (b) Allows current to flow freely (c) Blocks current (d) Acts as an open circuit

Answer(b) Allows current to flow freely Forward bias: p-side connected to +ve, n-side to -ve. Depletion region narrows → current flows. For Si diode: conduction begins at ~0.7V (threshold voltage). Ge diode: ~0.3V.

4. A half-wave rectifier converts: (a) DC to AC (b) AC to pulsating DC (using only one half of the AC cycle) (c) AC to smooth DC (d) High voltage to low voltage

Answer(b) AC to pulsating DC (using only one half of the AC cycle) Half-wave rectifier: one diode; conducts only during positive (or negative) half of AC. Full-wave rectifier: two or four diodes; uses both halves → smoother output; requires filter capacitor.

5. In a transistor used as an amplifier, the configuration with highest power gain is: (a) Common emitter (b) Common base (c) Common collector (d) All same

Answer(a) Common emitter Common emitter (CE) configuration: highest current gain (β or h_FE = IC/IB), high voltage gain, high power gain. Most widely used for amplification. Input at base, output at collector.

6. The output of an AND gate with inputs A=1, B=0 is: (a) 1 (b) 0 (c) 2 (d) Undefined

Answer(b) 0 AND: output = A × B = 1 × 0 = 0. AND output is 1 ONLY when ALL inputs are 1.

7. NAND gate is called a universal gate because: (a) It is the largest gate (b) Any logic function can be implemented using only NAND gates (c) It combines all gates in one chip (d) It uses the least power

Answer(b) Any logic function can be implemented using only NAND gates Universal gates: NAND and NOR. Any Boolean function (AND, OR, NOT, XOR) can be realised using only NAND gates → simplifies manufacturing (one type of gate needed).

II. Short Answer Questions

Q1. Explain the construction and working of a NPN transistor.

Answer

NPN transistor: three layers — N-type emitter	P-type base	N-type collector.

Construction:

Emitter: heavily doped N-type; emits electrons.
Base: thin, lightly doped P-type; controls transistor.
Collector: moderately doped N-type; collects electrons.

Working (as amplifier, CE configuration):

Emitter-base junction: forward biased (small VBE ~0.7V).
Collector-base junction: reverse biased (large VCC).
Small base current (IB) → controls large collector current (IC).
IC = β × IB (β = current gain, typically 50-300).

Applications: Amplifiers, switches, oscillators, digital logic.

Q2. Write the truth table for AND, OR, and NOT gates.

Answer

A	B	AND (A·B)	OR (A+B)	NOT A (Ā)
0	0	0	0	1
0	1	0	1	1
1	0	0	1	0
1	1	1	1	0

NAND truth table (NOT AND):

A	B	NAND (Ā·B̄)
0	0	1
0	1	1
1	0	1
1	1	0

Exam Checklist

p-type: holes majority; n-type: electrons majority.
Forward bias: current flows (Si: >0.7V); Reverse bias: blocks current.
Half-wave rectifier: 1 diode, one half used; Full-wave: 4 diodes (bridge), both halves.
Transistor CE: highest gain; IC = β × IB.
AND: all inputs 1 → output 1. OR: any input 1 → output 1. NOT: inverts input.
NAND and NOR: universal gates.
LED: forward biased diode that emits light; used in displays, indicators.

Frequently Asked Questions

p-n junction diode: formed by joining p-type (holes majority carrier, doped with trivalent atoms — boron) and n-type (electrons majority, doped with pentavalent atoms — phosphorus) semiconductors. Depletion region forms at junction. Forward bias (positive to p, negative to n): depletion region narrows, current flows. Reverse bias: depletion region widens, very little current (only reverse saturation current).