Question 1

Emitter-Coupled Logic (ECL) — identify the key characteristics Which option best summarizes the important features of ECL logic families used in high-speed digital design (consider voltage swing, supply polarity, noise margin, speed, and power)?

Accepted Answer

Introduction / Context: Emitter-Coupled Logic (ECL) is a classic high-speed bipolar logic family. Unlike CMOS or TTL, ECL operates transistors in an emitter-coupled differential configuration that avoids transistor saturation. This architectural choice dramatically reduces storage delay, delivering very short propagation times at the expense of power and voltage swing. Understanding these trade-offs is essential when choosing a logic family for fast clocking and low-skew systems. Given Data / Assumptions: ECL typically uses a negative supply (for example, –5.2 V) with small output swings around a reference. Speed is a primary design goal; power efficiency is secondary. Noise margin in absolute volts is comparatively small due to the small swing. Concept / Approach: ECL keeps transistors out of saturation, eliminating the time penalty of removing stored charge. It also uses constant-current biasing, which minimizes ground bounce and crosstalk at very high edge rates. The small output swing shortens charge and discharge times of interconnect capacitances, further improving speed. However, maintaining bias currents raises static power, and the small swing results in lower absolute noise margins than families with rail-to-rail signals. Step-by-Step Solution: Identify voltage operation: negative rail operation is a hallmark of many ECL families.Check voltage swing: intentionally small to increase speed.Assess speed: very fast, short propagation delay.Assess power: higher static power due to constant current biasing.Assess noise: lower absolute noise margin because of the small swing. Verification / Alternative check: Datasheets for classic ECL (e.g., 10K/100K series) show typical propagation delays in the sub-nanosecond to a few nanoseconds range, output swings of a few hundred millivolts, and static currents on the order of many milliamps per gate, matching the features summarized above. Why Other Options Are Wrong: Option b: claims low power; ECL is not low-power. Option c: claims slow propagation and high swing; these contradict ECL fundamentals. Option d: positive supply and low power are not representative of ECL. Option e: mixes CMOS/TTL traits and rail-to-rail swing, not ECL. Common Pitfalls: Confusing “good noise immunity” with large noise margin in volts; ECL is immune to certain supply disturbances but still has small swing. Final Answer: Low noise margin, low output voltage swing, negative voltage operation, fast, and high power consumption

Question 2

Buffers and drivers in digital circuits — what capability do they emphasize? Logic devices labeled as buffers, drivers, or buffer/drivers are primarily intended to provide which capability compared with ordinary logic gates?

Accepted Answer

Introduction / Context: In practical systems, some nodes must drive long traces, multiple loads, or external connectors. Standard logic gates may not safely supply that current or voltage swing into heavier capacitive or resistive loads. Buffer/driver devices exist to solve this by strengthening output drive while preserving logic functionality and timing integrity. Given Data / Assumptions: “Buffer,” “line driver,” and “bus driver” typically refer to increased output drive. Output parameters of interest include IOH, IOL, VOH under load, and VOL under load. Input characteristics (high impedance, hysteresis) sometimes improve, but output drive is the main purpose. Concept / Approach: A buffer/driver stage adds transistors and sometimes slew-rate control to source/sink higher currents without violating logic levels. Many parts include tri-state control for bus sharing. While inputs can be tolerant or Schmitt-triggered, the defining feature is robust output capability relative to simple logic gates like a basic inverter or NAND. Step-by-Step Solution: Compare datasheets: note larger IOH/IOL ratings and guaranteed VOH/VOL at higher currents.Recognize common uses: cable driving, bus transceiving, LED driving (within limits), and fan-out expansion.Conclude: buffers emphasize stronger output current/voltage capability. Verification / Alternative check: Typical “HC/HCT244” or “ALS245” style bus drivers specify tens of milliamps sink/source capability and tri-state outputs—significantly higher than simple logic gates—confirming the emphasis on outputs rather than inputs. Why Other Options Are Wrong: Option b/d: input capability is rarely the limiting factor; output drive is the purpose. Option c: smaller output capability contradicts the definition. Option e: raises input impedance only; this misses the driver’s raison d’être. Common Pitfalls: Overdriving loads like motors or high-power LEDs without dedicated drivers; ensure output power limits are respected. Final Answer: a greater current/voltage capability than an ordinary logic circuit.

Question 3

Decoupling and bypassing: Evaluate — “Power-supply decoupling uses a radio-frequency (RF) capacitor to shunt high-frequency noise and spikes to ground.”

Accepted Answer

Introduction / Context: Power-supply decoupling (bypass) is essential in digital systems to maintain stable logic levels and suppress switching noise. Small-value, low-ESL capacitors close to IC power pins provide a low-impedance path for high-frequency currents, thereby reducing voltage spikes on the supply rails. Given Data / Assumptions: Digital ICs draw transient currents at clock edges. Bypass capacitors present low impedance at high frequency. Placement close to IC power pins minimizes loop inductance. Concept / Approach: A ceramic RF bypass (e.g., 0.1 µF) from Vcc to ground shunts fast transients locally, maintaining a clean local supply. Larger bulk capacitors stabilize lower-frequency sag. Together they form a multi-decade impedance control strategy. Step-by-Step Solution: Identify high di/dt current pulses from logic switching.Provide a nearby low-impedance reservoir using small ceramic capacitors.Ensure short traces to reduce inductance so HF spikes are effectively shunted. Verification / Alternative check: Scope measurements show reduced supply ripple/spikes when decoupling is properly applied at each IC compared to an undeco upled board. Why Other Options Are Wrong: Incorrect / linear-only / >100 MHz-only / tantalum-only: Bypass principles are universal. Value and dielectric type are design choices; ceramics handle HF well, tantalum handles bulk energy but is not mandatory for HF shunting. Common Pitfalls: Placing capacitors far from pins; using only bulk capacitance; ignoring ground return path and plane quality; overlooking ESR/ESL effects at target frequencies. Final Answer: Correct

Question 4

TTL–CMOS pin and level compatibility: Evaluate — “The CMOS series that is pin-compatible with the TTL family is the 4000 series.”

Accepted Answer

Introduction / Context: Interfacing TTL and CMOS requires attention to pinout and logic-level compatibility. Although the classic 4000-series CMOS offers wide supply ranges, its pinouts and threshold levels are not generally drop-in compatible with the 74xx TTL family. The 74HC/74HCT families were introduced to address this compatibility need. Given Data / Assumptions: TTL family reference: 74xx numbering and pinouts. CMOS families: 4000 series (CD40xx), 74HC, 74HCT, etc. “Pin-compatible” typically implies identical functions, pinouts, and practical logic-level compatibility at 5 V. Concept / Approach: The 4000 series predated 74HC/HCT and uses different pinouts/functions for many parts. 74HC mirrors 74xx pinouts and functions (CMOS implementation), while 74HCT also aligns logic thresholds to be TTL-level compatible at 5 V. Hence, the statement that the 4000 series is the compatible series is incorrect. Step-by-Step Solution: Identify that 74HC/74HCT map to 74-series logic functions.Note 74HCT input thresholds support TTL-level driving at 5 V.Conclude the 4000 series is not the TTL-compatible drop-in family; the statement is incorrect. Verification / Alternative check: Part comparisons (e.g., 74HC00 vs. CD4011) show function similarity (NAND) but differing pinouts and electrical thresholds; 74HC00 matches 7400 pinout, confirming compatibility. Why Other Options Are Wrong: Correct / partial exceptions: A few devices (e.g., 4049/4050 level shifters) do not make the whole 4000 series pin-compatible. Mechanical-only compatibility: Even mechanical pinout often differs; logic thresholds certainly do. Common Pitfalls: Assuming all CMOS families are interchangeable; ignoring TTL input threshold requirements; overlooking supply voltage differences. Final Answer: Incorrect

Question 5

CMOS fan-out behavior: Evaluate — “The fan-out of CMOS gates is frequency dependent.”

Accepted Answer

Introduction / Context: CMOS inputs are high-impedance and draw negligible DC current, which suggests very high DC fan-out. However, at practical operating frequencies, input capacitance and trace capacitance demand dynamic charging/discharging current. This behavior makes effective fan-out frequency dependent. Given Data / Assumptions: CMOS inputs present capacitance (Cin), not significant DC current. Output stage has finite drive (Ron, Iout). Rise/fall times degrade as capacitive load increases with more loads or higher frequency. Concept / Approach: As frequency increases, the product of load capacitance and edge rate requirements forces more current, slowing transitions and possibly violating timing/noise margins. Therefore, the number of inputs that a CMOS output can reliably drive at a given frequency (i.e., effective fan-out) decreases as frequency rises. Step-by-Step Solution: Model each additional gate input as added capacitance.Compute dynamic current using I = C * dV/dt and power using P = C * V^2 * f.Observe that higher f or larger total C reduces timing margin, limiting practical fan-out. Verification / Alternative check: Vendor datasheets specify maximum capacitive loads for given rise/fall times and recommended loading guidelines versus clock frequency. Why Other Options Are Wrong: Incorrect / frequency-limited statements: The dependence exists across a wide frequency range; it is not restricted to a narrow band. Common Pitfalls: Equating DC fan-out (near infinite) with practical AC fan-out; ignoring trace capacitance and routing topology; overlooking that multiple receivers may switch simultaneously. Final Answer: Correct

Question 6

Saturated-switching in TTL: Evaluate — “The 74XX series TTL operates using saturated switching, where many conducting transistors enter saturation.”

Accepted Answer

Introduction / Context: Standard TTL (74XX) logic uses multi-emitter input transistors and relies on saturated switching in its internal stages. Saturation stores charge and limits speed; this motivated Schottky TTL variants to avoid deep saturation and improve performance. Given Data / Assumptions: “74XX” refers to the original TTL family, not CMOS (74HC/HCT). Saturation occurs when base–collector junction becomes forward-biased. Stored charge in saturation increases turn-off delay. Concept / Approach: In basic TTL, output transistors drive into saturation to achieve low VOL. The trade-off is speed versus simplicity and power. Variants like 74S/74LS include Schottky clamps to prevent deep saturation while preserving compatibility. Step-by-Step Solution: Recall that standard TTL outputs use a saturated NPN to pull output LOW.Recognize that such saturated operation is a defining characteristic of classic TTL.Thus, the statement accurately describes 74XX TTL operation. Verification / Alternative check: Timing diagrams and internal schematics in datasheets show saturated output devices for 74XX; Schottky families are advertised as “non-saturating” improvements. Why Other Options Are Wrong: Incorrect / HC/HCT: CMOS families (HC/HCT) are not TTL transistors; Schottky-only caveats miss the point that 74S/74LS specifically avoid deep saturation, unlike 74XX. Common Pitfalls: Confusing all 74-series as TTL (some are CMOS); assuming Schottky clamps are present in classic 74XX (they are not). Final Answer: Correct

Question 7

TTL output current specification: Evaluate — “The maximum current for a LOW output on a standard TTL gate is 100 µA.”

Accepted Answer

Introduction / Context: Understanding TTL output drive is crucial for reliable interfacing. Standard TTL can sink substantial current in the LOW state, which defines its fan-out capability. The claim of only 100 µA is drastically lower than actual specifications and would imply almost no fan-out. Given Data / Assumptions: Standard TTL IOL(max) is typically on the order of 16 mA for a logic LOW. TTL input LOW current (IIL) per input is around 1.6 mA in classic TTL. Fan-out ≈ IOL/IIL ≈ 10 for LOW state. Concept / Approach: If a TTL output LOW could only sink 100 µA, it would barely drive a single TTL input (which might demand around 1.6 mA). Actual TTL data contradicts this; therefore the statement is incorrect by more than an order of magnitude. Step-by-Step Solution: Check standard TTL specs: IOL(max) ≈ 16 mA at VOL spec.Compare to the claim: 0.1 mA is far smaller than 16 mA.Conclude the statement is incorrect. Verification / Alternative check: Referencing vendor datasheets for 7400 family confirms typical sink capability and corresponding loading calculations. Why Other Options Are Wrong: Correct / temperature-only / open-collector caveats: None reconcile the large discrepancy with standard specs; open-collector capability is determined by sink current and pull-up, not “no load.” Common Pitfalls: Confusing input current with output current; misreading µA vs. mA; using LS/ALS/HCT values without understanding the order of magnitude. Final Answer: Incorrect

Question 8

ECL interfacing drawback: Evaluate — “A major drawback when using ECL with TTL or MOS circuits is its negative supply and logic level conventions.”

Accepted Answer

Introduction / Context: While Emitter-Coupled Logic (ECL) offers very high speed due to non-saturating operation, it traditionally uses negative supply rails (e.g., VEE = −5.2 V, with ground as the positive rail), resulting in logic levels that are negative with respect to ground. Interfacing this directly with TTL or MOS logic families adds complexity. Given Data / Assumptions: ECL logic levels are negative-referenced; TTL and CMOS are usually ground-referenced positive supplies. Level translation or special interfacing devices are required. Power supply arrangements differ, increasing design complexity. Concept / Approach: Because logic thresholds and common-mode ranges differ markedly, translating between ECL and TTL/MOS often requires dedicated translator ICs or bias networks. Additionally, ECL’s constant-current operation and rail conventions complicate mixed-signal power distribution compared to single positive-rail systems. Step-by-Step Solution: Identify ECL negative-level convention and typical VEE.Contrast with TTL/CMOS positive supply and VIH/VIL thresholds.Conclude that negative supply/level differences are a primary interfacing drawback. Verification / Alternative check: Design guides for ECL-to-TTL/CMOS interfacing specify translators (e.g., MC10/100 series, PECL/LVPECL variants) to bridge level differences. Why Other Options Are Wrong: Incorrect / PECL-only / high-voltage-only / fan-out-only: The core issue is level convention and supply polarity, not merely drive strength or unusually high supply voltages. PECL uses positive supplies but still requires careful level management. Common Pitfalls: Assuming direct connection without translators; mixing grounds improperly; neglecting termination practices needed for high-speed ECL signals. Final Answer: Correct

Question 9

Integrated Injection Logic (I2L) vs. TTL fabrication and density: In integrated-circuit logic families, it is often stated that Integrated Injection Logic (I2L) offers very high component density and is simpler to fabricate than Transistor–Transisto…

Accepted Answer

Introduction / Context: Integrated Injection Logic (I2L) is a bipolar logic family that was developed to achieve extremely high integration density with very low power per gate. When comparing logic families such as I2L and Transistor–Transistor Logic (TTL), two recurring evaluation axes are component density and ease of fabrication. This question asks whether the common claim—“I2L offers high component density and is easier to fabricate than TTL”—is correct. Given Data / Assumptions: I2L uses current injection and simple lateral pnp devices with multi-collector npn transistors to realize logic. TTL uses multi-emitter input transistors and a more complex arrangement of bipolar devices per gate. We assume mainstream historical bipolar processes intended for high-volume digital logic. Concept / Approach: I2L was specifically optimized for gate density and low-power operation by simplifying device structures and leveraging current-injection techniques. The device count per logic function is small, interconnect can be compact, and the fabrication steps are fewer compared with full-feature TTL processes. TTL, while fast and robust, generally requires more complex device structures and interconnect per gate, reducing raw density and making fabrication comparatively more involved. Step-by-Step Solution: Identify comparative metric 1 (density): I2L packs many gates per unit area due to small device count and shared structures.Identify comparative metric 2 (fabrication): I2L can be realized with relatively simple bipolar steps; TTL demands multi-emitter structures and tighter matching, raising process complexity.Relate metrics to the statement: I2L excels at density and typically uses simpler, lower-power circuitry per gate than TTL.Draw conclusion: The statement is correct in general practice and historical literature. Verification / Alternative check: Survey historic bipolar logic overviews: I2L is often cited for ultra-high-density logic and low power, whereas TTL is known for speed and fan-out but with larger gate areas. Why Other Options Are Wrong: Incorrect: Contradicts widely documented characteristics of I2L vs. TTL.Depends on supply voltage only: Density and fabrication complexity are architectural, not simply supply-voltage effects.Cannot be determined without timing data: Timing is separate from density/fabrication questions. Common Pitfalls: Confusing I2L with ECL or TTL in terms of speed; mixing up density (gates per mm^2) with drive strength or noise margins. Also, assuming modern CMOS comparisons apply directly—this question is specifically about I2L vs. TTL. Final Answer: Correct

Question 10

Definition of noise immunity in logic circuits: Noise immunity describes a circuit’s ability to tolerate noise without producing spurious changes at the output. Evaluate the statement that it “tolerates noise by causing spurious charges in the outpu…

Accepted Answer

Introduction / Context: Noise immunity is a foundational specification in digital logic families (TTL, CMOS, ECL). A correct understanding is crucial for robust designs operating in real environments with crosstalk, supply ripple, and electromagnetic interference. Given Data / Assumptions: Noise immunity refers to the maximum noise the input can withstand without causing an unintended output change. Input threshold levels (VIL, VIH) define safe input voltage ranges for logical 0 and 1. Output integrity should be preserved despite reasonable noise within specified limits. Concept / Approach: The correct definition emphasizes resistance to noise-induced misinterpretation at the input. A logic family with good noise immunity ensures that small disturbances do not flip the interpreted logic level. The statement in question incorrectly describes the effect on the output as “causing spurious charges,” which reverses the idea: immunity prevents spurious changes, it does not cause them. Step-by-Step Solution: Clarify definition: Noise immunity = tolerance to noise without unintended output switching.Relate to thresholds: Adequate difference between VIH(min) and VIL(max) improves noise margin.Evaluate the statement: It claims spurious output changes are caused; this contradicts the definition.Conclude it is incorrect. Verification / Alternative check: Check standard datasheets and textbooks: noise margin = VIH(min) - VIL(max); higher margin implies better immunity and fewer spurious output transitions. Why Other Options Are Wrong: Correct: Opposite of the standard meaning.Only true for open-collector TTL / Only true when VIL = VIH: These are unrelated conditions and do not redefine noise immunity. Common Pitfalls: Confusing noise susceptibility (which could cause spurious changes) with noise immunity (which prevents them); assuming output filtering alone guarantees immunity. Final Answer: Incorrect

Question 11

Current sourcing vs. “pull-down” terminology: In digital outputs, a current-sourcing transistor is equivalent to a pull-up device, not a pull-down. Assess the correctness of calling a current-sourcing transistor a “pull-down” transistor.

Accepted Answer

Introduction / Context: In logic outputs, “sourcing” and “sinking” identify the direction of current flow when driving a load. Proper terminology helps avoid wiring mistakes and misinterpretation of fan-out and interfacing requirements. Given Data / Assumptions: A current-sourcing device provides current to the load when output is HIGH (typical of a pull-up path). A current-sinking device draws current from the load to ground when output is LOW (typical of a pull-down path). We consider standard TTL/CMOS totem-pole or complementary output stages. Concept / Approach: By definition, “pull-up” raises the node toward the positive rail and sources current; “pull-down” lowers the node toward ground and sinks current. Calling a sourcing transistor a “pull-down” reverses the meaning and is incorrect. Step-by-Step Solution: Define current sourcing: output provides positive current to the load at logical HIGH.Define current sinking: output conducts to ground at logical LOW.Map to devices: upper device (p-type in CMOS or transistor tied to Vcc in TTL) = pull-up; lower device (n-type or transistor to ground) = pull-down.Conclude the statement is incorrect. Verification / Alternative check: Review standard output stage diagrams: the top transistor is associated with pull-up (sourcing), the bottom with pull-down (sinking). Why Other Options Are Wrong: Correct: Contradicts definitions.Valid only in open-drain MOS outputs / wired-AND buses: These cases still preserve sourcing vs. sinking definitions. Common Pitfalls: Mixing up current direction conventions; assuming LED wiring polarity changes sourcing/sinking meaning—it does not. Final Answer: Incorrect

Question 12

Handling unused inputs on NAND gates (TTL/CMOS best practices): Evaluate the statement: “An unused NAND input can be left unconnected, or just pulled high, or tied together with another input without affecting the logic output.”

Accepted Answer

Introduction / Context: Proper termination of unused inputs is critical for noise immunity and predictable behavior. Floating inputs can assume indeterminate levels, inject noise, and increase power consumption, especially in CMOS where input gates have very high impedance. Given Data / Assumptions: NAND gate inputs must be at defined logic levels. TTL floating inputs may default high but are not guaranteed and are poor practice. CMOS floating inputs are especially problematic due to high input impedance and can oscillate or pick up noise. Concept / Approach: The safe approach is to tie unused inputs to a valid logic level: for NAND, tying to logic 1 through a proper connection (direct for CMOS with ESD safeguards or via resistor where recommended) maintains intended function. “Leaving unconnected” is never advised. Tying inputs together is acceptable only when both are driven by the same defined source—not to excuse leaving them floating. Step-by-Step Solution: Identify correct practice: never leave unused logic inputs floating.For NAND, tie unused inputs to logic 1 (or integrate into the logic expression) using proper wiring.Evaluate the statement: it endorses leaving unconnected, which is incorrect.Conclude that the statement is false; best practice demands defined levels. Verification / Alternative check: Datasheets and logic design guides consistently warn against floating inputs; application notes specify pull-ups/pull-downs or direct ties to rails as appropriate. Why Other Options Are Wrong: Correct: Contradicts datasheet recommendations.Correct only for CMOS at room temperature / only with Schmitt-trigger inputs: Temperature and input hysteresis do not legitimize floating inputs. Common Pitfalls: Relying on TTL’s default-high behavior; ignoring input protection and ESD structures; forgetting that floating inputs can inject switching noise into the system. Final Answer: Incorrect

Question 13

Key advantage of CMOS over TTL: Across logic families, a widely cited advantage of CMOS (Complementary Metal-Oxide-Semiconductor) over TTL is its very low static power consumption. Is this statement correct?

Accepted Answer

Introduction / Context: When choosing a logic family, designers balance power, speed, noise margins, and cost. CMOS logic is renowned for extremely low static power because, ideally, only leakage current flows when a gate is not switching. TTL, based on bipolar junction transistors, draws more static current by design. Given Data / Assumptions: CMOS static power is dominated by leakage; dynamic power scales with switching activity: P_dynamic = C_load * V^2 * f. TTL consumes notable static bias currents regardless of switching. We consider mainstream devices at nominal supply voltages (for example, 5 V legacy, 3.3 V/2.5 V/1.8 V modern CMOS). Concept / Approach: The fundamental CMOS inverter draws current primarily during transitions when both pMOS and nMOS briefly conduct and when charging/discharging capacitive loads. Thus, at low activity, total power is very low. TTL maintains bias currents even at DC, leading to higher static power consumption than CMOS. Step-by-Step Solution: Compare static power: CMOS (leakage) vs. TTL (bias currents).Compare dynamic behavior: CMOS power rises with frequency and load; TTL also increases but from a higher baseline.Evaluate claim: “very low power consumption” is a standard CMOS advantage over TTL.Conclude the statement is correct. Verification / Alternative check: Datasheet quiescent current (ICC) values show orders-of-magnitude difference in favor of CMOS families (for example, HC, HCT, LVC) compared to TTL. Why Other Options Are Wrong: Incorrect: Denies a well-established characteristic.Only true above 10 MHz / Only true below 1.8 V: Power advantage is not confined to these arbitrary conditions. Common Pitfalls: Confusing static with dynamic power; ignoring that heavy capacitive loads and high frequency can make CMOS power significant—yet its static power remains far below TTL. Final Answer: Correct

Question 14

Input-level abbreviations in logic families: The standard abbreviation for the required HIGH input voltage level is VIH. Assess the correctness of this notation.

Accepted Answer

Introduction / Context: Datasheets use standardized abbreviations to define logic thresholds and drive levels. Recognizing these acronyms is essential for interfacing different logic families safely and for computing noise margins. Given Data / Assumptions: VIH denotes the minimum input voltage guaranteed to be recognized as a logic HIGH. VIL denotes the maximum input voltage guaranteed to be recognized as a logic LOW. VOH and VOL are the guaranteed output HIGH and LOW levels at specified load currents. Concept / Approach: These four parameters—VIH, VIL, VOH, VOL—form the basis of noise margin calculations. Identifying VIH correctly ensures that the driving stage’s VOH meets or exceeds the receiving stage’s VIH under worst-case conditions. Step-by-Step Solution: Identify the abbreviation in question: VIH.Match definition: VIH = minimum HIGH input threshold.Cross-reference with common datasheets: TTL, CMOS, and LVC families all use VIH/VIL.Conclude the statement is correct. Verification / Alternative check: Any standard logic family datasheet glossary confirms the VIH/VIL conventions alongside VOH/VOL. Why Other Options Are Wrong: Incorrect: Conflicts with universal datasheet notation.Correct only for ECL or only for CMOS: The notation is widely used across families. Common Pitfalls: Confusing input thresholds with output levels; forgetting to include temperature and load conditions when checking compliance. Final Answer: Correct

Question 15

Meaning of CMOS and typical device mode: “CMOS” expands to “complementary metal-oxide-semiconductor,” and in mainstream logic processes the field-effect transistors are normally enhancement-mode devices. Is this statement correct?

Accepted Answer

Introduction / Context: CMOS stands for “complementary metal-oxide-semiconductor,” referring to the complementary use of p-channel and n-channel MOSFETs. Understanding the naming and the common operating mode of transistors in digital CMOS is fundamental to logic design. Given Data / Assumptions: Complementary means paired pMOS and nMOS devices form logic functions (for example, CMOS inverters, NAND, NOR). Standard digital CMOS uses enhancement-mode MOSFETs whose channels form when a gate voltage exceeds a threshold in the appropriate polarity. Depletion-mode devices exist but are uncommon in standard-cell digital logic paths. Concept / Approach: Digital CMOS relies on enhancement-mode devices so that, at steady state, either the pull-up or pull-down network is off, minimizing static current. The phraseology in the question is slightly pluralized, but the technical substance—expansion of CMOS and enhancement-mode usage—is correct. Step-by-Step Solution: Expand acronym: CMOS = complementary metal-oxide-semiconductor.Identify device mode: enhancement-mode MOSFETs for both pMOS and nMOS transistors.Relate to logic behavior: complementary networks minimize static current.Conclude the statement is correct. Verification / Alternative check: Semiconductor texts and process overviews confirm enhancement-mode devices in mainstream CMOS logic libraries. Why Other Options Are Wrong: Incorrect: Conflicts with standard terminology and device usage.Correct only for pMOS / only below 3.3 V: Enhancement-mode usage is not limited to a single polarity or narrow voltage range. Common Pitfalls: Confusing analog specialty processes (where depletion loads may appear) with mainstream digital logic; misremembering the expansion of CMOS. Final Answer: Correct

Question 16

TTL source current vs. sink current capability: Evaluate the statement: “In TTL outputs, the HIGH-level source current is higher than the LOW-level sink current.”

Accepted Answer

Introduction / Context: Classic TTL output stages are asymmetric: they can sink substantially more current when driving a LOW than they can source when driving a HIGH. Recognizing this asymmetry matters for fan-out planning and interfacing to LEDs or other loads. Given Data / Assumptions: Typical TTL VOH specification guarantees only a small source current (for example, -400 microampere) at a valid HIGH. Typical TTL VOL specification allows significant sink current (for example, 8–16 milliampere) at a valid LOW. Exact values vary by subfamily, but the asymmetry persists. Concept / Approach: Because TTL can sink much more current than it can source, it is common to wire LEDs to Vcc with the TTL output sinking current when LOW. The statement claims the opposite relationship and is therefore incorrect. Step-by-Step Solution: Recall TTL output specs: small source current at HIGH, large sink current at LOW.Compare magnitudes: I_sink(LOW) >> I_source(HIGH).Evaluate statement: It claims source > sink, which is false.Conclude the statement is incorrect. Verification / Alternative check: Any TTL or 74LS/74ALS datasheet confirms the asymmetry; application notes recommend sinking loads for better current capability and logic-level compliance. Why Other Options Are Wrong: Correct: Contradicts datasheet values.Only true for Schottky TTL / open-collector: Even in these variants, the fundamental asymmetry remains; open-collector can sink but not source through the transistor. Common Pitfalls: Assuming symmetric drive because CMOS outputs are often near-symmetric; forgetting that TTL’s design heritage is different. Final Answer: Incorrect

Question 17

Are PMOS and NMOS circuit implementations “identical except for voltage polarity”?  Consider common digital design practices and device physics to evaluate this statement.

Accepted Answer

Introduction / Context: PMOS and NMOS are complementary device types with different carrier mobilities and threshold behaviors. While logical functions can be mirrored with polarity changes, practical circuit implementations are not truly identical due to performance and device-physics differences. Given Data / Assumptions: Electron mobility (nMOS) exceeds hole mobility (pMOS), affecting conductivity for the same geometry. Historic PMOS and NMOS families used different load strategies and had different speed/power characteristics. In CMOS, pull-up (pMOS) and pull-down (nMOS) networks are complementary but sized differently. Concept / Approach: The statement claims identity except for voltage polarity. In reality, even if the logic function can be inverted or mirrored, designers must compensate for mobility differences, threshold voltages, body effects, and leakage by sizing devices and choosing topologies appropriately. Thus, the implementations are not identical. Step-by-Step Solution: Compare device physics: nMOS vs. pMOS mobilities and thresholds.Note practical design choices: device sizing ratios for equal rise/fall times.Observe historic logic families: PMOS and NMOS single-polarity families used different loads and achieved different speeds.Conclude the statement is incorrect. Verification / Alternative check: Process and standard-cell documentation show different width/length recommendations for pMOS and nMOS to balance drive strength and timing. Why Other Options Are Wrong: Correct: Overgeneralizes and ignores physics.Correct only at 5 V or only for depletion-load logic: Voltage level or a specific load style does not negate inherent device differences. Common Pitfalls: Equating symbolic Boolean duality with physical identity; forgetting that timing symmetry requires unequal device sizing. Final Answer: Incorrect

Question 18

Rise time vs. propagation delay in digital waveforms: The interval for a square wave to move from 10% to 90% of its final level is called rise time, not propagation delay. Evaluate the correctness of calling it “propagation delay.”

Accepted Answer

Introduction / Context: Two frequently cited timing parameters are rise time (10% to 90% transition at the same node) and propagation delay (time from an input transition to the corresponding 50% output crossing). Mixing these definitions leads to errors when budgeting timing and edges in digital systems. Given Data / Assumptions: Rise time (tr): time for a waveform to go from 10% to 90% of its final value at a given node. Fall time (tf): time for a waveform to go from 90% to 10%. Propagation delay (tpd): delay from input event to output response, typically measured between 50% points (or specified points) at input and output. Concept / Approach: The question’s statement labels a 10%–90% interval as “propagation delay,” which is a definition error. That interval is rise time (or fall time). Propagation delay depends on both device internal delays and load effects and is measured between different nodes (input to output), not within a single edge at one node. Step-by-Step Solution: Identify the metric in the statement: 10%–90% transition.Map to correct term: rise time (or fall time for a falling edge).Contrast with propagation delay: input-to-output timing, commonly 50% crossings.Conclude the statement is incorrect. Verification / Alternative check: Timing diagrams and datasheets consistently separate tr/tf (edge rates) from tpd (input-to-output delay). Simulation tools and oscilloscopes use the same conventions. Why Other Options Are Wrong: Correct: Conflicts with standard timing definitions.Correct only for TTL outputs or high fan-out: Device family or loading does not change definitions. Common Pitfalls: Using propagation delay as a synonym for “how fast the edge looks”; ignoring that slow edges can still have short input-to-output delay, and vice versa. Final Answer: Incorrect

Question 19

IC logic-family benchmarking A common and practical figure of merit used to measure and compare the overall performance of an integrated-circuit (IC) logic family is the speed–power product (also called the energy–delay product for one switching eve…

Accepted Answer

Introduction / Context: When evaluating digital IC logic families (for example, TTL, CMOS, ECL), designers want a single number that captures both how fast gates can switch and how much power they consume doing so. The speed–power product provides exactly this: it multiplies typical propagation delay by typical power dissipation to estimate the energy per switching event at a given operating point, enabling apples-to-apples comparisons across families and subfamilies. Given Data / Assumptions: The statement claims that the speed–power product is a common means to compare overall performance. Propagation delay (t_pd) and power dissipation (P) are available on every logic-family data sheet. We assume worst-case or typical values are chosen consistently when computing the product. Concept / Approach: The figure of merit is defined as SPP = t_pd * P. Lower is better because it indicates a gate that both switches quickly and wastes little power. While no single number covers every nuance (drive strength, noise margin, fan-out, voltage range), the speed–power product is widely used to characterize efficiency across families and process generations. Step-by-Step Solution: Identify the two primary attributes of interest: switching speed and power.Consult a logic family’s data sheet for t_pd (ns) and P (mW) at a reference load/supply.Compute SPP = t_pd * P to get an energy-like metric (for example, ns*mW).Compare SPP values for different families (e.g., LS-TTL vs HC CMOS) to gauge which family achieves better speed for a given power budget. Verification / Alternative check: Designers also look at energy per transition derived from dynamic power formulas (P ≈ C * V^2 * f). For a fixed load and voltage, lower t_pd often correlates with lower energy per operation in advanced families, reinforcing the usefulness of a combined metric like SPP. Why Other Options Are Wrong: “Incorrect” ignores the broad industry usage of this metric. “Applies only to analog ICs” is irrelevant; SPP is a digital logic metric. “Meaningless without fan-out” is misleading; while fan-out influences delay and power, SPP remains a standard comparative measure when referenced to specified test conditions. Common Pitfalls: Treating SPP as the only criterion and ignoring noise margin, I/O drive, voltage range, or cost. Another pitfall is mixing typical and worst-case numbers across families, which can skew comparisons. Final Answer: Correct

Question 20

TTL noise margins refresher The direct-current (dc) noise margin for standard TTL logic is 0.8 V.

Accepted Answer

Introduction / Context: Noise margin quantifies how much unwanted voltage (noise) a logic signal can tolerate and still be interpreted correctly. For TTL, manufacturers publish guaranteed logic-level thresholds and output levels that allow us to compute worst-case noise margins for logic LOW and logic HIGH. Given Data / Assumptions: Standard TTL input thresholds: V_IL(max) ≈ 0.8 V, V_IH(min) ≈ 2.0 V. Standard TTL output guarantees: V_OL(max) ≈ 0.4 V, V_OH(min) ≈ 2.4 V. Noise margin LOW = V_IL(max) - V_OL(max); noise margin HIGH = V_OH(min) - V_IH(min). Concept / Approach: Using worst-case data-sheet values ensures margins hold across temperature, process, loading, and supply variations. The computed margins for classic TTL families are typically about 0.4 V for both LOW and HIGH, not 0.8 V. Step-by-Step Solution: Compute LOW margin: 0.8 V - 0.4 V = 0.4 V.Compute HIGH margin: 2.4 V - 2.0 V = 0.4 V.Compare with the claim of 0.8 V and observe the mismatch.Conclude the statement is incorrect; TTL dc noise margins are typically about 0.4 V worst-case. Verification / Alternative check: Some “typical” conditions may show larger margins, but specifications and robust design use guaranteed worst-case values, which are approximately 0.4 V. Why Other Options Are Wrong: “Correct” conflicts with the computation. “True only for LS-TTL” is wrong; LS-TTL also lists similar guaranteed thresholds, yielding ~0.4 V. “True only at Vcc = 6 V” is invalid; TTL is not specified to operate at 6 V. Common Pitfalls: Confusing input threshold (0.8 V) with noise margin. The 0.8 V figure is an input limit, not the margin itself. Final Answer: Incorrect

Integrated-Circuit Logic Families Questions