1. Which logic gate gives a HIGH output only when all its inputs are HIGH?
Correct Answer: b) AND
Explanation: The AND gate follows the logical conjunction principle where the output is HIGH (1) only when all inputs are HIGH (1). For any other combination of inputs (where one or more inputs are LOW/0), the output will be LOW (0). This behavior makes the AND gate fundamental for implementing logical conditions where all criteria must be satisfied simultaneously.
2. What is the output of a NAND gate when both inputs are 1?
Correct Answer: a) 0
Explanation: A NAND gate is an AND gate followed by a NOT gate (inverter). When both inputs are 1: 1. The AND operation gives 1 (since 1 AND 1 = 1) 2. The NOT operation inverts this to 0 Therefore, NAND(1,1) = 0. This is why NAND is called a "universal gate" - the fact that it can output 0 when both inputs are 1 is crucial for constructing other logic functions.
3. Which gate is also known as the universal gate?
Correct Answer: d) NAND
Explanation: NAND gates are called universal gates because any other logic gate (AND, OR, NOT, etc.) can be constructed using only NAND gates. Similarly, NOR gates are also universal gates. This property is fundamental in digital electronics because it means entire circuits can be built using just one type of gate, simplifying manufacturing and design. For example: - A NOT gate can be made by connecting both inputs of a NAND gate together - An AND gate is a NAND followed by a NOT (which itself can be made from a NAND)
4. Which logic gate produces an output that is the inverse of the input?
Correct Answer: c) NOT
Explanation: The NOT gate (also called an inverter) is the simplest logic gate with one input and one output. Its function is to invert the input signal: - If input is 1, output is 0 - If input is 0, output is 1 This operation is fundamental in digital logic and is represented by a bar over the variable (Ā) or a prime (A'). All other gates mentioned have two or more inputs and different truth tables.
5. Which gate provides a HIGH output when one (but not both) of the inputs is HIGH?
Correct Answer: b) XOR
Explanation: The XOR (exclusive OR) gate has the unique property of outputting 1 when the inputs are different: - 0 XOR 0 = 0 - 0 XOR 1 = 1 - 1 XOR 0 = 1 - 1 XOR 1 = 0 This "difference detector" behavior makes XOR gates essential in applications like binary addition (where it calculates the sum bit), parity generation, and error detection circuits. The output is HIGH only when exactly one input is HIGH.
6. Which gate can be used to construct any other logic gate?
Correct Answer: b) NOR
Explanation: NOR gates, like NAND gates, are universal gates that can be used to construct any other logic gate. Here's how: - NOT gate: Connect both inputs together (A NOR A = NOT A) - OR gate: NOR followed by NOT (using NORs) - AND gate: Using De Morgan's laws with NORs This universality property is why some early computers used only NOR gates in their design. The ability to build any logic function from just one type of gate simplifies manufacturing and reduces component variety.
7. CMOS logic gates use which type of transistors?
Correct Answer: b) MOSFETs
Explanation: CMOS (Complementary Metal-Oxide-Semiconductor) technology uses MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) exclusively. The "complementary" part refers to using both: - NMOS (n-channel MOSFET) transistors for the pull-down network - PMOS (p-channel MOSFET) transistors for the pull-up network This complementary arrangement is what gives CMOS its extremely low static power consumption, as one type is always off when the other is on, preventing direct current paths between power and ground.
8. What is the primary advantage of CMOS over TTL logic?
Correct Answer: b) Lower power consumption
Explanation: The key advantage of CMOS is its extremely low static power consumption because: 1. In steady state (not switching), there's no direct path between VDD and ground 2. Current only flows during switching to charge/discharge capacitances 3. Input impedance is very high (MOSFET gates are essentially capacitors) While modern CMOS can be fast (option a), this wasn't always true. TTL typically has faster edge rates but much higher power consumption. CMOS also scales better to smaller geometries, enabling modern high-density ICs.
9. Which type of MOSFET is used in CMOS technology?
Correct Answer: c) Both NMOS and PMOS
Explanation: CMOS technology uses complementary pairs of: - NMOS transistors (n-channel MOSFETs): Good at passing 0s (strong pull-down) - PMOS transistors (p-channel MOSFETs): Good at passing 1s (strong pull-up) This complementary arrangement ensures: 1. Rail-to-rail output swing (0V to VDD) 2. High noise margins 3. Low static power consumption The combination allows for efficient implementation of logic functions where one network is on while the other is off.
10. What happens when both NMOS and PMOS transistors in a CMOS gate are conducting?
Correct Answer: a) There is a direct short-circuit
Explanation: When both NMOS and PMOS transistors in a CMOS gate conduct simultaneously: 1. A low-resistance path forms between VDD and ground 2. This creates a short-circuit condition called "shoot-through" 3. Large current flows, potentially damaging the circuit 4. Power dissipation spikes dramatically This condition should be avoided by: - Proper input signal timing - Ensuring non-overlapping clock phases - Careful design of transition states Well-designed CMOS circuits minimize this condition to brief moments during switching.
11. Which factor significantly affects the speed of CMOS circuits?
Correct Answer: a) Gate capacitance
Explanation: The speed of CMOS circuits is primarily determined by how quickly the gate capacitance can be charged and discharged. This capacitance includes: - The gate capacitance of MOSFETs themselves - The interconnect capacitance between components - The load capacitance of connected gates The time constant τ = RC, where R is the transistor's on-resistance and C is the total capacitance, directly affects switching speed.
12. What is the main disadvantage of CMOS technology?
Correct Answer: b) Large area requirement
Explanation: CMOS technology requires more silicon area compared to other technologies because: 1. It uses complementary pairs of NMOS and PMOS transistors 2. PMOS transistors typically need to be larger than NMOS for equal current drive 3. Well and substrate contacts consume additional area While modern CMOS has addressed speed and power issues, the area overhead remains a fundamental characteristic.
13. According to Boolean algebra, A + A = ?
Correct Answer: c) A
Explanation: This demonstrates the Idempotent Law in Boolean algebra, which states that for any Boolean variable A: - A + A = A (OR form) - A · A = A (AND form) Intuitively, saying "A OR A" is the same as just saying "A". This law differs from conventional algebra where x + x = 2x.
14. What is De Morgan's first theorem?
Correct Answer: a) (A + B)' = A'B'
Explanation: De Morgan's Theorems are fundamental duality principles in Boolean algebra. The first theorem states: "The complement of a sum is equal to the product of complements" Mathematically: (A + B)' = A' · B' This theorem is crucial for converting between AND and OR forms and simplifying complex Boolean expressions.
15. Which Boolean expression represents the identity law?
Correct Answer: a) A + 0 = A
Explanation: The Identity Law states that a Boolean variable combined with an identity element leaves it unchanged: - A + 0 = A (OR identity) - A · 1 = A (AND identity) These laws parallel arithmetic where x + 0 = x and x × 1 = x.
16. The complement of (A + B)' using De Morgan's Theorem is:
Correct Answer: a) A'B'
Explanation: This question demonstrates the application of De Morgan's Theorem: 1. Original expression: (A + B)' 2. Apply De Morgan's: A' · B' The complement of a sum becomes the product of complements. This transformation is essential for converting between NAND and NOR implementations.
17. Which Boolean algebra property is represented by: A + AB = A?
Correct Answer: b) Absorption Law
Explanation: The Absorption Law states that: - A + AB = A - A(A + B) = A This law "absorbs" redundant terms. In the first case, if A is true, the whole expression is true regardless of B. If A is false, AB is also false.
18. In Boolean algebra, A + A'B simplifies to:
Correct Answer: a) A + B
Explanation: This simplification uses the consensus theorem: 1. Original expression: A + A'B 2. Can be rewritten as: (A + A')(A + B) [by distributive law] 3. A + A' = 1 (complement law) 4. 1 · (A + B) = A + B (identity law) This shows how a term (A) can "absorb" another term (A'B) when they share a relationship through complements.
19. Which Boolean theorem states that A · A = A?
Correct Answer: c) Idempotent Law
Explanation: The Idempotent Law states that: - A · A = A (AND form) - A + A = A (OR form) This means applying the operation multiple times with the same input doesn't change the result. It differs from conventional algebra where x·x = x². In Boolean algebra, there are only two possible values (0 and 1), and for both: 0·0=0 and 1·1=1.
20. Which of the following expressions is always equal to 1?
Correct Answer: a) A + A'
Explanation: This represents the Complement Law in Boolean algebra: - A + A' = 1 (OR form) - A · A' = 0 (AND form) The first case (option a) is always true because for any Boolean variable A: - If A is 1, then A + A' = 1 + 0 = 1 - If A is 0, then A + A' = 0 + 1 = 1 This law forms the basis for many simplification techniques and proves useful in digital circuit design.