Question 1

Average power in a digital gate (50% duty) A gate draws 1.8 µA when the output is HIGH and 3.3 µA when the output is LOW. With VCC = 5 V and a 50% duty cycle, what is the average power dissipation (PD)?

Accepted Answer

Introduction / Context: Digital CMOS and TTL families specify supply current in different logic states. Average power dissipation over time depends on the time spent in each state. Here, we compute average current over a 50% duty cycle and multiply by supply voltage. Given Data / Assumptions: I_H (HIGH-state current) = 1.8 µA. I_L (LOW-state current) = 3.3 µA. Duty cycle = 50% HIGH, 50% LOW. VCC = 5 V. Ignore dynamic switching losses for this simple estimate. Concept / Approach: Average current is the time-weighted mean of state currents. With equal time in both states, I_avg = (I_H + I_L) / 2. Average power PD_avg = VCC * I_avg. Step-by-Step Solution: I_avg = (1.8 µA + 3.3 µA) / 2 = 2.55 µA.PD = VCC * I_avg = 5 V * 2.55 µA.PD = 12.75 µW. Verification / Alternative check: Because the duty cycle is symmetric, arithmetic mean is valid. If the duty cycle were not 50%, a weighted average would be used. Why Other Options Are Wrong: 2.55 µW: mistakenly multiplies current average by 1 V or omits VCC.1.27 µW and 5 µW: arithmetic/units errors.5 µW is a round number guess with no basis here. Common Pitfalls: Forgetting to multiply by VCC, mixing µA with mA, or ignoring dynamic (capacitive) power when edge rates are high (not required in this static-state problem). Final Answer: 12.75 µW

Question 2

TTL output stages Which TTL output structure is commonly referred to as a “totem-pole” arrangement?

Accepted Answer

Introduction / Context: TTL logic families use specific output stage topologies to achieve low output impedance and fast transitions. One of the most recognized forms is the totem-pole output. Given Data / Assumptions: Standard TTL outputs often use a push-pull (active pull-up and active pull-down) stack of transistors. This stack is colloquially named “totem-pole.” Concept / Approach: A totem-pole stage places two active devices in series between VCC and ground, one sourcing during HIGH, the other sinking during LOW. This yields low Rout and fast edges, unlike open-collector which requires an external pull-up. Step-by-Step Solution: Identify options: only one is an established TTL output term—“totem-pole.”Select it as the correct descriptor of the push-pull TTL output. Verification / Alternative check: TTL datasheets (e.g., 74LSxx) describe outputs as totem-pole (standard) or open-collector variants. Why Other Options Are Wrong: Diode arrangement: too vague; not the accepted output term.JBT arrangement: not a recognized TTL topology.Base, emitter, collector arrangement: generic transistor anatomy, not an output style. Common Pitfalls: Confusing totem-pole with open collector or tri-state; only totem-pole actively drives both directions (except in high-Z tri-state variants). Final Answer: totem-pole arrangement

Question 3

Nominal logic supply rails What is the nominal dc supply voltage used by classic TTL and many CMOS families in 5 V systems?

Accepted Answer

Introduction / Context: Digital logic families are associated with standard supply voltages. Classic 5 V logic remains common in legacy and mixed-signal designs even as lower-voltage CMOS (3.3 V, 1.8 V) has proliferated. Given Data / Assumptions: Question refers to “TTL and CMOS” without qualifiers, implying the ubiquitous 5 V era. Concept / Approach: Standard TTL (74xx, 74LSxx) is specified at 5 V ± tolerance. Early CMOS 4000B series operated over a wide range (3–15 V) but was frequently used at 5 V for compatibility. Many 74HC/HCT families also target 5 V systems. Step-by-Step Solution: Identify the conventional supply level shared across many TTL and CMOS parts: +5 V. Verification / Alternative check: Look at datasheets for 74LS00 or 74HC00: recommended VCC ≈ 5 V. Why Other Options Are Wrong: +3 V: common in modern low-voltage CMOS but not “nominal” for TTL.+9 V and +12 V: used in analog or power domains, not standard for TTL logic rails. Common Pitfalls: Assuming all CMOS is 3.3 V; many CMOS families are 5 V compatible or specified at 5 V. Final Answer: +5 V

Question 4

Static HIGH-state dissipation Given ICCH = 1.1 mA at VCC = 5 V and the gate is held in a static HIGH output state (no switching), what is the power dissipation PD?

Accepted Answer

Introduction / Context: Static power in logic is simply supply voltage times the specified static current in a particular state. Manufacturers often list ICCH (HIGH) and ICCL (LOW) currents separately. Given Data / Assumptions: ICCH = 1.1 mA. VCC = 5 V. No switching (ignore dynamic power). Concept / Approach: Use PD = VCC * ICC for the given static state. Step-by-Step Solution: PD = 5 V * 1.1 mA = 5.5 mW. Verification / Alternative check: Units: volts * amperes = watts. 5 * 0.0011 = 0.0055 W = 5.5 mW. Why Other Options Are Wrong: 5.5 W: off by a factor of 1000 (mA vs A).5 mW and 1.1 mW: arithmetic errors. Common Pitfalls: Dropping the milli prefix or mixing µA/mA leads to thousandfold mistakes. Final Answer: 5.5 mW

Question 5

ESD and CMOS handling practice Which of the following is NOT a recommended precaution when handling CMOS devices?

Accepted Answer

Introduction / Context: CMOS inputs are susceptible to electrostatic discharge. Proper ESD practice prevents latent or immediate device damage. Some habits reduce charge buildup; others make it worse. Given Data / Assumptions: ESD can destroy gate oxides via high voltage spikes. Grounding and power-off insertion are standard precautions. Certain materials (e.g., wool) promote charge generation. Concept / Approach: Best practice includes working on grounded surfaces, using wrist straps, keeping humidity reasonable, and never hot-plugging unprotected CMOS. Insulative clothing and carpets increase ESD risk and are discouraged. Step-by-Step Solution: Identify clearly bad advice: wearing wool promotes static.Other listed items match standard ESD guidelines (ground everything, power off before insertion). Verification / Alternative check: ESD control manuals recommend conductive mats, wrist straps, and avoiding static-generating materials like wool or some synthetics. Why Other Options Are Wrong: Grounding surfaces and tools reduces ESD risk.Pins-down on grounded surface equalizes potential.Inserting with power off avoids latch-up and transients. Common Pitfalls: Forgetting that humidity affects ESD, and neglecting personal grounding measures when moving around the lab. Final Answer: Wear wool clothing at all times.

Question 6

Handling unused TTL inputs safely: Evaluate the statement: “Unused TTL logic inputs should be tied LOW.”

Accepted Answer

Introduction / Context: Proper termination of unused logic inputs prevents noise pickup, oscillation, and excess current. TTL and CMOS families have different input structures, so the best practice is family-dependent. This question checks safe handling for unused TTL inputs. Given Data / Assumptions: Standard TTL input structure exhibits a default bias toward logic HIGH when left open (due to multi-emitter transistor input network). We want to avoid unnecessary current draw and erratic behavior. We are not discussing open inputs for CMOS, which must never be left floating. Concept / Approach: Unused TTL inputs are commonly tied HIGH, not LOW. Tying LOW often sinks current through the input transistor network, increasing power dissipation. If leaving open is acceptable per the datasheet for certain gates, tying HIGH (directly or via a pull-up) is still preferred for noise immunity. CMOS, by contrast, must never be left floating; it should be tied definitively to HIGH or LOW through a resistor, but that is a separate rule. Step-by-Step Solution: Recognize TTL bias: a floating TTL input tends toward logic HIGH.Select safe default: tie HIGH to reduce current and improve noise immunity.Conclude that “tie LOW” is not recommended; therefore the statement is incorrect. Verification / Alternative check: Classic TTL datasheets and application notes recommend tying unused inputs to VCC (often via a resistor) or connecting them to another driven input to ensure a defined HIGH. Why Other Options Are Wrong: Correct: Contradicts TTL best practice.Series resistor caveat: Even with a resistor, tying LOW is not standard for minimizing current.CMOS-specific claim: The prompt is about TTL, not CMOS. Common Pitfalls: Applying CMOS rules to TTL; assuming all logic inputs behave the same when left floating. Final Answer: Incorrect

Question 7

Logic-level parameters across families: Is it correct that digital IC families specify four logic-level parameters: VIL, VIH, VOL, and VOH?

Accepted Answer

Introduction / Context: Understanding logic-level thresholds and output drive limits is fundamental for interfacing between IC families. Standard datasheet parameters define input recognition levels and guaranteed output levels for logic 0 and logic 1. This question checks familiarity with those four canonical parameters. Given Data / Assumptions: VIL and VIH are input-level thresholds defining recognized LOW and HIGH ranges. VOL and VOH are guaranteed output voltage levels for LOW and HIGH outputs under specified load. Applies to TTL, CMOS, and other families with different numeric values. Concept / Approach: Datasheets nearly always present VIL, VIH, VOL, and VOH. Designers ensure margin by comparing an output's VOL/VOH against the next stage's VIL/VIH. While the numeric values differ (for example, TTL vs. CMOS vs. LVCMOS), the categories remain consistent and enable safe logic-level interfacing and noise-margin analysis. Step-by-Step Solution: Identify input thresholds: VIL (max LOW input), VIH (min HIGH input).Identify output guarantees: VOL (max LOW output), VOH (min HIGH output).Confirm universality across common logic families. Verification / Alternative check: Any standard datasheet for 74-series TTL or 4000-series CMOS lists these parameters, confirming the statement. Why Other Options Are Wrong: Incorrect: Conflicts with standard datasheet conventions.TTL-only or ECL-only claims: These parameters are used widely, not limited to one family. Common Pitfalls: Ignoring load conditions that affect VOL/VOH; always check the test current and supply when comparing levels. Final Answer: Correct

Question 8

Speed comparison of logic families: Evaluate the statement: “Emitter-Coupled Logic (ECL) IC technology is faster than TTL.”

Accepted Answer

Introduction / Context: Logic families balance speed, power, noise margins, and voltage swings differently. Emitter-Coupled Logic is renowned for high speed due to small voltage swings and transistors that do not saturate. This item checks whether you recognize ECL’s speed advantage over traditional TTL families. Given Data / Assumptions: ECL avoids transistor saturation, reducing stored charge and enabling rapid switching. TTL uses saturated switching (with variants like Schottky TTL to reduce storage delay). We compare families in their typical operating regimes. Concept / Approach: ECL’s differential, constant-current topology produces fast transitions with low, fixed voltage swings, which directly translates to higher propagation speed and bandwidth. TTL families improved speed via Schottky clamping and advanced processes, but mainstream ECL remains faster in raw propagation delay, historically used in high-speed communications and clock distribution. Step-by-Step Solution: Identify saturation behavior: ECL does not saturate; TTL generally does.Relate voltage swing to speed: smaller swings reduce transition time.Conclude ECL typically outperforms TTL in speed. Verification / Alternative check: Prop-delay specs for classic ECL (hundreds of picoseconds to a few nanoseconds) beat standard TTL and many Schottky TTL parts, confirming the statement. Why Other Options Are Wrong: Incorrect: Opposes well-established characteristics.Temperature or Schottky caveats: While conditions matter, the general statement holds. Common Pitfalls: Ignoring ECL’s higher power and negative supply requirements; faster does not mean universally preferable. Final Answer: Correct

Question 9

Using speed-power product for logic choice: Does the speed-power product provide a useful basis to compare logic families when both propagation delay and power dissipation matter?

Accepted Answer

Introduction / Context: When selecting a logic family, engineers often juggle speed and power. A single metric that multiplies typical propagation delay by power dissipation helps compare options on an equal footing. This question checks whether you recognize the speed-power product as that comparative figure of merit. Given Data / Assumptions: Propagation delay reflects switching speed per gate. Power dissipation includes static and dynamic components under specified conditions. We seek a simple, comparative metric, not an exhaustive characterization. Concept / Approach: The speed-power product (sometimes called the energy per transition proxy) is defined as power dissipation * propagation delay. Lower values indicate greater efficiency at delivering speed per watt. While it abstracts away many details (load, supply, temperature), it remains a useful first-pass metric for family selection and system-level budgeting. Step-by-Step Solution: Compute SPP = P_diss * t_pd for each candidate family.Compare results: lower SPP suggests better speed-per-power trade-off.Select the family that meets timing within power limits. Verification / Alternative check: Datasheets and textbooks frequently introduce SPP as a way to summarize logic-family efficiency; it complements, not replaces, detailed timing and power analyses. Why Other Options Are Wrong: Incorrect: Disagrees with established practice.CMOS-only or 5 V-only: The concept applies broadly; numeric results vary with supply and process but the metric remains meaningful. Common Pitfalls: Relying solely on SPP without checking drive strength, fanout, and dynamic loading; SPP is a starting point, not the whole story. Final Answer: Correct

Question 10

Terminating open-collector TTL outputs: Evaluate the statement: “A pull-down resistor must be used with open-collector TTL circuits.”

Accepted Answer

Introduction / Context: Open-collector outputs are a classic TTL feature used for wired-AND logic and level interfacing. Because the transistor can sink current but cannot source it, an external resistor is required to define the logic HIGH level when the transistor is off. This question checks whether you know which direction that resistor goes. Given Data / Assumptions: Open-collector output can pull the line LOW (sink), but cannot drive it HIGH. An external resistor determines the inactive line level. Loads may include multiple OC outputs sharing a bus (wired-AND). Concept / Approach: The correct termination for an open-collector line is a pull-up resistor to the positive supply. When the transistor is off, the resistor pulls the node HIGH; when on, the transistor sinks current and forces the node LOW. A pull-down resistor would not create a valid HIGH level and would fight the intended operation. Step-by-Step Solution: Recognize one-sided drive: OC can only pull LOW.Provide the missing source: add a pull-up to VCC.Conclude that “pull-down” is incorrect terminology and use. Verification / Alternative check: Application notes and interface schematics consistently show pull-up resistors on open-collector outputs, often with values sized to meet rise time and current specs. Why Other Options Are Wrong: Correct: Contradicts the basic OC principle.Wired-AND caveat: Even on wired-AND busses, the resistor is a pull-up.CMOS-load caveat: CMOS inputs still require a defined HIGH, supplied via a pull-up, not a pull-down. Common Pitfalls: Choosing too-strong a pull-up that wastes power or too-weak that causes slow rise times; balance with bus capacitance and speed. Final Answer: Incorrect

Question 11

Noise immunity vs. power: Evaluate the statement: “Power dissipation is a measure of a circuit’s noise immunity.”

Accepted Answer

Introduction / Context: Noise immunity quantifies how robustly a logic circuit tolerates unwanted disturbances on its inputs and power rails. Power dissipation, by contrast, measures energy consumption. While some high-power families may incidentally enjoy larger noise margins, power alone is not the metric for immunity. This question separates these concepts. Given Data / Assumptions: Noise immunity is commonly evaluated using input noise margins: NMH = VOH(min) − VIH(min) and NML = VIL(max) − VOL(max). Power dissipation depends on static leakage and dynamic switching losses under specified load and frequency. Different logic families show varied relationships between power and margins. Concept / Approach: Noise immunity derives from logic-level thresholds and output drive guarantees, not directly from watts consumed. Designers compare VOH/VOL against VIH/VIL to calculate noise margins. Although higher supply voltages or stronger drivers (which may increase power) can improve margins, the definitive measure remains the margin equations, not power itself. Step-by-Step Solution: Identify correct measures: NMH and NML quantify immunity.Note that power is orthogonal: it reflects efficiency, thermal load, and battery life.Conclude that stating “power dissipation is a measure of noise immunity” is incorrect. Verification / Alternative check: Datasheets list noise margins and logic thresholds in separate tables from power characteristics, demonstrating that they are distinct specifications evaluated differently. Why Other Options Are Wrong: Correct/ECL-only/thresholded claims: These confuse correlation with causation; immunity is not measured in watts. Common Pitfalls: Assuming a hotter, higher-power device is automatically more robust; always compute noise margins using the specified voltage levels and loads. Final Answer: Incorrect

Question 12

Integrated-circuit families – in modern digital systems and VLSI, is complementary metal–oxide–semiconductor (CMOS) generally the more dominant IC technology compared with classic TTL logic, considering power, density, and scalability?

Accepted Answer

Introduction / Context: Over decades of digital design, technology has shifted from bipolar transistor–transistor logic (TTL) to complementary metal–oxide–semiconductor (CMOS). This question asks whether CMOS is indeed the dominant technology today and why that is the case in the context of logic ICs and large-scale integration (VLSI). Given Data / Assumptions: Dominance is judged by market share, integration level, power efficiency, and scalability. CMOS here includes all modern process nodes used in microcontrollers, CPUs, FPGAs, ASICs, and standard logic. TTL refers to classic bipolar families (74, 74LS, 74ALS, etc.). Concept / Approach: CMOS offers extremely low static power (ideally near zero when not switching), very high integration density, and superb scalability with process shrinks. Leakage and dynamic power are managed by device engineering and clock gating. In contrast, TTL uses bipolar transistors with higher static currents and does not scale to nanometer nodes for VLSI. Modern “TTL-compatible” logic levels are actually implemented in CMOS (for example, 74HC/74HCT/74LVC), reflecting CMOS’ practical dominance while maintaining TTL-level interfacing when needed. Step-by-Step Solution: 1) Compare power: CMOS static power ≪ TTL; dynamic power proportional to C * V^2 * f.2) Compare density: CMOS scales with lithography; TTL cannot match transistor counts.3) Compare performance: modern CMOS achieves very high speeds with lower voltages.4) Industry adoption: virtually all CPUs, SoCs, memory, and logic today are CMOS. Verification / Alternative check: Data books for current logic families show 74HC/HCT/LVC/AUP/etc., all CMOS. TTL (LS/ALS) is legacy and niche due to higher power and limited speed/voltage flexibility relative to modern CMOS options. Why Other Options Are Wrong: “Incorrect” contradicts industry reality. “Only for 4000-series” is too narrow: modern CMOS spans many families beyond 4000-series. Frequency and analog qualifiers do not define dominance across digital ICs. Common Pitfalls: Confusing “TTL-compatible voltage levels” with TTL technology; many CMOS parts accept TTL-level inputs without being TTL internally. Final Answer: Correct

Question 13

Output loading in logic families – as more load-gate inputs are connected to a driver output, does the total sink current required by the driving gate decrease, increase, or stay the same?

Accepted Answer

Introduction / Context: Fan-out describes how many inputs a logic output can reliably drive. The electrical consequence of attaching more inputs to one output is a classic consideration in digital electronics. This question focuses specifically on sink current and how it changes as additional loads are attached. Given Data / Assumptions: The driver output must meet guaranteed logic-level voltages under load. Each load input presents some current requirement (DC and dynamic). Sink current refers to current flowing into the driver when it pulls the line to logic 0. Concept / Approach: In TTL, each input sources a small current when high and demands current be sunk by the driver when low. More connected inputs add to the total current that must be sunk by the driver to hold a valid low level. Even with CMOS inputs (which have negligible DC input current), dynamic currents due to input capacitance increase with more loads, raising transient sink/source current and slowing edges if the driver is not strong enough. Step-by-Step Solution: 1) Model each input as drawing/sourcing a small current plus capacitance.2) Sum currents for N loads: Itotal ≈ N * Iinput (plus dynamic terms).3) Recognize the driver must sink this total when asserting a low.4) Conclude that increasing the number of loads increases required sink current. Verification / Alternative check: Datasheets specify IOH/IOL (source/sink limits) and fan-out ratings. Exceeding these limits degrades VOL/VOH margins and rise/fall times, confirming the increased current requirement with more loads. Why Other Options Are Wrong: Decrease is the opposite of reality; “stays the same” ignores additive loading; oscillation or zero current claims are nonphysical for practical logic families. Common Pitfalls: Confusing CMOS near-zero DC input current with zero total demand; dynamic switching still requires charging/discharging input capacitances, increasing transient current with more loads. Final Answer: It increases because additional inputs require more current to sink

Question 14

Timing vs speed – does a greater propagation delay (tpd) imply a higher maximum operating frequency, or does it limit the achievable clock rate?

Accepted Answer

Introduction / Context: Propagation delay is the time a signal takes to travel through a combinational element from input to output. Maximum operating frequency is the fastest clock or toggle rate a system can sustain while maintaining correct logic operation. Understanding their relationship is critical for timing closure and reliable design. Given Data / Assumptions: We consider basic synchronous systems where combinational delay plus setup/hold constraints bound the clock period. Propagation delay refers to typical or worst-case path delays within logic devices or nets. Clock period T is the reciprocal of frequency f. Concept / Approach: The maximum frequency is constrained by the slowest (critical) path delay. As propagation delay increases, the minimum required clock period must also increase to allow signals to settle and meet setup times. Therefore, higher delay reduces maximum frequency. In formula form: Tmin ≥ tpd(critical) + tsetup + margins and fmax ≤ 1 / Tmin. This holds for logic families across TTL, CMOS, and others, though absolute numbers vary with technology and supply voltage. Step-by-Step Solution: 1) Identify critical path delay through combinational logic.2) Add register setup/clock skew/jitter margins.3) Compute Tmin; infer fmax = 1 / Tmin.4) Observe: increasing tpd increases Tmin → decreases fmax. Verification / Alternative check: Timing analysis tools (STA) explicitly compute fmax from cell/net delays and constraints; device datasheets show families with lower tpd achieving higher toggle and clock rates. Why Other Options Are Wrong: Saying delay raises frequency inverts the relationship; claiming no relation ignores synchronous timing fundamentals; supply voltage affects delay but does not break the inverse relationship. Common Pitfalls: Confusing throughput improvements from pipelining (which shortens critical paths) with raw per-stage delay; pipelines increase fmax by reducing per-stage tpd, not by increasing it. Final Answer: Greater delay lowers the maximum frequency

Question 15

CMOS device physics – are metal–oxide–semiconductor field-effect transistors (MOSFETs) the active switching elements used to implement logic in CMOS circuits (paired as nMOS and pMOS networks)?

Accepted Answer

Introduction / Context: CMOS stands for complementary MOS, highlighting its use of n-channel and p-channel MOSFETs in complementary pairs. This question checks whether MOSFETs are indeed the active devices switching logic levels inside CMOS integrated circuits. Given Data / Assumptions: CMOS logic uses pull-up (pMOS) and pull-down (nMOS) networks. Gate terminals are insulated by a thin oxide, controlling channel conduction. Logic families considered include basic gates, transmission gates, and complex VLSI blocks. Concept / Approach: In CMOS, MOSFETs operate as controlled switches. The complementary arrangement ensures that for any valid logic input, only one network (pull-up or pull-down) conducts strongly, minimizing static current. This leads to low static power, high noise margins, and scalability. Transmission gates are simply parallel nMOS/pMOS pairs controlled by complementary signals to pass both logic 0 and 1 effectively; they, too, are MOSFET-based. Step-by-Step Solution: 1) Identify CMOS gate topology: series/parallel arrangements of nMOS and pMOS devices.2) Note conduction paths form logic functions (AND via series, OR via parallel, etc.).3) Recognize negligible static current when not switching due to complementary action.4) Conclude MOSFETs are the switching elements in CMOS. Verification / Alternative check: Process names (for example, “28 nm CMOS”) and PDK device libraries explicitly provide nMOS and pMOS transistors for logic synthesis and layout; bipolar transistors are not used for standard CMOS logic switching. Why Other Options Are Wrong: Bipolar-only and JFET-only statements contradict CMOS’ definition; limiting MOSFET use to transmission gates ignores that all CMOS gates are built from MOSFETs. Common Pitfalls: Confusing CMOS (device technology) with TTL (bipolar); assuming “MOSFETs only pass analog signals”—they implement both logic and analog functions. Final Answer: Correct

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