Question 1

Supply range for classic 74-series TTL A typical data sheet statement for 74-series TTL devices claims that Vcc may range from 4.5 V to 5.5 V.

Accepted Answer

Introduction / Context: TTL devices in the classic 74-series families were designed around a nominal 5 V supply. While different subfamilies exist (standard, LS, ALS, AS, F), their recommended operating supply ranges are tightly centered near 5 V to guarantee specified thresholds, noise margins, and speeds. Given Data / Assumptions: Classic bipolar 74-series TTL typically specifies Vcc ≈ 5 V nominal. Recommended operating range is often given as 4.75 V to 5.25 V (±5%) in many data sheets. Some CMOS 74HCT parts list 4.5–5.5 V; pure TTL generally does not rely on that wide a span for guaranteed specs. Concept / Approach: Verify whether the stated 4.5–5.5 V is a general rule for “74-series TTL” (bipolar). Historically, TTL guarantees are tied to ±5% around 5 V. Wider ranges are more associated with CMOS subfamilies (e.g., HCT) that are TTL-input compatible but not bipolar TTL. Step-by-Step Solution: Identify the target family: bipolar 74-series TTL, not HC/HCT CMOS.Review the commonly published recommended Vcc range: ≈ 4.75–5.25 V.Note that 4.5–5.5 V appears on many HCT CMOS sheets, not classic TTL.Conclude the blanket statement is not correct for “the 74 series of TTL ICs.” Verification / Alternative check: Cross-reference multiple vendor sheets (e.g., 74LS00, 74F00); recommended ranges cluster around ±5% of 5 V. Absolute maximum ratings are wider but are not operating recommendations. Why Other Options Are Wrong: “Correct” overgeneralizes; the 4.5–5.5 V range commonly applies to HCT CMOS, not to bipolar TTL. “True only for 74HC/74HCT” is partly right for HCT but 74HC usually supports 2–6 V; hence this option is imprecise. “True only for ECL” is unrelated; ECL uses different supply conventions. Common Pitfalls: Confusing absolute-maximum or extended test conditions with recommended operation, and mixing CMOS HCT specifications with bipolar TTL. Final Answer: Incorrect

Question 2

TTL noise-margin interpretation The dc noise margins calculated from guaranteed (worst-case) values in a standard TTL data sheet represent worst-case margins; typical dc noise margins are usually somewhat larger.

Accepted Answer

Introduction / Context: Data sheets publish guaranteed worst-case limits that cover device variation, temperature, and loading. Designers compute dc noise margins using these limits to ensure robust operation under all specified conditions. Typical parts often perform better than these worst-case guarantees, so real-world noise margins are commonly higher. Given Data / Assumptions: TTL worst-case thresholds: V_IL(max), V_IH(min); outputs: V_OL(max), V_OH(min). Noise margin LOW = V_IL(max) - V_OL(max). Noise margin HIGH = V_OH(min) - V_IH(min). Concept / Approach: Worst-case values guarantee operation even under adverse conditions. Typical device behavior—often documented in “typical characteristics”—exceeds these guarantees, thereby increasing computed noise margins in practice. Step-by-Step Solution: Compute worst-case margins from guaranteed values; for classic TTL they are about 0.4 V.Observe that typical V_OH is higher than the guaranteed minimum and typical V_OL is lower than the guaranteed maximum.Higher V_OH and lower V_OL increase both HIGH and LOW noise margins beyond worst-case calculations.Therefore, the statement is correct. Verification / Alternative check: Typical-characteristics graphs in vendor literature show distributions that exceed minimum/maximum limits, confirming that the worst-case margins are conservative. Why Other Options Are Wrong: “Incorrect” ignores the standard meaning of guaranteed limits. “True only for CMOS” is wrong; the logic applies to TTL as well. “False unless Vcc is 5.5 V” is irrelevant to the principle. Common Pitfalls: Designing to typical values rather than worst-case guarantees can cause field failures when devices or conditions drift toward limits. Final Answer: Correct

Question 3

Fastest logic family claim check The statement “The logic family with the highest maximum clock frequency is HS-TTL (high-speed TTL)” is being evaluated.

Accepted Answer

Introduction / Context: Different logic families trade speed for power, voltage swing, and noise margin. Historically, Emitter-Coupled Logic (ECL) has offered the highest toggle rates among mainstream families, thanks to its non-saturating design and small voltage swings, whereas HS-TTL improves over standard TTL but does not surpass ECL in maximum frequency capability. Given Data / Assumptions: HS-TTL is faster than standard TTL/LS-TTL but still saturates transistors. ECL uses differential, non-saturating stages with small output swings (~0.8 V), enabling very high speed. Modern CMOS with advanced processes can also be extremely fast, but the classical comparison favors ECL for peak frequency. Concept / Approach: Saturation in TTL requires charge removal when switching, limiting maximum speed. ECL avoids saturation, drastically reducing storage delay and enabling multi-hundred-MHz to GHz operation in certain families and processes. Step-by-Step Solution: Identify the claim: HS-TTL is the fastest logic family.Recall ECL’s defining features: non-saturating, constant-current, small swing signals.Compare published maximum toggle rates; ECL typically exceeds HS-TTL by a wide margin.Conclude the statement is incorrect. Verification / Alternative check: Consult family data: 10H/100K ECL series show very high propagation speeds. HS-TTL improves over standard TTL but does not achieve ECL-class frequencies. Why Other Options Are Wrong: “Correct” contradicts the accepted performance hierarchy. The qualifiers “below 1 MHz” or “tri-state outputs only” are irrelevant to inherent family limits. Common Pitfalls: Equating “high-speed TTL” labeling with absolute fastest among all families. Also, overlooking small-swing signaling advantages in ECL. Final Answer: Incorrect

Question 4

ECL characteristics Evaluate the statement: “Emitter-Coupled Logic (ECL) gates are noted for very high frequency capability and a small output voltage swing.”

Accepted Answer

Introduction / Context: Emitter-Coupled Logic is a differential, non-saturating logic family designed to achieve very high switching speeds. Its hallmark is a relatively small output swing centered around a reference level, which minimizes the time required to charge and discharge node capacitances. Given Data / Assumptions: ECL does not drive transistors into saturation, avoiding storage delay. Output swing is small (roughly hundreds of millivolts to about 0.8 V, depending on subfamily). Constant-current operation stabilizes delays over supply and temperature within the designed range. Concept / Approach: Smaller voltage swings reduce dynamic power and allow faster edge rates. Non-saturating devices further shorten propagation delay. Together, these design choices give ECL superior high-frequency performance compared to saturated logic families like TTL. Step-by-Step Solution: Identify what makes logic fast: no saturation and small voltage transitions.Note ECL uses differential pairs that switch currents rather than charging deep into saturation.Recognize the modest swing leads directly to higher toggle rates.Therefore, the statement is correct. Verification / Alternative check: Speed specifications for classic ECL families routinely exceed those of TTL at comparable process nodes, consistent with the stated characteristics. Why Other Options Are Wrong: “Incorrect” ignores well-documented ECL behavior. The options referencing 3.3 V or open-collector are irrelevant; ECL’s small swing/high-speed traits are fundamental and not tied to those conditions. Common Pitfalls: Assuming small swing means poor noise immunity; ECL systems use appropriate terminations and differential techniques to maintain robustness. Final Answer: Correct

Question 5

Totem-pole TTL output behavior Consider a TTL gate with a totem-pole output stage. The statement claims: “The upper (pull-up) transistor is OFF when the gate output is LOW.”

Accepted Answer

Introduction / Context: TTL gates often use a totem-pole (push-pull) output stage: an upper transistor sources current to drive a HIGH, and a lower transistor sinks current to drive a LOW. Proper operation avoids both devices conducting heavily at the same time, which would waste power and stress the devices. Given Data / Assumptions: In a LOW output state, the output node is pulled toward ground. To achieve a strong LOW, the lower (pull-down) transistor turns ON to sink current. To prevent shoot-through, the upper (pull-up) transistor must be OFF (or minimally conducting during transitions) when the output is LOW. Concept / Approach: The totem-pole arrangement alternately sources or sinks current. Logic design ensures complementary action: pull-up drives HIGH, pull-down drives LOW. When the output is LOW, keeping the pull-up off prevents contention and reduces static power dissipation. Step-by-Step Solution: Assess the LOW state: output near ground.Confirm pull-down path is active while pull-up is inactive.Recognize this aligns with typical TTL totem-pole behavior.Conclude the statement is correct. Verification / Alternative check: Timing diagrams show brief crossover currents during transitions, but in steady LOW the upper device is OFF and the lower device holds the node LOW. Why Other Options Are Wrong: “Incorrect” contradicts standard totem-pole operation. Open-collector outputs lack the upper transistor entirely, making option “True only for open-collector” inapplicable. Floating inputs are unrelated to the output topology’s intended steady state. Common Pitfalls: Confusing transient overlap during switching with steady-state conditions. The question concerns the steady LOW state. Final Answer: Correct

Question 6

TTL vs. CMOS propagation delay Evaluate the claim: “The major advantage of TTL logic circuits over CMOS is lower propagation delay.”

Accepted Answer

Introduction / Context: In classic textbook comparisons, TTL (especially Schottky variants like LS, AS, F) offered significantly shorter propagation delays than early CMOS 4000-series devices, whose strengths were ultra-low static power and very wide supply ranges rather than speed. Although modern CMOS families can be extremely fast, the historical “TTL vs. early CMOS” comparison underpins this statement. Given Data / Assumptions: Compare representative families at 5 V: TTL LS/AS/F vs. CMOS 4000-series (CD4xxx). Early CMOS 4000-series had longer t_pd but consumed orders of magnitude less static power. We interpret the statement in the classic educational context where it typically appears. Concept / Approach: Propagation delay is influenced by device physics and circuit topology. Schottky-clamped TTL avoids deep saturation, reducing storage delay; correspondingly, early CMOS prioritized low power and high noise immunity over raw speed. Hence, TTL’s major advantage in that era was lower propagation delay. Step-by-Step Solution: Identify baseline families: LS-TTL or F-TTL vs. 4000-series CMOS.Look up typical t_pd values: TTL often single-digit nanoseconds to tens of ns; 4000-series CMOS often tens to hundreds of ns at 5 V with heavy loads (historical typicals).Observe that TTL is faster under these conditions.Therefore, the claim holds in the commonly taught context. Verification / Alternative check: Modern CMOS (HC/HCT, AHC, LVC) can outperform TTL, but the question compares broad families historically, not best-of-breed modern CMOS versus old TTL. Why Other Options Are Wrong: “Incorrect” ignores the standard historical comparison. “True only for the 4000-series CMOS” is close but over-qualifies; the educational point is general. “True only above 10 V” is irrelevant to TTL-CMOS comparisons. Common Pitfalls: Extrapolating the statement to all modern CMOS, which can be faster than TTL; context matters. Final Answer: Correct

Question 7

MOS vs. TTL primary advantages Evaluate the statement: “The principal advantage of MOS ICs over TTL ICs is their fast operating speed.”

Accepted Answer

Introduction / Context: MOS (and especially CMOS) technologies revolutionized digital ICs primarily by offering extremely low static power consumption and very high integration density, not by raw speed in their earliest incarnations. TTL traditionally offered shorter propagation delays than early MOS families, while consuming more power. Given Data / Assumptions: Early MOS/CMOS: low power, high noise margins, high input impedance, wide supply ranges. TTL: faster in classic comparisons (e.g., LS/AS/F series) but with higher static power and fixed 5 V operation. Modern CMOS can be very fast due to process scaling, but the historical “principal advantage” remains low power and density. Concept / Approach: When teaching logic families, the “principal advantage” of MOS/CMOS is low power and ease of scaling integration density. Speed leadership historically belonged to ECL and, relative to early CMOS, to Schottky-TTL variants. Thus, claiming speed as the principal advantage of MOS over TTL is inaccurate. Step-by-Step Solution: Identify key MOS advantages: low static power and scalability.Contrast with TTL advantages: faster switching in classic comparisons, robust drive in 5 V systems.Conclude the statement is incorrect because MOS’s hallmark advantage is not speed.Note the nuance: modern CMOS performance can surpass TTL, but that is not the “principal advantage” historically. Verification / Alternative check: Review standard textbooks that present logic family tradeoffs; they consistently highlight MOS/CMOS low power and high density as the central benefits. Why Other Options Are Wrong: “Correct” misstates the classic comparison. The options invoking ECL-MOS hybrids or cryogenic operation are off-topic. Common Pitfalls: Confusing present-day high-performance CMOS (fast) with the general historical rationale for MOS adoption (power/density). Final Answer: Incorrect

Question 8

Interconnecting CMOS and TTL Evaluate the statement: “Because CMOS logic draws extremely low power, any number of CMOS and TTL gates can be interconnected without concern.”

Accepted Answer

Introduction / Context: While CMOS inputs draw negligible static current, practical digital design is still constrained by fan-out limits, input capacitance, rise/fall times, logic-level compatibility, and supply differences between families (TTL vs. CMOS). Therefore, “any number” is never a safe assumption. Given Data / Assumptions: CMOS inputs have high impedance but non-zero capacitance that must be driven during transitions. TTL outputs have specified source/sink current limits and logic-level thresholds that may not align with CMOS without level translation (e.g., V_OH(min) vs. V_IH(min)). Interfacing sometimes requires pull-ups, level shifters, or the HCT family to ensure compatibility. Concept / Approach: Fan-out is determined not just by static current but also by dynamic loading (C_load). As the number of driven inputs increases, edge rates slow and timing can fail. Furthermore, TTL HIGH output minimum (≈ 2.4 V) may not meet CMOS HIGH input minimum at 5 V for some families, necessitating HCT or buffers for safe interfacing. Step-by-Step Solution: Recognize that “any number” ignores fan-out and dynamic capacitance.Check logic-level compatibility between the chosen TTL and CMOS families.Apply proper buffering or use HCT to ensure V_OH/V_IH compliance.Conclude the statement is incorrect. Verification / Alternative check: Data sheets specify maximum fan-out and input capacitance; timing analysis demonstrates degraded rise/fall times as loads increase, confirming limits exist. Why Other Options Are Wrong: “Correct” disregards practical constraints. Open-drain or higher supply voltages do not eliminate level and loading concerns by themselves. Common Pitfalls: Assuming static input current is the only factor; in reality, capacitive loading and threshold compatibility often dominate. Final Answer: Incorrect

Question 9

In standard TTL (transistor–transistor logic) families, the majority of logic gates employ a totem-pole (push–pull) output stage to provide low output impedance, fast edges, and active drive for both logic HIGH and logic LOW. Evaluate this statement.

Accepted Answer

Introduction / Context: Totem-pole (also called push–pull) outputs are a hallmark of most standard TTL logic gates. This arrangement actively drives the output HIGH and LOW using a pair of transistors stacked like a totem pole, delivering sharper transitions and better fan-out than passive pull-ups. Given Data / Assumptions: The statement claims most TTL gates use a totem-pole output. We are talking about mainstream 74xx TTL families, not special variants. Open-collector and tri-state outputs exist but are options, not the default for most basic gates. Concept / Approach: A totem-pole output uses an upper transistor (source current when HIGH) and a lower transistor (sink current when LOW). This design reduces output impedance, shortens rise/fall times, and improves speed compared to resistor pull-ups. Classic 74, 74LS, and 74S families commonly use totem-pole outputs for standard gates like AND, OR, NAND, NOR, and inverters. Step-by-Step Solution: Identify the common default TTL output stage → totem-pole.Note special cases (open-collector, tri-state) are variants for bus-ORing or bus sharing.Compare prevalence: standard gates ship with totem-pole by default.Conclude the statement is correct for most TTL gates. Verification / Alternative check: Datasheets for 74LS00, 74LS04, etc., show push–pull output stages with specified I_OH (source) and I_OL (sink), confirming the default totem-pole configuration. Why Other Options Are Wrong: Open-collector TTL is a special variant, not “most.” Tri-state outputs support bus sharing but are not the default for basic gates. “Schottky CMOS” is a mismatched technology label. Common Pitfalls: Assuming wired-OR capability on totem-pole outputs; that is unsafe unless using open-collector/tri-state with proper resistors and control. Final Answer: Correct

Question 10

In digital I/O terminology, when a gate output is at logic HIGH and can deliver current to an external load, this capability is referred to as the output's source current. Assess this definition.

Accepted Answer

Introduction / Context: Digital outputs are characterized by how much current they can source (provide) at a HIGH level and sink (absorb) at a LOW level. Knowing the correct terms matters for interfacing logic devices and sizing pull-ups, LEDs, and other loads. Given Data / Assumptions: The statement defines source current for a HIGH output. Standard TTL and CMOS push–pull outputs provide both sourcing and sinking capability within specified limits. We are not discussing specialized outputs like open-collector (which cannot source in the active sense). Concept / Approach: “Source current” means current flowing out of the output pin into a load when the pin is at logic HIGH. Conversely, “sink current” is current flowing into the output pin from a load tied to Vcc when the pin is at logic LOW. Datasheets specify I_OH (source) and I_OL (sink) with corresponding guaranteed output voltages (V_OH, V_OL). Step-by-Step Solution: Check the definition: HIGH output delivering current → source current.Confirm against I/O conventions used in TTL/CMOS datasheets.Conclude the statement aligns with standard usage. Verification / Alternative check: Typical TTL specs: I_OH is smaller than I_OL, reflecting stronger sink capability; nonetheless, I_OH is the rated source current at logic HIGH. Why Other Options Are Wrong: Limiting it to open-collector or CMOS is inaccurate; the term is technology-agnostic. Logic level 12 V is unrelated to TTL/CMOS standard levels. Common Pitfalls: Confusing current direction; some beginners think “source” applies to LOW. Remember: source at HIGH, sink at LOW for push–pull drivers. Final Answer: Correct

Question 11

For standard TTL input thresholds, the maximum voltage guaranteed to be recognized as a logic LOW (V_IL(max)) is asserted to be 2.0 V. Evaluate the correctness of this claim.

Accepted Answer

Introduction / Context: Recognizing valid logic levels is crucial for reliable interfacing. TTL has long-established input thresholds that define what voltages are treated as LOW or HIGH. Misstating these thresholds can cause design errors and intermittent faults. Given Data / Assumptions: Family: standard TTL operating at 5 V nominal. Claim: V_IL(max) = 2.0 V. We must compare the claim with standard TTL thresholds. Concept / Approach: Standard TTL defines V_IL(max) (maximum recognized LOW) typically as 0.8 V, and V_IH(min) (minimum recognized HIGH) typically as 2.0 V. The noise margin exists between these thresholds. Therefore, 2.0 V is the HIGH threshold, not the LOW maximum. Step-by-Step Solution: Recall standard thresholds: V_IL(max) ≈ 0.8 V; V_IH(min) ≈ 2.0 V.Compare with the statement assigning 2.0 V to LOW → mismatch.Conclude the statement is incorrect for TTL. Verification / Alternative check: Any 74xx TTL datasheet I/O characteristics table will list V_IL(max) around 0.8 V and V_IH(min) around 2.0 V at 25 °C, 5 V supply. Why Other Options Are Wrong: “Correct” contradicts the standard. “Correct only for LS-TTL/3.3 V/CMOS” are distractors; LS-TTL uses essentially the same thresholds, and 3.3 V CMOS has very different, technology-dependent thresholds. Common Pitfalls: Swapping V_IL and V_IH; designing interfaces that drive inputs at around 1.5 V, which is undefined and risks metastability. Final Answer: Incorrect

Question 12

Consider wiring practices for TTL outputs: if you tie the outputs of several standard TTL totem-pole gates directly together, does this safely increase available fan-out? Judge the statement.

Accepted Answer

Introduction / Context: Digital outputs are not meant to be tied together arbitrarily. Standard TTL gates use totem-pole outputs that actively drive both HIGH and LOW. Shorting multiple drivers can cause destructive contention currents and undefined logic levels. Given Data / Assumptions: Devices: standard TTL with push–pull outputs. Practice in question: tying outputs together to “increase fan-out.” Fan-out properly refers to one output driving multiple inputs, not multiple outputs tied together. Concept / Approach: Fan-out is achieved by ensuring one driver meets the total input current of all loads. You do not parallel outputs unless they are specifically designed for current sharing (rare in logic ICs). For wired-OR functionality, open-collector (TTL) or open-drain (CMOS) with a pull-up is used; totem-pole outputs must not be tied. Step-by-Step Solution: Identify output type → totem-pole drives both states aggressively.Consider tying two drivers → contention whenever they disagree.Result → potential damage, excessive current, undefined logic; does not “increase fan-out.” Verification / Alternative check: Datasheets warn against connecting outputs together except with open-collector/open-drain architectures and proper pull-ups or bus transceivers using tri-state control. Why Other Options Are Wrong: “Correct” is unsafe. “Valid only for open-collector…” is a different topology, not “standard TTL outputs tied directly.” Tri-state also requires bus control to ensure only one active driver at a time. Common Pitfalls: Confusing “one output to many inputs” (fan-out) with “many outputs to one node” (bus contention risk). Final Answer: Incorrect

Question 13

Select the logic family that is ideally suited for battery-powered or battery-backup applications due to its very low static power consumption.

Accepted Answer

Introduction / Context: Portable and battery-backed designs need logic that consumes minimal power when idle and scales well with supply voltage. The choice of logic family directly impacts battery life and thermal performance. Given Data / Assumptions: Goal: minimal static (quiescent) power draw. Application: battery or battery-backup operation. Candidates: MOS variants and CMOS. Concept / Approach: CMOS (complementary MOS) uses complementary p-channel and n-channel devices; ideally, only switching events consume dynamic power, while static power is near zero (aside from leakage). That makes CMOS the dominant choice for low-power digital circuits. Pure NMOS/PMOS families draw static current through load devices, raising idle power. Step-by-Step Solution: Compare static power: CMOS ≪ NMOS/PMOS.Consider supply range and noise margins: CMOS scales to low voltages common in battery systems.Select CMOS as the best fit for battery operation. Verification / Alternative check: Modern microcontrollers, SoCs, and memories are overwhelmingly CMOS due to power efficiency and integration density. Why Other Options Are Wrong: NMOS/PMOS consume higher static power; “MOS” is non-specific; BiCMOS can optimize speed/analog features but not minimum static power. Common Pitfalls: Overlooking leakage in deep-submicron CMOS; although nonzero, it remains far below the static currents of classic NMOS/PMOS logic families. Final Answer: CMOS

Question 14

In a dual in-line package (DIP), what is the typical center-to-center pin spacing (pitch) along a row of leads on the package?

Accepted Answer

Introduction / Context: Understanding package dimensions is essential for PCB layout and prototyping. The DIP form factor has standardized pin spacing that matches breadboards and through-hole PCBs. Given Data / Assumptions: Package: DIP (dual in-line package). Asked dimension: pitch along a single row (adjacent pins). Common imperial units: mils (1 mil = 0.001 inch). Concept / Approach: DIP packages use a 0.1 inch pitch along each row, which equals 100 mils (2.54 mm). Across-row spacing (body width) is commonly 300 mils or 600 mils depending on pin count and package variant, but the question concerns pin-to-pin pitch along a row. Step-by-Step Solution: Recall DIP standard: 0.1 inch pitch → 100 mils.Map mils to inches: 100 mils = 0.1 inch.Select 100 mils as the typical DIP row pitch. Verification / Alternative check: Mechanical drawings for 14-pin and 16-pin DIPs confirm 2.54 mm (100 mils) pitch. Why Other Options Are Wrong: 5 and 10 mils are trace/spacing scales, not pin pitch. 50 mils is used in compact headers but not DIP lead pitch. 200 mils is too large for in-row pitch (though across-row spacings can be 300 or 600 mils). Common Pitfalls: Confusing along-row pitch (100 mils) with across-row spacing (300/600 mils body sizes). Final Answer: 100 mils

Question 15

A logic probe is placed on the output of a digital circuit and the probe's indicator is dim or flickering rather than solid ON or OFF. What does this observation most likely indicate about the signal at that node?

Accepted Answer

Introduction / Context: Logic probes provide a quick visual indication of the logic state at a node. Many include pulse-stretch or pulse-detect features that cause the indicator to appear dim or flicker when the signal is toggling rapidly. Given Data / Assumptions: The probe lamp/LED is dim or flickering, not a steady HIGH or LOW. We assume a typical logic probe with pulse capture capability. No additional faults (like missing ground) are reported. Concept / Approach: A steady bright indication corresponds to a solid HIGH (or LOW for a separate indicator). A dim, rapidly flickering, or alternating glow typically indicates that the node is switching—often faster than the eye can resolve—so the probe integrates the activity and appears dim. Some probes explicitly label this as a “pulse” or “toggle” indication. Step-by-Step Solution: Interpret dim/flicker as time-averaged activity.Common cause: high-frequency pulse train or toggling node (clock, data line).Conclude the most likely explanation is a high-frequency pulse train. Verification / Alternative check: Checking the same node with an oscilloscope would show periodic transitions or activity bursts correlating with the flicker. Why Other Options Are Wrong: “Open/bad logic level” is more likely to produce an indeterminate or floating reading, often flagged differently. A simple “high level output” should be steady, not dim. Low supply voltage would affect the whole system, not just one node. A “latched tri-state” normally reads as high-impedance and is often indicated differently by the probe. Common Pitfalls: Assuming dim means “weak HIGH”; in probes with pulse detect, it commonly means “fast toggling.” Final Answer: a high-frequency pulse train

Question 16

When a digital output pin is at logic LOW and absorbs current from an external load into the device, this LOW-state output current is commonly called the ________ current.

Accepted Answer

Introduction / Context: Correctly naming current directions at digital outputs is essential for interpreting datasheets and ensuring loads are driven within ratings. Outputs can source or sink current depending on their logic state and topology. Given Data / Assumptions: Output is in the LOW state. Current flows from the load into the output pin and then to ground. We are using standard digital terminology. Concept / Approach: “Sink current” refers to current flowing into the output (the device “sinks” it) when the pin is LOW. TTL traditionally provides strong sink capability (I_OL) and more limited source capability (I_OH). Datasheets specify maximum sink current along with the guaranteed V_OL level. Step-by-Step Solution: Identify current direction: into the output at LOW.Match to terminology: sink current.Confirm with datasheet symbols: I_OL (output low current) corresponds to sinking. Verification / Alternative check: Vendor I/O specs list I_OL and the corresponding maximum V_OL to guarantee a valid logic LOW under load. Why Other Options Are Wrong: “Ground current” and “return current” are vague and not the formal term for this rating. “Exit current” is nonstandard. “Fan-out” counts loads and is unrelated to current naming. Common Pitfalls: Swapping source and sink definitions; remember that at LOW, the pin usually sinks current. Final Answer: sink current

Question 17

The term that denotes how many standard logic inputs can be driven by a single gate output without exceeding the gate’s rated source/sink currents is called ________.

Accepted Answer

Introduction / Context: When interconnecting logic gates, one output often feeds multiple inputs. The allowable number of such inputs is limited by current and voltage specifications. This limit is formalized with a specific term. Given Data / Assumptions: One gate output drives several standard inputs. The limit is determined by I_OH/I_OL and input currents I_IH/I_IL. We seek the formal term for this limit. Concept / Approach: Fan-out = allowable number of standard inputs that a single output can drive while still meeting V_OH and V_OL guarantees. For TTL, fan-out is often asymmetric because sinking capability is higher than sourcing. Exceeding fan-out risks slow edges and invalid logic levels. Step-by-Step Solution: Identify the metric: number of inputs per output.Match to the standard term: fan-out.Relate to datasheet currents: fan-out ≈ I_OL/I_IL (for LOW) and I_OH/I_IH (for HIGH). Verification / Alternative check: Datasheets sometimes state fan-out directly or provide the currents to calculate it. Why Other Options Are Wrong: Propagation relates to delay; dissipation is power; SSI classifies integration scale (small-scale integration); noise margin is voltage headroom against noise. Common Pitfalls: Ignoring differing HIGH vs LOW drive limits; the smaller of the two determines practical fan-out. Final Answer: fan-out

Question 18

For a standard TTL output to be recognized as a valid logic HIGH by another TTL input, the output voltage must be at least ________ under load.

Accepted Answer

Introduction / Context: Interfacing TTL devices safely requires meeting input threshold requirements. The output HIGH voltage under load must exceed the receiving device’s V_IH(min) to ensure a reliable HIGH. Given Data / Assumptions: Family: standard 5 V TTL (e.g., 74LS/74ALS classes). We seek the guaranteed minimum HIGH output voltage specification. Concept / Approach: Typical TTL thresholds: V_IH(min) ≈ 2.0 V; V_IL(max) ≈ 0.8 V. To guarantee that a driven input sees a HIGH, the driver’s V_OH(min) is specified at or above 2.4 V at a defined I_OH. Hence, 2.4 V is the standard minimum HIGH output voltage rating for compatibility. Step-by-Step Solution: Recall TTL thresholds: V_IH(min) 2.0 V → driver must exceed this.Datasheets specify V_OH(min) 2.4 V at rated source current.Therefore, the correct minimum HIGH level is 2.4 V. Verification / Alternative check: Examine any 74LSxx datasheet: V_OH(min) commonly 2.4 V at I_OH = –0.4 mA (example value). Why Other Options Are Wrong: 2.0 V is the input HIGH threshold, not output guarantee. 0.8 V is the input LOW maximum. 5 V is the supply, not the minimum. 3.3 V is unrelated to 5 V TTL standards. Common Pitfalls: Confusing input thresholds with output guarantees; always compare V_OH(min) (driver) against V_IH(min) (receiver) to ensure margin. Final Answer: 2.4 V

Question 19

CMOS dynamic power behavior and scaling: In complementary metal–oxide–semiconductor (CMOS) integrated circuits, total power dissipation is dominated by dynamic switching. As the activity of the circuit changes, what is the correct qualitative depend…

Accepted Answer

Introduction / Context: CMOS ICs are prized for their extremely low static (DC) power, but when they switch, they consume dynamic power to charge and discharge node capacitances. This question tests whether you recognize the qualitative dependence of CMOS power on operating frequency and on the physical size/capacitance of gates. Given Data / Assumptions: Total CMOS power is the sum of dynamic (switching) power and leakage (static) power. Dynamic power is often modeled as Pdyn = α * Cload * V^2 * f, where α is the activity factor, Cload the switched capacitance. Gate size and interconnect scale affect the effective capacitance Cload. Supply voltage V is treated as constant for this qualitative question. Concept / Approach: With Pdyn proportional to both the switched capacitance and the operating frequency, any increase in frequency or node capacitance leads directly to higher power. Larger transistors and larger fan-out typically imply larger capacitances, while slowing the clock (lower f) lowers dynamic power. Leakage can matter at very small geometries, but the core relationship above remains fundamental. Step-by-Step Solution: Express dynamic power: Pdyn = α * Cload * V^2 * f.Hold α, V roughly constant; examine dependence on f → Pdyn increases as f increases.Examine dependence on Cload → Cload grows with gate size and fan-out, increasing Pdyn.Therefore, statements that power increases with frequency (and with gate size) are qualitatively correct. Verification / Alternative check: Power measurements on CMOS systems scale near-linearly with clock frequency when voltage and workload are fixed. Why Other Options Are Wrong: decrease with frequency: Contradicts Pdyn ∝ f.decrease with gate size: Larger devices mean higher capacitance; power does not decrease.increase with gate size: This is also qualitatively true, but the prompt asks which effect regarding frequency is correct; among the provided single best answers, “increase with frequency” is the direct frequency relationship. Common Pitfalls: Confusing static leakage with dynamic power; in classic CMOS, dynamic dominates at moderate technology nodes.Ignoring activity factor α; even with low α, the f and Cload relationships hold. Final Answer: increase with frequency

Question 20

TTL input threshold recognition: For a standard transistor–transistor logic (TTL) gate, what is the minimum input voltage that is guaranteed to be recognized as a logic HIGH (valid 1)?

Accepted Answer

Introduction / Context: Digital interfacing requires knowing the guaranteed logic thresholds. TTL families define input high and low thresholds so that designers can compute noise margins and safe interfacing to other families such as CMOS (HC/HCT). Given Data / Assumptions: Standard TTL input thresholds: VIL(max) ≈ 0.8 V, VIH(min) ≈ 2.0 V. Standard TTL output levels: VOL(max) ≈ 0.4 V, VOH(min) ≈ 2.4 V. Supply voltage is 5 V nominal. Concept / Approach: The minimum input voltage recognized as HIGH is VIH(min). Values below this may not be reliably interpreted as a 1. Values at or above VIH(min) must be recognized as HIGH by any compliant TTL input, allowing some positive noise margin from the VOH(min) of a driving TTL gate. Step-by-Step Solution: List TTL thresholds: VIH(min) = 2.0 V, VIL(max) = 0.8 V.Compare to options: 2.0 V is the minimum guaranteed HIGH threshold.Therefore the correct answer is 2.0 V. Verification / Alternative check: TTL datasheets for 74xx families consistently specify VIH(min) ≈ 2.0 V. Why Other Options Are Wrong: 2.4 V: This is VOH(min) for TTL outputs, not the input threshold.0.8 V: That is VIL(max), the maximum LOW input voltage.5.0 V: That is the supply, not the threshold. Common Pitfalls: Confusing VOH(min) (output) with VIH(min) (input).Assuming thresholds are symmetric around half the supply; TTL is not symmetric. Final Answer: 2.0 V

Integrated-Circuit Logic Families Questions