Question 1

TTL input sink current calculation A TTL NAND gate specifies IIL(max) = –1.6 mA per input. If its output in the LOW state drives eight standard TTL inputs, how much current must the driving output sink?

Accepted Answer

Introduction / Context: Fan-out in TTL logic is limited by the amount of current a driving output can sink (LOW) or source (HIGH) while maintaining valid logic levels. Understanding the IIL(max) specification per input allows you to compute the total sink requirement placed on a driver's LOW output level. Given Data / Assumptions: IIL(max) per TTL input = –1.6 mA (current into the input when it is receiving a LOW). Number of inputs driven = 8. Driver output is LOW (thus it must sink the sum of input currents). Concept / Approach: When a TTL output is LOW, current flows from each receiving gate into the driver. The total sink current is the sum of all input currents. Multiply the per-input IIL(max) by the number of inputs to get the worst-case sink demand on the driver. Step-by-Step Solution: Per-input sink current = 1.6 mA (magnitude).Number of loads = 8.Total sink current = 1.6 mA * 8 = 12.8 mA.Sign convention (negative for current into the pin): –12.8 mA. Verification / Alternative check: Compare to standard TTL driver capability (IOL(max) for 74xx is typically around 16 mA). The computed 12.8 mA is within this limit, consistent with a fan-out rating near 10 for many families. Why Other Options Are Wrong: –8 mA: assumes only five loads or a smaller IIL. –1.6 mA: counts a single load, not eight. –25.6 mA: doubles the correct value; would exceed many standard TTL driver limits. Common Pitfalls: Forgetting that the worst-case calculation uses IIL(max), not a typical value. Mixing source and sink specs; LOW requires sinking, HIGH requires sourcing (often much smaller current in TTL). Final Answer: –12.8 mA

Question 2

Propagation delay vs. maximum operating frequency As the propagation delay of a digital logic family increases, what happens to its maximum usable clock frequency?

Accepted Answer

Introduction / Context: Propagation delay determines how quickly a logic signal traverses from input to output. System clock rates must respect these delays with adequate margin, making the relationship between delay and maximum frequency fundamental to timing design and static timing analysis. Given Data / Assumptions: Propagation delay (tpd) is the time from a valid input transition to a valid output transition. Maximum frequency is limited by the slowest path delay in the clocked data path. Setup/hold and contamination delays are respected, but focus is on the basic inverse relation. Concept / Approach: In synchronous systems, the clock period must exceed the sum of logic propagation delays and register setup times. As propagation delay gets larger, the minimum safe clock period increases, which directly lowers the maximum clock frequency (fmax = 1 / Tmin). Step-by-Step Solution: Let total worst-case path delay be Tpath.Clock period Tmin ≥ Tpath + margins.Maximum frequency fmax = 1 / Tmin.If Tpath increases (greater delay), then Tmin increases, so fmax decreases. Verification / Alternative check: Empirical observation across logic families: ECL (short delays) allows high frequencies; older standard TTL or CMOS with larger delays supports lower fmax, confirming the inverse relationship. Why Other Options Are Wrong: Higher maximum frequency: contradicts the inverse relation. Maximum frequency unaffected: ignores timing reality. Minimum frequency unaffected: minimum frequency is generally not the limiting factor in synchronous logic; maximum frequency is. Common Pitfalls: Ignoring routing/interconnect delay, which also contributes to Tpath. Forgetting to include setup time and clock skew in the timing budget. Final Answer: lower the maximum frequency

Question 3

Prevalent TTL subfamily in practice Among standard TTL subfamilies, which form is most widely used in modern TTL-based designs?

Accepted Answer

Introduction / Context: Transistor–Transistor Logic (TTL) includes numerous subfamilies optimized for speed, power, or interface features. Knowing which variant is most common informs expectations about speed, power, and interface behavior in legacy and mixed-technology systems. Given Data / Assumptions: We are discussing TTL subfamilies such as standard, Schottky (S/LS/ALS), open-collector, and tristate outputs. “Used today” refers to common parts historically and in ongoing TTL-compatible systems. Concept / Approach: Schottky TTL integrates Schottky diodes to prevent transistor saturation, reducing storage time and improving speed without the complexity of ECL. Variants like LS (Low-power Schottky) and ALS further reduce power while maintaining speed, making them the dominant TTL forms compared with plain open-collector or tristate as primary “families.” Step-by-Step Solution: Identify typical parts in catalogs → 74LSxx, 74ALSxx widely available and used.These are Schottky-based families providing balanced speed/power.Therefore, the most widely used form is Schottky TTL. Verification / Alternative check: Legacy designs, textbooks, and supplier inventories frequently cite 74LS/74ALS as the workhorse TTL families. Why Other Options Are Wrong: Tristate TTL and open-collector TTL describe output configurations, not the dominant internal family across most parts. Low-power TTL without Schottky (L) is slower and less common than LS/ALS variants. Common Pitfalls: Confusing output features (tristate, open-collector) with core subfamilies (S, LS, ALS). Final Answer: Schottky TTL

Question 4

Hybrid logic families Which logic technology combines the advantages of CMOS (low power, high input impedance) with the speed or drive of bipolar TTL devices?

Accepted Answer

Introduction / Context: Modern integrated circuits often blend device types to optimize speed, power, and drive. The term for a technology that integrates both bipolar and CMOS transistors on the same die is crucial when evaluating families that promise “best of both worlds.” Given Data / Assumptions: CMOS benefits: very low static power, high noise margins, excellent scaling. Bipolar (TTL-like) benefits: strong drive currents, fast analog behavior in certain regions. Goal: combine these advantages. Concept / Approach: BiCMOS technology integrates Bipolar Junction Transistors (BJTs) with CMOS on a single chip. CMOS handles most logic functions efficiently, while bipolar devices provide high-current output stages or fast analog interfaces, enabling improved speed and drive without losing CMOS efficiency. Step-by-Step Solution: Identify hybrid process → “Bi” (bipolar) + “CMOS”.Match to benefits → low power + higher drive/speed.Select BiCMOS as the correct logic family. Verification / Alternative check: Common parts like high-speed buffers and microprocessors historically employed BiCMOS output drivers to achieve strong drive while keeping core logic CMOS. Why Other Options Are Wrong: TTL/CMOS or TTL/MOS are informal phrases, not established process names. ECL is a distinct high-speed bipolar family with constant-current operation, not CMOS-hybrid. Common Pitfalls: Assuming any TTL-compatible input means it is a “hybrid”; compatibility does not define the process. Final Answer: BiCMOS

Question 5

Inside ECL gates Which of the following circuit features is not part of emitter-coupled logic (ECL) gate design?

Accepted Answer

Introduction / Context: Emitter-coupled logic (ECL) is a very high-speed bipolar logic family that avoids transistor saturation by using differential pairs biased with constant current. Its internal building blocks are characteristic and differ from TTL structures such as totem-pole outputs. Given Data / Assumptions: ECL uses differential pairs, reference biasing, and emitter followers for buffering and level shifting. Totem-pole outputs are typical of TTL, not ECL. Concept / Approach: ECL logic levels are produced by steering current in a differential amplifier stage. Outputs are typically buffered by emitter followers to provide low output impedance and fast edges. A bias network establishes reference voltages. There is no need for the saturated pull-up/pull-down arrangement (totem-pole) used in TTL. Step-by-Step Solution: List ECL blocks → differential pair, bias reference, emitter follower.Compare with TTL totem-pole → absent in ECL by design.Therefore, “Totem-pole circuit” is not part of ECL. Verification / Alternative check: Standard ECL schematics confirm differential pairs and emitter followers; TTL diagrams show totem-pole NPN stages—clearly different approaches. Why Other Options Are Wrong: Differential amplifier, bias circuit, emitter follower are hallmark ECL elements that enable its speed. Common Pitfalls: Assuming all high-speed logic uses totem-pole outputs; ECL does not. Final Answer: Totem-pole circuit

Question 6

Transistor devices in CMOS logic Which transistor type is the fundamental active device used to implement complementary MOS (CMOS) logic gates?

Accepted Answer

Introduction / Context: CMOS stands for Complementary Metal-Oxide-Semiconductor and is the dominant logic technology in modern integrated circuits. The specific transistor device at its core determines switching behavior, input impedance, and power characteristics. Given Data / Assumptions: CMOS logic uses complementary pairs of transistors. The options include generic and specific transistor categories. Concept / Approach: CMOS is built from MOSFETs—specifically n-channel MOSFETs (NMOS) and p-channel MOSFETs (PMOS) arranged in complementary networks. While “FET” is a broader term that includes JFETs and MOSFETs, CMOS specifically relies on MOSFET devices with insulated gates (oxide), providing extremely high input impedance and very low static power consumption. Step-by-Step Solution: Identify device acronym in “CMOS” → MOS (Metal-Oxide-Semiconductor).Complementary pairing → NMOS + PMOS MOSFETs.Therefore, the correct device is “MOSFET.” Verification / Alternative check: Process technology nodes, SPICE models, and CMOS gate schematics confirm NMOS/PMOS MOSFET transistors form the pull-down and pull-up networks, respectively. Why Other Options Are Wrong: FET: too generic; CMOS specifically uses MOSFETs. Bipolar: used in TTL/ECL, not CMOS logic cores. Unijunction: used in timing/trigger circuits, not logic gate cores. Common Pitfalls: Answering “FET” because it is technically true that MOSFETs are FETs; the precise correct term here is MOSFET. Final Answer: MOSFET

Question 7

MOSFET terminals Identify the terminal name that does not belong to a MOSFET device.

Accepted Answer

Introduction / Context: MOSFETs are three-terminal field-effect devices used extensively in digital and analog circuits. Knowing their terminal names is fundamental to reading schematics, data sheets, and analyzing circuits correctly. Given Data / Assumptions: MOSFET standard terminals: Gate (control), Drain, Source. “Base” is a terminal name associated with bipolar junction transistors (BJTs). Concept / Approach: The MOSFET uses an insulated gate to modulate channel conductivity between source and drain. There is no “base” terminal in MOSFET terminology; that belongs to BJTs (emitter, base, collector). Therefore, among the listed options, “Base” does not belong to a MOSFET. Step-by-Step Solution: List MOSFET terminals → Gate, Drain, Source.Compare each option with the list.“Base” is not a MOSFET terminal → correct choice. Verification / Alternative check: Any MOSFET datasheet (discrete or IC) labels pins as G, D, S (and sometimes body/bulk). BJTs use E, B, C; hence “Base” clearly identifies a BJT, not a MOSFET. Why Other Options Are Wrong: Gate, Drain, Source are the canonical MOSFET terminals. Common Pitfalls: Confusing body/bulk connection (often tied to source in many technologies) with the named three external terminals. Final Answer: Base

Question 8

Noise margin formulas in logic families Which of the following equations correctly expresses the high-level noise margin of a logic family?

Accepted Answer

Introduction / Context: Noise margins quantify how much unwanted voltage can be superimposed on a logic level without causing an error. There are two margins: high-level (NMH or VNH) and low-level (NML or VNL). Correct formulas are essential for robust interface design and mixed-family connections. Given Data / Assumptions: VOH(min): minimum guaranteed output HIGH voltage of a driver. VIH(min): minimum input voltage that will be recognized as HIGH by a receiver. VIL(max): maximum input voltage that will be recognized as LOW. VOL(max): maximum output LOW voltage of a driver. Concept / Approach: High-level noise margin is the “headroom” between what the driver guarantees at HIGH (VOH(min)) and what the receiver needs to see to accept a HIGH (VIH(min)). Thus, VNH = VOH(min) – VIH(min). Low-level noise margin is VNL = VIL(max) – VOL(max). Step-by-Step Solution: Compute high margin: VNH = VOH(min) – VIH(min).Compute low margin (for reference): VNL = VIL(max) – VOL(max).Select the equation that matches the high-level definition. Verification / Alternative check: Check any logic family datasheet; noise margins are commonly listed using these definitions, ensuring interoperability between drivers and receivers. Why Other Options Are Wrong: VNL = VIL(max) + VOL(max): incorrect operation and sign. VNH = VOH(min) + VIH(min): adding values makes no physical sense for margin. VNL = VOH(min) – VIH(min): that expression is the high-level margin, not low-level. Common Pitfalls: Swapping high and low symbols; remember “H” pairs with VOH/VIH, “L” with VOL/VIL. Final Answer: VNH = VOH(min) – VIH(min)

Question 9

Standard TTL totem-pole output capability For a standard 74xx TTL gate with a totem-pole output, what is the typical maximum sink current (IOL(max)) in the LOW state?

Accepted Answer

Introduction / Context: Designers often need to know how much current a logic output can safely sink or source while keeping valid logic levels. In standard TTL (74xx), the totem-pole configuration provides defined current capabilities that determine fan-out and LED/relay driving limits (with proper resistors). Given Data / Assumptions: Standard TTL family (not LS/ALS) with totem-pole output stage. Asking for IOL(max): maximum current the output can sink while LOW. Concept / Approach: Standard TTL typically specifies IOL(max) ≈ 16 mA and IOH(max) ≈ –0.4 mA. These asymmetric limits reflect transistor configurations and ensure VOL and VOH remain within guaranteed levels under load. Newer LS/ALS have different limits, but the classic “standard TTL” figure is 16 mA for sinking in the LOW state. Step-by-Step Solution: Identify family → standard 74xx TTL.Recall spec → IOL(max) around 16 mA at VOL(max) conditions.Choose 16 mA from the options. Verification / Alternative check: Consult legacy datasheets and textbooks; they consistently cite 16 mA sink capability for standard TTL. LS versions are lower (around 8 mA), reinforcing that 16 mA is the “standard TTL” figure. Why Other Options Are Wrong: 20 mA: higher than typical standard TTL specification. 16 µA / 20 µA: off by three orders of magnitude; those currents are far too small. Common Pitfalls: Confusing standard 74xx TTL with 74LSxx values (which differ). Assuming source current equals sink current; in TTL, sourcing is much smaller. Final Answer: 16 mA

Question 10

Digital electronics (TTL logic families): Why is it considered bad practice to leave unused TTL inputs unconnected (left open)? Explain the underlying reason related to TTL input behavior and noise susceptibility.

Accepted Answer

Introduction / Context: In transistor–transistor logic (TTL) families, unused inputs are sometimes assumed to default to a logic HIGH. However, leaving TTL inputs floating is poor engineering practice. This question probes your understanding of TTL input structure and why noise can induce unintended switching if inputs are not tied to a defined logic level. Given Data / Assumptions: Standard TTL input structure is a multi-emitter NPN transistor front end. A floating TTL input may bias near a HIGH, but its state is not guaranteed across temperature, noise, and coupling. Reliable logic design requires each input to be at a defined DC level with adequate noise margin. Concept / Approach: TTL inputs left open can act like small antennas. Capacitive and inductive coupling from nearby switching traces can inject transient currents. Because the input threshold region is finite, noise can drive the input through the threshold, causing spurious toggling, increased power consumption, and electromagnetic interference. The cure is simple: tie unused inputs to a valid logic level (usually through a resistor if the gate allows) or use available pull-up/pull-down networks recommended by the manufacturer. Step-by-Step Solution: Identify the family: TTL input defaults are not hard-guaranteed.Recognize the risk: floating inputs can drift or pick up noise.Conclude: the main reason to avoid open TTL inputs is noise sensitivity and undefined logic levels. Verification / Alternative check: On a scope, a floating TTL input often shows random transitions when adjacent signals toggle. Once tied to VCC (through an appropriate resistor) or GND as specified, the spurious transitions disappear. Why Other Options Are Wrong: Low-current requirement: TTL inputs actually source/sink specific currents; floating does not meet requirements.Open-collector outputs and tristate construction relate to outputs, not input behavior.Input clamp diodes are for protection; they do not solve undefined floating states. Common Pitfalls: Assuming “floats HIGH” equals “safe.” In practice, noise margins and EMI demand defined input levels. Final Answer: noise sensitivity

Question 11

CMOS logic families: For a CMOS gate, which listed speed–power product (energy per operation) represents the best performance?

Accepted Answer

Introduction / Context: The speed–power product is an energy-per-operation metric widely used to compare digital logic families and device technologies. For CMOS gates, a smaller speed–power product indicates faster switching for less energy. This question asks you to pick the best (lowest) value among options. Given Data / Assumptions: Speed–power product is expressed in joules (commonly picojoules for logic). Lower energy-per-transition is better for performance per watt and thermal efficiency. All options are comparable units (pJ). Concept / Approach: Since all numeric values are in the same unit, the optimal speed–power product is simply the smallest number. In energy terms, each logic transition costs less energy, reducing dynamic power at a given activity factor and frequency (Pdynamic ≈ C * V^2 * f). With lower energy per switch, a design can achieve higher speed for the same power or the same speed at lower power. Step-by-Step Solution: List values: 1.4, 1.6, 2.4, 3.3 pJ.Select the minimum value.Conclude that 1.4 pJ is best. Verification / Alternative check: Compare any two entries: 1.4 pJ < 1.6 pJ, and 1.4 pJ < 2.4 pJ, 3.3 pJ, so 1.4 pJ is indeed the minimum. Why Other Options Are Wrong: Larger pJ values correspond to higher energy per operation, which is less desirable. Common Pitfalls: Confusing “best” with “largest.” In speed–power product metrics, smaller is better. Final Answer: 1.4 pJ

Question 12

Open-collector TTL outputs: What external component is required for proper logic-level operation on the output node?

Accepted Answer

Introduction / Context: Open-collector TTL outputs present an NPN transistor collector at the pin. The device can pull the line LOW (sink current) but cannot drive it HIGH. This topology enables wired-AND connections and level shifting, but it requires an external component to establish a valid logic HIGH when the transistor is off. Given Data / Assumptions: Open-collector output transistor can only sink current. No internal path to VCC is provided in this mode. Logic HIGH must be created externally. Concept / Approach: To produce a HIGH when the transistor is off, a resistor connects the output node to VCC, supplying a weak current that pulls the node up. When the transistor turns on, it sinks current through this resistor, forcing a LOW. The resistor value must balance rise time, fan-out, and power dissipation. Step-by-Step Solution: Identify output type: open-collector has no active pull-up.Add external element to create HIGH: a pull-up resistor.Confirm behavior: transistor off → HIGH; transistor on → LOW. Verification / Alternative check: Datasheets specify recommended pull-up ranges to meet VOH and rise-time requirements given bus capacitance and fan-out. Why Other Options Are Wrong: Pull-down resistor would hold the line LOW, defeating logic-high generation.“No resistor” cannot generate HIGH; the node would float.Arbitrary resistor or diode does not guarantee valid logic levels. Common Pitfalls: Using a pull-up that is too strong (excessive current when LOW) or too weak (slow rise time). Final Answer: a pull-up resistor

Question 13

TTL technology remains widely used. In which application segment is TTL particularly common and valued for robustness?

Accepted Answer

Introduction / Context: Transistor–transistor logic (TTL) is an established digital logic technology. While CMOS dominates modern low-power devices, TTL and TTL-compatible families remain relevant where noise immunity, simplicity, and established interfacing are prized. This question highlights the sector where TTL continues to be commonly applied. Given Data / Assumptions: Industrial environments involve electrical noise, long cables, and legacy equipment. Robust signaling and known characteristics are valued over ultra-low power. Maintenance and retrofitting often require TTL compatibility. Concept / Approach: Industrial control systems, PLC interfacing, and instrumentation often retain TTL-level I/O or TTL-compatible signaling for compatibility and ruggedness. Designers may choose TTL or TTL-compatible transceivers for robust edges and predictable thresholds, even if CMOS gates are inside. Step-by-Step Solution: Identify application domains with legacy and noise.Recognize that industrial settings match these constraints.Select “industrial applications.” Verification / Alternative check: Review catalogs for industrial I/O cards and sensors—TTL-compatible I/O remains common, particularly at 5 V interfaces. Why Other Options Are Wrong: Educational and commercial applications use both TTL and CMOS, but robustness needs are less dominant.Military may use radiation-hardened CMOS or specialized logic families.Wearables prioritize ultra-low power CMOS, not TTL. Common Pitfalls: Equating “popular” with “everywhere.” TTL persists chiefly where its strengths align with environmental demands and legacy systems. Final Answer: industrial applications

Question 14

In TTL (Transistor–Transistor Logic) families, which active switching device is used inside the basic gate structures?

Accepted Answer

Introduction / Context: Different logic families are defined by their internal switching devices. TTL is historically important and uses bipolar technology. Recognizing the underlying transistor type helps you understand speed, power, and interfacing characteristics. Given Data / Assumptions: TTL dates back to widespread bipolar processes. Gate cores employ multiple-emitter BJTs, Schottky clamping in some variants, and totem-pole outputs. Device physics of BJTs give TTL its switching behavior and currents. Concept / Approach: Because TTL is a bipolar logic family, its elementary gates (NAND as a primitive) use BJTs for input, switching, and output stages. This contrasts with CMOS, which uses complementary MOSFETs, and ECL, which uses differential BJT pairs biased in the active region. Step-by-Step Solution: Identify the family: TTL.Recall: “Transistor–Transistor Logic” refers to BJTs implementing both logic and amplification.Select “bipolar junction transistor (BJT).” Verification / Alternative check: Open any TTL gate schematic (e.g., 74xx NAND): you will find multi-emitter input BJT and BJT stages forming the totem-pole output. Why Other Options Are Wrong: FET/MOSFET are used in CMOS and NMOS/PMOS families, not TTL cores.Unijunction or tunnel diodes are not used as the active switch in TTL logic gates. Common Pitfalls: Confusing TTL-compatible CMOS (HCT families) with original TTL; compatibility refers to voltage levels, not the internal device type. Final Answer: bipolar junction transistor (BJT)

Question 15

TTL fan-out and output HIGH level: In a TTL circuit, if too many load gate inputs are connected, what happens to the driver’s HIGH-level output?

Accepted Answer

Introduction / Context: Fan-out is the number of standard inputs a logic output can reliably drive. Exceeding fan-out loads the driver beyond its guaranteed current sourcing/sinking capability, risking violation of specified logic levels. This question targets the effect on the output HIGH level of a TTL driver overloaded with inputs. Given Data / Assumptions: Each TTL input requires a defined input current at HIGH (IIH) and at LOW (IIL). The driving output specifies VOH(min) at a guaranteed IOH(max). Connecting excessive inputs increases current demand above IOH(max). Concept / Approach: When IOH demand exceeds the driver’s rating, the output transistor and pull-up network cannot maintain the guaranteed HIGH voltage. Therefore, the actual VOH falls below its guaranteed minimum (VOH(min)), potentially entering the undefined region between VIH(min) and VIL(max) and causing logic errors down the chain. Step-by-Step Solution: Compute total IIH = IIH(per input) * number of inputs.Compare to IOH(max) of the driving gate.If total IIH > IOH(max), the driver VOH sags below VOH(min). Verification / Alternative check: Datasheet fan-out = IOH(max) / IIH(max). Observe VOH on a scope under overload; it droops, confirming the violation. Why Other Options Are Wrong: Option A reverses the relationship; VOH(min) is a spec limit, not a measured value that “drops.”Options C and D contradict the overload effect.Option E is an input threshold parameter, not directly affected by the driver’s overload. Common Pitfalls: Ignoring IIH at HIGH and considering only LOW-state fan-out; both states must be checked for reliable design margins. Final Answer: VOH drops below VOH(min)

Question 16

Process technologies: PMOS and NMOS logic families were historically used most extensively in which integration scale?

Accepted Answer

Introduction / Context: Before CMOS dominated modern designs, PMOS and NMOS processes were widely used to build large-scale integrated (LSI) devices, such as early microprocessors and memory chips. This question checks historical and architectural awareness of where PMOS/NMOS found their greatest use. Given Data / Assumptions: MSI = medium-scale integration (dozens to hundreds of gates). LSI = large-scale integration (thousands of gates and beyond). Early CPUs, ROMs, and RAMs commonly used NMOS or PMOS prior to CMOS maturity. Concept / Approach: NMOS and PMOS technologies allowed dense digital logic and memory prior to the low-leakage, high-speed advantages of CMOS becoming standard. Consequently, these single-polarity MOS processes were the workhorses of LSI/VLSI-era parts before CMOS replaced them due to power and scaling benefits. Step-by-Step Solution: Identify process families: PMOS and NMOS.Recall their historical deployment: early microprocessors/memories (LSI).Select “LSI functions.” Verification / Alternative check: Classic NMOS microprocessors (e.g., early 8-bit CPUs) and RAM/ROM chips exemplify LSI implementation in single-polarity MOS. Why Other Options Are Wrong: MSI was also present but the dominant association for PMOS/NMOS advancement was LSI/VLSI.Diode or TTL functions describe different device families.Vacuum tubes are pre-MOS era. Common Pitfalls: Conflating logic families (TTL vs. MOS) with integration scales; here the question asks where PMOS/NMOS were largely used. Final Answer: LSI functions

Question 17

Tri-state (three-state) logic outputs: Which of the following is not a valid output state?

Accepted Answer

Introduction / Context: Three-state (tri-state) outputs allow a pin to be driven HIGH, driven LOW, or placed in a high-impedance (Hi-Z) state, effectively disconnecting it from the bus. Understanding these states is essential for bus sharing and multiplexed interfaces. Given Data / Assumptions: Valid states: logic HIGH, logic LOW, and High-impedance (Hi-Z). Hi-Z allows multiple devices to share a line by de-asserting their drivers. “Low-Z” is not a distinct logic state; it describes impedance magnitude, not logic function. Concept / Approach: The hallmark of tri-state outputs is the third state: High-Z. This state makes the output behave like an open circuit, enabling wired bus architectures. Any term that implies simply “low impedance” is not a defined tri-state mode, because both driven HIGH and driven LOW are low-impedance drive states by design. Step-by-Step Solution: List tri-state modes: HIGH, LOW, High-Z.Evaluate each option.Identify “Low-Z” as not a logical state but a characteristic of active drive. Verification / Alternative check: Datasheets describe output enable/disable controls resulting in Hi-Z when disabled; no device lists “Low-Z” as a third state. Why Other Options Are Wrong: HIGH and LOW are driven states with low output impedance.High-Z is the legitimate disconnected state.“Hi-Z with weak pull” is still conceptually a High-Z state plus biasing, not a fourth state. Common Pitfalls: Confusing impedance descriptors with logic states; only three logical output states exist in tri-state logic. Final Answer: Low-Z

Question 18

Fan-out calculation: A TTL NAND gate specifies IIH(max) = 40 µA per input at logic HIGH. If it drives ten standard TTL inputs, how much source current must its output provide in the HIGH state?

Accepted Answer

Introduction / Context: Fan-out quantifies how many inputs a logic output can drive while still meeting guaranteed logic levels. Calculations rely on datasheet currents: IIH for inputs at HIGH and IIL for inputs at LOW. This problem asks you to compute the required source current for a driver at logic HIGH. Given Data / Assumptions: IIH(max) per input = 40 µA. Number of loads N = 10. All loads are at logic HIGH simultaneously. Concept / Approach: Total current the driver must source at HIGH equals the sum of the IIH currents of all connected inputs. Ensuring the driver’s IOH(max) exceeds this sum preserves VOH ≥ VOH(min). If the total exceeds capability, VOH droops and logic margins are violated. Step-by-Step Solution: Compute total IIH: Itotal = IIH(max) * N.Substitute values: Itotal = 40 µA * 10.Calculate: Itotal = 400 µA = 0.4 mA.Compare to driver IOH(max) from datasheet to confirm compliance. Verification / Alternative check: If fan-out rating is given, it equals IOH(max) / IIH(max). Rearranging confirms the same current requirement. Why Other Options Are Wrong: 40 µA or 200 µA underestimate the sum; 800 µA and 4 mA overestimate for the stated loads. Common Pitfalls: Forgetting that fan-out must be evaluated for both HIGH and LOW states; IIL at LOW can be significantly larger and may become the limiting case in TTL. Final Answer: 400 µA

Question 19

CMOS outputs: An open-drain CMOS gate is the direct counterpart of which TTL output style?

Accepted Answer

Introduction / Context: Output structures in logic families often have analogous forms. In CMOS, the “open-drain” configuration mirrors the TTL “open-collector.” Understanding this equivalence is important for bus wiring, wired-AND/OR functions, and level translation using pull-up resistors. Given Data / Assumptions: Open-drain CMOS exposes the drain of an NMOS to the pin; it can pull LOW but not drive HIGH. Open-collector TTL exposes the collector of an NPN; it can pull LOW but not drive HIGH. Both require an external pull-up to generate a logic HIGH. Concept / Approach: Because both topologies only sink current, they are interchangeable conceptually. Using a resistor to VCC, the line idles HIGH. Multiple outputs can be “wire-ORed” (wired-AND in active LOW logic) on a shared bus. Rise time depends on the pull-up and bus capacitance, so resistor sizing is critical. Step-by-Step Solution: Identify the CMOS structure: open-drain.Identify the TTL analog: open-collector.Conclude they are counterparts requiring a pull-up for HIGH. Verification / Alternative check: Examine datasheets: both specify VOL at a sink current, but VOH depends on the external pull-up, not an internal transistor. Why Other Options Are Wrong: Tristate TTL can actively drive HIGH/LOW or Hi-Z, unlike open structures.ECL and BJT entries describe different logic paradigms, not open-sink outputs. Common Pitfalls: Forgetting to add a pull-up; without it, the output floats and may never reach a valid logic HIGH level. Final Answer: an open-collector TTL gate

Question 20

CMOS digital logic loading Which one of the following factors does NOT influence the loading (fan-out/speed) of a CMOS gate that is driving several other CMOS inputs?

Accepted Answer

Introduction / Context: CMOS loading determines how many inputs a single CMOS output can reliably drive at the desired speed. It is primarily a capacitive loading problem because CMOS inputs behave like small capacitors that must be charged and discharged through finite output resistances. Given Data / Assumptions: Driving device: one CMOS gate output. Loads: multiple CMOS gate inputs on the same net. We consider rise/fall timing and fan-out constraints. Concept / Approach: Propagation delay and edge rates in CMOS are dominated by the RC time constant formed by the driver’s output resistance (Rout) and the total load capacitance (Cload). Cload is the sum of the input capacitances of all receiving gates plus parasitics from traces and packages. Load gates do not present “output capacitance” to the net—only their inputs do. Step-by-Step Solution: Identify charging effect: charging time depends on Rout_high * Cload.Identify discharging effect: discharging time depends on Rout_low * Cload.Cload is primarily the sum of input capacitances of the receiving gates and interconnect capacitance.“Output capacitance of the load gates” is not connected to the driven net; therefore it does not affect the driver’s loading. Verification / Alternative check: CMOS data sheets specify Cin (input capacitance) and IOH/IOL with corresponding Rout or drive strength; edge rate calculations use Cin, not the output capacitance of other devices. Why Other Options Are Wrong: Charging time via driver output resistance: directly impacts rising edges.Discharging time via driver output resistance: directly impacts falling edges.Input capacitance of loads: the dominant component of Cload. Common Pitfalls: Confusing input and output capacitances, and forgetting PCB trace capacitance which also adds to Cload. Final Answer: Output capacitance of the load gates

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