Q: Operational condition for NOR: For a multi-input NOR gate, under which input condition will the output be 0 (LOW)?

Introduction / Context: For reliable logic design, one must know the precise conditions for NOR outputs. NOR is widely used in active-low logic and for functionally complete implementations. Given Data / Assumptions: NOR = NOT(OR). Inputs can be any combination of 0 and 1. Concept / Approach: If any input is 1, OR outputs 1. Inverting that, NOR outputs 0 whenever at least one input is 1. NOR outputs 1 only when all inputs are 0. Step-by-Step Solution: 1) Evaluate OR for input pattern.2) Invert to get NOR.3) Conclude output 0 occurs whenever any input = 1. Verification / Alternative check: Construct a truth table for a two-input NOR; generalizes to n-input. Why Other Options Are Wrong: “Whenever a 0…” contradicts NOR; “Only when all inputs = 0” is NOR = 1 case; “Only when all inputs = 1” is not special for NOR output 0 because any single 1 already forces 0. Common Pitfalls: Confusing NOR with NAND; mixing up the OR trigger condition. Final Answer: Whenever a 1 is present at an input

Q: TTL troubleshooting scenario: An inverter feeds one input of a 2-input AND gate. If the inverter’s input pin is left open (no drive), and logic pulses are applied to the other AND input (point B), what appears at the AND output?

Introduction / Context: In TTL, a floating input tends to read as HIGH. When that floating node is the input to an inverter, the inverter’s output becomes a constant LOW. Understanding this lets you predict downstream gate behavior during faults. Given Data / Assumptions: Standard TTL inverter driving one input of a two-input AND gate. The inverter’s input is open (floating). Point B (the other AND input) carries logic pulses. Concept / Approach: A floating TTL input reads HIGH; therefore the inverter outputs a steady LOW. The AND gate thus sees inputs (LOW, pulses), and LOW AND anything is LOW, independent of the pulses. Step-by-Step Solution: 1) Floating TTL input → reads HIGH.2) Inverter HIGH in → LOW out.3) AND truth: 0 * X = 0, so output is a steady LOW. Verification / Alternative check: Check truth tables and TTL input bias behavior to confirm the constant LOW at the inverter output and, consequently, the AND output. Why Other Options Are Wrong: “a steady HIGH” contradicts AND with a constant LOW input; “pulses” would occur only if the other input were a constant HIGH; “an undefined level” reflects CMOS-like floating behavior, not typical TTL input biasing. Common Pitfalls: Assuming floating equals undefined in TTL; ignoring the inversion before the AND stage. Final Answer: a steady LOW

Q: Operational condition for NAND: For a multi-input NAND gate, under which input condition will the output be 1 (HIGH)?

Introduction / Context: NAND is ubiquitous because it is functionally complete. Knowing when its output is HIGH clarifies many design shortcuts and fault predictions. Given Data / Assumptions: NAND = NOT(AND). Inputs can be any pattern of 0 and 1. Concept / Approach: AND outputs 1 only when all inputs are 1. Therefore, NAND outputs 0 only when all inputs are 1; in every other case (i.e., when any input is 0), NAND outputs 1. Step-by-Step Solution: 1) Evaluate AND for the input pattern.2) Invert to obtain NAND.3) Conclude output 1 occurs whenever at least one input is 0. Verification / Alternative check: Construct the two-input NAND table and observe the three rows producing 1 versus the single all-1 row producing 0. Why Other Options Are Wrong: “Only when all inputs = 0” confuses NOR with NAND; “Whenever a 1…” is AND-centric; “Only when all = 1” is exactly the LOW case for NAND. Common Pitfalls: Mixing NOR and NAND rules; forgetting the inversion step relative to AND. Final Answer: Whenever a 0 is present at an input

Q: Among TTL, CMOS, and ECL logic families, which key advantage is typically attributed to Emitter-Coupled Logic (ECL) compared to TTL and CMOS?

Introduction / Context: Logic families trade off speed, power, and noise margins. TTL and CMOS are common in general-purpose designs, whereas ECL is chosen when very high-speed switching is paramount. Given Data / Assumptions: ECL operates transistors in the active region, avoiding saturation delays. Comparative families: TTL (bipolar, saturating) and CMOS (field-effect, low static power). Concept / Approach: By avoiding transistor saturation and using differential pairs, ECL achieves extremely fast transitions at the expense of higher static power consumption and smaller logic swings, which also aids speed but reduces noise margins. Step-by-Step Solution: 1) TTL: moderate speed, moderate power.2) CMOS: low static power, wide supply range, speed improves with scaling.3) ECL: highest speed (shortest propagation delays) due to non-saturating operation and constant-current biasing. Verification / Alternative check: Datasheets show ECL propagation delays markedly lower than comparable TTL/CMOS of similar eras. Why Other Options Are Wrong: “low power dissipation” is not an ECL hallmark; “both…” conflicts with ECL’s higher power; “neither…” ignores ECL’s defining speed advantage. Common Pitfalls: Assuming CMOS is always fastest; conflating modern high-speed CMOS with classic family comparisons. Final Answer: high speed

Q: Digital Electronics — For a 4-input logic gate, how many distinct binary input combinations (covering every 0/1 state across all four inputs) are possible?

Introduction: Digital logic gates respond to all possible binary input patterns. For a gate with n inputs, every input can be 0 or 1, so the total number of distinct input combinations equals the count of all 0/1 sequences of length n. This question checks your grasp of that fundamental counting idea for a 4-input gate. Given Data / Assumptions: Number of inputs n = 4 Each input can be 0 or 1 only We assume ideal binary logic without undefined or high-impedance states Concept / Approach: The number of binary strings of length n is 2^n because each position has 2 choices (0 or 1) and choices multiply across positions. This applies regardless of the specific gate type (AND, OR, XOR, etc.); the gate type affects outputs, not the count of input patterns. Step-by-Step Solution: Step 1: Each input has 2 states: 0 or 1.Step 2: With n independent inputs, total combinations = 2^n.Step 3: Substitute n = 4 to get 2^4 = 16. Verification / Alternative check: List examples mentally: 0000, 0001, 0010, ..., 1111. Counting from 0 to 15 in binary yields 16 patterns in total. Why Other Options Are Wrong: 8: This is 2^3, which would apply to a 3-input gate, not 4.32: This is 2^5, which corresponds to 5 inputs.4: This equals n, not 2^n; it ignores binary branching per input. Common Pitfalls: Confusing number of inputs (n) with number of combinations (2^n).Assuming gate type changes the count; it does not. Final Answer: 16

Q: Logic Behavior — A is LOW or B is LOW (or both) implies X is LOW; only when A and B are both HIGH does X become HIGH. Which 2-input gate follows this rule?

Introduction: The prompt describes an output that is HIGH exclusively when both inputs are HIGH, and LOW otherwise. Recognizing this verbal truth table is a core skill in digital electronics for mapping behaviors to gate names. Given Data / Assumptions: Inputs: A and B are binary (LOW = 0, HIGH = 1) Output: X is binary X is HIGH only if A = 1 and B = 1 Concept / Approach: The AND function outputs 1 only when all inputs are 1. If any input is 0, the output is 0. This statement precisely matches the verbal rule provided. Step-by-Step Solution: Step 1: Translate text to logic: X = 1 only when A = 1 and B = 1.Step 2: Standard AND truth table: 00 → 0, 01 → 0, 10 → 0, 11 → 1.Step 3: The rule matches the AND truth table exactly. Verification / Alternative check: Compare to OR: OR would output 1 when any input is 1, which contradicts the prompt. XOR outputs 1 when inputs differ, also not matching. NAND outputs 0 only for 11, which is the inverse of the described behavior. Why Other Options Are Wrong: XOR gate: High for unequal inputs, not both high.OR gate: High if any input is high; contradicts rule for single highs.NAND gate: Inverts AND; output would be LOW when both inputs are HIGH. Common Pitfalls: Misreading the phrase "or both are LOW" as an OR description; the key is output LOW in those cases.Confusing NAND with AND due to similar names. Final Answer: AND gate

Q: Controlled Inversion — Which 2-input logic gate can pass a digital waveform unchanged at some times and invert it at other times (using one input as a control)?

Introduction: Many digital designs use a gate as a conditional inverter. The idea is to feed the data signal on one input and a control signal on the other so that sometimes the output equals the input and other times it equals the inverted input. This question asks which 2-input gate provides exactly that behavior. Given Data / Assumptions: Two inputs: Data D and control C Goal: Output X equals D when C = 0 and equals NOT D when C = 1 Concept / Approach: XOR implements X = D XOR C. When C = 0, XOR acts as a buffer (X = D). When C = 1, XOR acts as an inverter (X = NOT D). This is why XOR is often described as a controlled inverter or programmable inverter. Step-by-Step Solution: Step 1: Define X = D XOR C.Step 2: If C = 0, then X = D XOR 0 = D.Step 3: If C = 1, then X = D XOR 1 = NOT D. Verification / Alternative check: XNOR (the complement) would pass when C = 1 and invert when C = 0, which is the opposite control convention but not what is usually meant by controlled inversion from the question wording. Why Other Options Are Wrong: XNOR gate: Acts as pass when control is 1 and invert when control is 0; opposite polarity.NAND gate: Does not provide clean pass vs invert based solely on a single control bit.OR gate: Cannot invert the data based on a single control in this simple configuration. Common Pitfalls: Confusing XOR with XNOR; pay attention to control polarity.Assuming any two-input gate can be wired to behave like a controlled inverter without extra logic. Final Answer: XOR gate

Q: XOR Property — The output of a 2-input XOR gate is HIGH under which input relationship?

Introduction: The exclusive OR (XOR) gate is essential in parity generation, adders, and control logic. Understanding when its output goes HIGH is fundamental for designing and analyzing digital circuits. Given Data / Assumptions: 2 inputs: A and B Binary levels only (0 and 1) Ideal logic with no undefined states in normal operation Concept / Approach: XOR outputs 1 if and only if exactly one input is 1. In other words, the inputs must differ. If both inputs are equal (00 or 11), XOR outputs 0. Step-by-Step Solution: Step 1: Write XOR condition: X = 1 when A != B.Step 2: Evaluate pairs: 00 → 0, 01 → 1, 10 → 1, 11 → 0.Step 3: Conclude output is HIGH only for unequal inputs (01 and 10). Verification / Alternative check: Truth table enumeration or algebraic identity: A XOR B = A NOT B + NOT A B; both terms cannot be true at the same time when inputs are equal. Why Other Options Are Wrong: Both inputs are 0: Produces 0 for XOR.Both inputs are 1: Produces 0 for XOR.Both inputs are undefined: Standard XOR truth table does not rely on undefined states. Common Pitfalls: Confusing XOR with OR; OR is high for 01, 10, and 11.Assuming equality creates a high output; that is XNOR, not XOR. Final Answer: Inputs are unequal

Question 1

TTL fault diagnosis: An inverter (NOT gate) has an internal open circuit. What DC level would you most likely measure directly at the inverter’s output pin in such a fault condition?

Accepted Answer

Introduction / Context: Diagnosing TTL faults often involves recognizing characteristic voltages. A healthy TTL LOW is typically below about 0.4 V, and a healthy TTL HIGH is typically above about 2.4 V. Faults can yield mid-levels near 1.6 V, which are indeterminate for logic. Given Data / Assumptions: Standard TTL inverter with a totem-pole output stage. Internal open circuit disrupts the normal pull-up/pull-down path. No external pull resistors clamping the node. Concept / Approach: With an internal open, the output transistor network cannot drive a valid HIGH or LOW. The node may sit at a mid-rail potential due to leakage and internal bias, commonly around 1.6 V in TTL, which is outside guaranteed logic thresholds and thus undefined for reliable operation. Step-by-Step Solution: 1) Identify normal TTL VOH >= ~2.4 V, VOL <= ~0.4 V.2) An internal open removes a solid current path, leaving the node to float to a mid-level.3) The common observed mid-level is near 1.6 V. Verification / Alternative check: Service manuals and TTL family datasheets discuss undefined mid-levels under fault conditions; scope measurements typically show unstable or mid-rail DC. Why Other Options Are Wrong: “Less than 0.4 V” and “Greater than 2.4 V” imply a healthy LOW or HIGH; “All of the above” cannot be correct because these are mutually exclusive ranges. Common Pitfalls: Assuming every failure rails the output; confusing TTL thresholds with CMOS levels. Final Answer: 1.6 V

Question 2

Operational condition for NOR: For a multi-input NOR gate, under which input condition will the output be 0 (LOW)?

Accepted Answer

Introduction / Context: For reliable logic design, one must know the precise conditions for NOR outputs. NOR is widely used in active-low logic and for functionally complete implementations. Given Data / Assumptions: NOR = NOT(OR). Inputs can be any combination of 0 and 1. Concept / Approach: If any input is 1, OR outputs 1. Inverting that, NOR outputs 0 whenever at least one input is 1. NOR outputs 1 only when all inputs are 0. Step-by-Step Solution: 1) Evaluate OR for input pattern.2) Invert to get NOR.3) Conclude output 0 occurs whenever any input = 1. Verification / Alternative check: Construct a truth table for a two-input NOR; generalizes to n-input. Why Other Options Are Wrong: “Whenever a 0…” contradicts NOR; “Only when all inputs = 0” is NOR = 1 case; “Only when all inputs = 1” is not special for NOR output 0 because any single 1 already forces 0. Common Pitfalls: Confusing NOR with NAND; mixing up the OR trigger condition. Final Answer: Whenever a 1 is present at an input

Question 3

TTL troubleshooting scenario: An inverter feeds one input of a 2-input AND gate. If the inverter’s input pin is left open (no drive), and logic pulses are applied to the other AND input (point B), what appears at the AND output?

Accepted Answer

Introduction / Context: In TTL, a floating input tends to read as HIGH. When that floating node is the input to an inverter, the inverter’s output becomes a constant LOW. Understanding this lets you predict downstream gate behavior during faults. Given Data / Assumptions: Standard TTL inverter driving one input of a two-input AND gate. The inverter’s input is open (floating). Point B (the other AND input) carries logic pulses. Concept / Approach: A floating TTL input reads HIGH; therefore the inverter outputs a steady LOW. The AND gate thus sees inputs (LOW, pulses), and LOW AND anything is LOW, independent of the pulses. Step-by-Step Solution: 1) Floating TTL input → reads HIGH.2) Inverter HIGH in → LOW out.3) AND truth: 0 * X = 0, so output is a steady LOW. Verification / Alternative check: Check truth tables and TTL input bias behavior to confirm the constant LOW at the inverter output and, consequently, the AND output. Why Other Options Are Wrong: “a steady HIGH” contradicts AND with a constant LOW input; “pulses” would occur only if the other input were a constant HIGH; “an undefined level” reflects CMOS-like floating behavior, not typical TTL input biasing. Common Pitfalls: Assuming floating equals undefined in TTL; ignoring the inversion before the AND stage. Final Answer: a steady LOW

Question 4

Operational condition for NAND: For a multi-input NAND gate, under which input condition will the output be 1 (HIGH)?

Accepted Answer

Introduction / Context: NAND is ubiquitous because it is functionally complete. Knowing when its output is HIGH clarifies many design shortcuts and fault predictions. Given Data / Assumptions: NAND = NOT(AND). Inputs can be any pattern of 0 and 1. Concept / Approach: AND outputs 1 only when all inputs are 1. Therefore, NAND outputs 0 only when all inputs are 1; in every other case (i.e., when any input is 0), NAND outputs 1. Step-by-Step Solution: 1) Evaluate AND for the input pattern.2) Invert to obtain NAND.3) Conclude output 1 occurs whenever at least one input is 0. Verification / Alternative check: Construct the two-input NAND table and observe the three rows producing 1 versus the single all-1 row producing 0. Why Other Options Are Wrong: “Only when all inputs = 0” confuses NOR with NAND; “Whenever a 1…” is AND-centric; “Only when all = 1” is exactly the LOW case for NAND. Common Pitfalls: Mixing NOR and NAND rules; forgetting the inversion step relative to AND. Final Answer: Whenever a 0 is present at an input

Question 5

Among TTL, CMOS, and ECL logic families, which key advantage is typically attributed to Emitter-Coupled Logic (ECL) compared to TTL and CMOS?

Accepted Answer

Introduction / Context: Logic families trade off speed, power, and noise margins. TTL and CMOS are common in general-purpose designs, whereas ECL is chosen when very high-speed switching is paramount. Given Data / Assumptions: ECL operates transistors in the active region, avoiding saturation delays. Comparative families: TTL (bipolar, saturating) and CMOS (field-effect, low static power). Concept / Approach: By avoiding transistor saturation and using differential pairs, ECL achieves extremely fast transitions at the expense of higher static power consumption and smaller logic swings, which also aids speed but reduces noise margins. Step-by-Step Solution: 1) TTL: moderate speed, moderate power.2) CMOS: low static power, wide supply range, speed improves with scaling.3) ECL: highest speed (shortest propagation delays) due to non-saturating operation and constant-current biasing. Verification / Alternative check: Datasheets show ECL propagation delays markedly lower than comparable TTL/CMOS of similar eras. Why Other Options Are Wrong: “low power dissipation” is not an ECL hallmark; “both…” conflicts with ECL’s higher power; “neither…” ignores ECL’s defining speed advantage. Common Pitfalls: Assuming CMOS is always fastest; conflating modern high-speed CMOS with classic family comparisons. Final Answer: high speed

Question 6

Digital Electronics — For a 4-input logic gate, how many distinct binary input combinations (covering every 0/1 state across all four inputs) are possible?

Accepted Answer

Introduction: Digital logic gates respond to all possible binary input patterns. For a gate with n inputs, every input can be 0 or 1, so the total number of distinct input combinations equals the count of all 0/1 sequences of length n. This question checks your grasp of that fundamental counting idea for a 4-input gate. Given Data / Assumptions: Number of inputs n = 4 Each input can be 0 or 1 only We assume ideal binary logic without undefined or high-impedance states Concept / Approach: The number of binary strings of length n is 2^n because each position has 2 choices (0 or 1) and choices multiply across positions. This applies regardless of the specific gate type (AND, OR, XOR, etc.); the gate type affects outputs, not the count of input patterns. Step-by-Step Solution: Step 1: Each input has 2 states: 0 or 1.Step 2: With n independent inputs, total combinations = 2^n.Step 3: Substitute n = 4 to get 2^4 = 16. Verification / Alternative check: List examples mentally: 0000, 0001, 0010, ..., 1111. Counting from 0 to 15 in binary yields 16 patterns in total. Why Other Options Are Wrong: 8: This is 2^3, which would apply to a 3-input gate, not 4.32: This is 2^5, which corresponds to 5 inputs.4: This equals n, not 2^n; it ignores binary branching per input. Common Pitfalls: Confusing number of inputs (n) with number of combinations (2^n).Assuming gate type changes the count; it does not. Final Answer: 16

Question 7

Logic Behavior — A is LOW or B is LOW (or both) implies X is LOW; only when A and B are both HIGH does X become HIGH. Which 2-input gate follows this rule?

Accepted Answer

Introduction: The prompt describes an output that is HIGH exclusively when both inputs are HIGH, and LOW otherwise. Recognizing this verbal truth table is a core skill in digital electronics for mapping behaviors to gate names. Given Data / Assumptions: Inputs: A and B are binary (LOW = 0, HIGH = 1) Output: X is binary X is HIGH only if A = 1 and B = 1 Concept / Approach: The AND function outputs 1 only when all inputs are 1. If any input is 0, the output is 0. This statement precisely matches the verbal rule provided. Step-by-Step Solution: Step 1: Translate text to logic: X = 1 only when A = 1 and B = 1.Step 2: Standard AND truth table: 00 → 0, 01 → 0, 10 → 0, 11 → 1.Step 3: The rule matches the AND truth table exactly. Verification / Alternative check: Compare to OR: OR would output 1 when any input is 1, which contradicts the prompt. XOR outputs 1 when inputs differ, also not matching. NAND outputs 0 only for 11, which is the inverse of the described behavior. Why Other Options Are Wrong: XOR gate: High for unequal inputs, not both high.OR gate: High if any input is high; contradicts rule for single highs.NAND gate: Inverts AND; output would be LOW when both inputs are HIGH. Common Pitfalls: Misreading the phrase "or both are LOW" as an OR description; the key is output LOW in those cases.Confusing NAND with AND due to similar names. Final Answer: AND gate

Question 8

Controlled Inversion — Which 2-input logic gate can pass a digital waveform unchanged at some times and invert it at other times (using one input as a control)?

Accepted Answer

Introduction: Many digital designs use a gate as a conditional inverter. The idea is to feed the data signal on one input and a control signal on the other so that sometimes the output equals the input and other times it equals the inverted input. This question asks which 2-input gate provides exactly that behavior. Given Data / Assumptions: Two inputs: Data D and control C Goal: Output X equals D when C = 0 and equals NOT D when C = 1 Concept / Approach: XOR implements X = D XOR C. When C = 0, XOR acts as a buffer (X = D). When C = 1, XOR acts as an inverter (X = NOT D). This is why XOR is often described as a controlled inverter or programmable inverter. Step-by-Step Solution: Step 1: Define X = D XOR C.Step 2: If C = 0, then X = D XOR 0 = D.Step 3: If C = 1, then X = D XOR 1 = NOT D. Verification / Alternative check: XNOR (the complement) would pass when C = 1 and invert when C = 0, which is the opposite control convention but not what is usually meant by controlled inversion from the question wording. Why Other Options Are Wrong: XNOR gate: Acts as pass when control is 1 and invert when control is 0; opposite polarity.NAND gate: Does not provide clean pass vs invert based solely on a single control bit.OR gate: Cannot invert the data based on a single control in this simple configuration. Common Pitfalls: Confusing XOR with XNOR; pay attention to control polarity.Assuming any two-input gate can be wired to behave like a controlled inverter without extra logic. Final Answer: XOR gate

Question 9

XOR Property — The output of a 2-input XOR gate is HIGH under which input relationship?

Accepted Answer

Introduction: The exclusive OR (XOR) gate is essential in parity generation, adders, and control logic. Understanding when its output goes HIGH is fundamental for designing and analyzing digital circuits. Given Data / Assumptions: 2 inputs: A and B Binary levels only (0 and 1) Ideal logic with no undefined states in normal operation Concept / Approach: XOR outputs 1 if and only if exactly one input is 1. In other words, the inputs must differ. If both inputs are equal (00 or 11), XOR outputs 0. Step-by-Step Solution: Step 1: Write XOR condition: X = 1 when A != B.Step 2: Evaluate pairs: 00 → 0, 01 → 1, 10 → 1, 11 → 0.Step 3: Conclude output is HIGH only for unequal inputs (01 and 10). Verification / Alternative check: Truth table enumeration or algebraic identity: A XOR B = A NOT B + NOT A B; both terms cannot be true at the same time when inputs are equal. Why Other Options Are Wrong: Both inputs are 0: Produces 0 for XOR.Both inputs are 1: Produces 0 for XOR.Both inputs are undefined: Standard XOR truth table does not rely on undefined states. Common Pitfalls: Confusing XOR with OR; OR is high for 01, 10, and 11.Assuming equality creates a high output; that is XNOR, not XOR. Final Answer: Inputs are unequal

Logic Gates Questions