Q: Non-saturating logic family — avoiding transistor saturation Which logic family is characterized by operation that prevents transistor saturation during switching?

Introduction / Context: Switching speed is heavily influenced by whether devices enter saturation. When transistors saturate, stored charge must be removed before the device turns off, increasing delay. A logic family that avoids saturation can therefore switch faster at the cost of higher power. Given Data / Assumptions: ECL (Emitter-Coupled Logic) uses differential transistor pairs biased in the active region. TTL and IIL are bipolar families that can involve saturation or current-steering differently. MOS is a field-effect technology, not a single “family.” Concept / Approach: ECL keeps BJTs from saturating by using constant current sources and emitter-coupled pairs. Voltages swing over a small range and never drive the transistors into saturation, minimizing storage delay and enabling very short propagation times. TTL variants (even Schottky TTL) reduce but do not fully eliminate saturation effects; IIL is current-mode but distinct in behavior. MOS refers broadly to PMOS/NMOS/CMOS and does not answer the saturation-specific question for bipolar BJTs. Step-by-Step Solution: Identify family known for non-saturating operation: ECL.Recall ECL's differential, constant-current design.Conclude ECL avoids saturation and achieves fast switching.Select ECL from options. Verification / Alternative check: Classic 10K/100K ECL datasheets highlight small-swing, non-saturating characteristics as the source of speed. Why Other Options Are Wrong: TTL: Schottky clamps reduce saturation depth but do not fully avoid it. MOS: Not a bipolar family; the question targets bipolar saturation behavior. IIL: Uses current injection; not characterized primarily by “no saturation” like ECL. Common Pitfalls: Assuming “MOS” implies non-saturation; the concept here is about BJT saturation avoidance. Final Answer: ECL

Q: TTL Best Practice — Why a decoupling capacitor is needed and where to connect it In transistor–transistor logic (TTL) circuits, what is the purpose of a local decoupling (bypass) capacitor and where should it be connected for best results?

Introduction / Context: Every TTL IC switches current quickly, creating sharp edges that momentarily draw high transient currents. Without local decoupling, these transients can cause ground bounce, supply droop, and unintended logic errors. Decoupling capacitors provide a low-impedance local energy reservoir near each IC. Given Data / Assumptions: IC supply is typically VCC = +5 V with ground reference. Fast edges cause di/dt and voltage spikes on supply traces. A small ceramic capacitor (for example 0.1 µF) is physically placed close to the VCC and GND pins. Concept / Approach: The decoupling capacitor shunts high-frequency noise to ground and locally supplies transient current, stabilizing the IC’s supply voltage. The correct connection is directly between the power pin and ground pin, with the shortest possible loop (minimal lead/trace inductance). Step-by-Step Solution: Identify noise source → switching currents and trace inductance.Provide local energy → place a capacitor near the IC.Define connection → from VCC to GND at the IC pins.Result → reduced supply ripple, fewer false toggles, improved signal integrity. Verification / Alternative check: Scope measurements with and without local 0.1 µF ceramics show visibly reduced supply ringing and ground bounce at the IC; adding bulk capacitance (for example 10 µF per board region) further helps low-frequency transients. Why Other Options Are Wrong: a, b: Connecting to inputs does not address supply transients and can even degrade signal integrity. d: Not applicable; the meaningful, standard practice is VCC-to-GND decoupling. Common Pitfalls: Using long traces to the capacitor or omitting one per IC; both undermine the capacitor’s ability to handle high-frequency spikes. Final Answer: to reduce the effects of noise, connect between power supply and ground

Q: Open-Collector Outputs — Source vs. sink capability Complete the statement correctly: An open-collector output can ________ current, but it cannot ________.

Introduction / Context: Open-collector (OC) outputs are widely used for wired-AND interconnections, level shifting, and driving loads that require voltages different from the logic family’s supply. Understanding what an OC stage can and cannot do is essential for correct pull-up selection and load interfacing. Given Data / Assumptions: An OC output is the collector of an NPN transistor left “open” (no internal pull-up). The external circuit must provide a pull-up resistor to a suitable positive supply for a logic HIGH level. The transistor can pull the output LOW by conducting to ground (sinking current). Concept / Approach: An OC stage works by sinking current through the transistor to ground when ON. When the transistor is OFF, it does not source current; the pull-up resistor sources current to raise the output HIGH. Thus, OC outputs can sink current but cannot source it. Step-by-Step Solution: Transistor ON → current flows from pull-up through load into collector → to emitter (ground): sink operation.Transistor OFF → output is “released”; pull-up resistor provides the sourcing path, not the IC.Therefore, fill blanks as “sink, source current.” Verification / Alternative check: Datasheets specify IOL (sink current) ratings for OC outputs and do not give IOH (source) ratings; the HIGH level depends on the external pull-up. Why Other Options Are Wrong: b: Reverses the roles; OC cannot source. c, d: Phrase “source/sink voltage” is incorrect usage; devices source or sink current. Common Pitfalls: Choosing too weak a pull-up (slow rise, susceptibility to noise) or too strong (excessive sink current and power when LOW). Final Answer: sink, source current

Q: TTL Output Drive — Compare sourcing at HIGH vs. sinking at LOW Evaluate the statement: “The TTL HIGH-level source current is higher than the LOW-level sinking current.” Is it true or false in standard TTL practice?

Introduction / Context: TTL totem-pole outputs are asymmetric: they typically can sink substantially more current when LOW than they can source when HIGH. Knowing this asymmetry guides correct fan-out calculations and pull-up/pull-down choices. Given Data / Assumptions: Standard TTL logic HIGH source current capability is modest (for example, around –0.4 mA). Standard TTL logic LOW sink current capability is much larger (for example, around 16 mA for LS-TTL families). Exact numbers vary by subfamily, but the asymmetry persists. Concept / Approach: Compare typical IOH (source at HIGH) to IOL (sink at LOW). Since IOH is much smaller in magnitude than IOL, the statement claiming HIGH-level source current is higher is incorrect. Step-by-Step Solution: Identify IOH (small) vs. IOL (large).Evaluate statement: “HIGH-level source current higher than LOW sink” → contradicts specs.Therefore, the statement is False. Verification / Alternative check: Check a 74LSxx datasheet: IOH around –0.4 mA; IOL around 8–16 mA. The asymmetry is clear and consistent. Why Other Options Are Wrong: “True” would only apply to a hypothetical family with symmetric or reversed capabilities, not standard TTL. Common Pitfalls: Designers sometimes assume symmetric drive strength; ensure loads are oriented so the logic level that sinks current handles the heavier load when possible. Final Answer: False

Q: Common TTL Series Identification — Which family code is most prevalent? Among the listed options, select the designation that refers to the classic, widely used TTL logic IC series.

Introduction / Context: The “7400” designation identifies the ubiquitous TTL logic family that defined small-scale and medium-scale integration for decades. Many subfamilies (LS, ALS, S, AS, H, F, etc.) expand on the basic 7400 lineage, each optimizing for speed, power, or drive. Given Data / Assumptions: Standard TTL parts are commonly denoted 74xx for commercial temperature ranges (and 54xx for military). Examples: 7400 (quad NAND), 74138 (3-to-8 decoder), 7474 (dual D flip-flop). Other strings in the options are not family designations. Concept / Approach: Recognize that “7400” is a canonical family marker for TTL logic ICs. Terms like “E-MOSFET” describe device physics, not a TTL series. “AC00” refers to an advanced CMOS (74AC) variant—not TTL. “quad” describes a package function count, not a series. Step-by-Step Solution: Identify a family indicator among options.Match to standard TTL naming → “7400.”Exclude descriptive or CMOS-family terms. Verification / Alternative check: Datasheets and catalogs list 74xx series parts as TTL; 74HC/AC families are CMOS with TTL-compatible inputs but are distinct from original TTL. Why Other Options Are Wrong: E-MOSFET: Device type, not a TTL family. quad: Merely indicates four gates/units; not a series. AC00: Part of 74AC CMOS family, not TTL. Common Pitfalls: Assuming all “74xx” parts are TTL; some modern “74xx” are CMOS (HC/HCT/AC/LCX). Check family suffixes. Final Answer: 7400

Question 1

Interfacing logic families with different VCC — appropriate solution When two IC logic families operate at different supply voltages (VCC), which interface component is typically used to ensure correct logic-level translation?

Accepted Answer

Introduction / Context: Modern systems often mix 5 V TTL-compatible parts with 3.3 V or lower-voltage CMOS devices. Directly connecting such parts can violate VIH/VIL/VOH/VOL requirements and even damage inputs. Proper level translation preserves signal integrity and device reliability. Given Data / Assumptions: Different VCC rails exist (e.g., 5 V and 3.3 V). Logic-high thresholds differ across families. Bidirectional or unidirectional translation may be needed. Concept / Approach: A level shifter (level translator) converts logic levels from one voltage domain to another. Solutions range from simple transistor or MOSFET-based shifters to dedicated translator ICs with direction control and fail-safe inputs. Pull-ups/downs and capacitors do not perform true level translation across domains by themselves. Step-by-Step Solution: Identify mismatch: VIH(min)/VOH(min) between domains do not align.Insert proper translator: use a level-shifter that guarantees output levels meeting the target family specs.Verify directionality (uni/bidirectional) and speed to meet timing.Confirm power sequencing and protection features as needed. Verification / Alternative check: Examine VIH/VIL tables for both families; ensure the translated levels meet or exceed target thresholds with margin. Why Other Options Are Wrong: Tristate shifter: “tristate” refers to output enable, not level translation. Decoupling capacitor: filters noise; does not shift levels. Pull-down resistor: sets default logic state; cannot raise VOH across domains. Common Pitfalls: Relying on resistor dividers at high speed (poor edge integrity) or forgetting overvoltage tolerance on low-VCC inputs. Final Answer: level-shifter

Question 2

CMOS dynamic power — why frequency matters for dissipation Why is the operating frequency a critical factor in determining power dissipation for CMOS logic?

Accepted Answer

Introduction / Context: CMOS logic exhibits very low static power, but dynamic (switching) power dominates as frequency rises. Understanding the C * V^2 * f relationship is essential for low-power design and for sizing decoupling networks and regulators in fast digital systems. Given Data / Assumptions: CMOS gates have parasitic input and interconnect capacitances. Each logic transition charges or discharges these capacitances. Supply current spikes occur on transitions, not during static states. Concept / Approach: Each transition moves charge Q = C * V between the rails. The energy drawn per transition is approximately E = C * V^2. With f transitions per second (scaled by toggle probability and activity factor), average dynamic power is P ≈ C * V^2 * f * alpha. Therefore, higher f linearly increases dynamic power for a given voltage and capacitance budget. Step-by-Step Solution: Recognize charging/discharging of gate capacitance on each edge.Compute energy per transition proportional to C * V^2.Multiply by transitions per second (frequency * activity) → power scales with f.Conclude that higher frequency increases dissipation due to repeated capacitance charging. Verification / Alternative check: Measure supply current vs. clock rate on a CMOS device; current rises roughly linearly with frequency for a fixed activity factor. Why Other Options Are Wrong: A: At low frequency, dynamic component decreases, not increases. B and D: “70.7 % of rated power” is unrelated to CMOS switching power. Common Pitfalls: Ignoring activity factor (alpha) and short-circuit current during transitions, which also contribute but still increase with f. Final Answer: At high frequencies, charging and discharging the gate capacitance will draw a heavy current from the power supply and thus increase power dissipation.

Question 3

Lowest propagation delay among logic families Which logic family is known for the shortest propagation delay (i.e., the fastest switching) among the options below?

Accepted Answer

Introduction / Context: Propagation delay defines how quickly a logic family transitions from input change to output change. For high-speed data paths and clock distribution, selecting a fast family is key. Emitter-Coupled Logic (ECL) is historically recognized for its very low delays due to its non-saturating operation. Given Data / Assumptions: Comparison across common families: CMOS, BiCMOS, ECL, Schottky TTL (74S). Metric: shortest propagation delay. Typical operating conditions assumed (standard supply and load). Concept / Approach: ECL uses differential transistor pairs biased to avoid saturation, maintaining transistors in their active region. This minimizes charge storage delays and yields extremely fast switching, at the cost of higher static power and negative supply usage in some variants. CMOS and BiCMOS can be very fast in modern processes, but classic family comparisons still place ECL as the fastest among these choices. Step-by-Step Solution: List families and basic characteristics.Recall non-saturating nature of ECL → minimal storage delay.Conclude ECL provides the shortest propagation delay.Select ECL from options. Verification / Alternative check: Datasheets for classic ECL (e.g., 10K/100K families) show sub-nanosecond to a few-ns delays, outperforming 74S and general-purpose CMOS of the same era. Why Other Options Are Wrong: CMOS: efficient and scalable but not inherently the fastest among listed families. BiCMOS: blends strengths; still slower than classic ECL in propagation delay. 74SXX: Schottky TTL improves over standard TTL but trails ECL. Common Pitfalls: Confusing modern deep-submicron CMOS ASIC speed (process-dependent) with classic family characteristics. Final Answer: ECL

Question 4

Understanding PMOS and NMOS logic families PMOS and NMOS logic families primarily:

Accepted Answer

Introduction / Context: Before complementary MOS (CMOS) dominated, digital ICs were often implemented using only one transistor polarity: PMOS or NMOS. Knowing this historical and architectural distinction helps in understanding device behavior, speed, and power characteristics compared to CMOS. Given Data / Assumptions: PMOS uses only p-channel MOSFETs in logic gates. NMOS uses only n-channel MOSFETs in logic gates. CMOS uses both p- and n-channel devices in complementary pairs. Concept / Approach: Single-polarity MOS logic (PMOS/NMOS) simplifies fabrication but suffers from static power dissipation (through resistive or depletion loads) and limited noise margins compared to CMOS. CMOS leverages both polarities to reduce static power and improve switching characteristics. The 74HC series refers to high-speed CMOS, not PMOS/NMOS-only logic. Step-by-Step Solution: Identify key difference: single-polarity versus complementary.Match definition: PMOS/NMOS gates are built exclusively from one channel type.Exclude incorrect associations with 74HC (a CMOS family).Select the statement that captures exclusive-channel implementation. Verification / Alternative check: Classic NMOS microprocessors (e.g., early 6502 variants) and PMOS logic families illustrate single-polarity construction. Why Other Options Are Wrong: B: 74HC is CMOS, not PMOS/NMOS-only. C: “positive/negative” and differential-supply claims are unrelated to PMOS/NMOS definitions. D: Not applicable since A is correct. Common Pitfalls: Confusing PMOS/NMOS with CMOS; forgetting that CMOS combines both channel types. Final Answer: represent MOSFET devices utilizing either P-channel or N-channel devices exclusively within a given gate

Question 5

Interfacing TTL outputs to CMOS inputs — solving VOH(min) mismatch If the VOH(min) of a TTL IC is too low to meet the VIH(min) requirement of a CMOS input, what is the usual simple fix?

Accepted Answer

Introduction / Context: Interfacing TTL (5 V) outputs to CMOS inputs can fail when the TTL high-level output voltage (VOH) does not reach the CMOS input high threshold (VIH). A simple passive technique can often fix the DC level without active translators when speeds and loads allow. Given Data / Assumptions: TTL VOH(min) may be around 2.4 V under load. CMOS VIH(min) (at 5 V CMOS) is typically ~0.7 * VCC ≈ 3.5 V. The interface is unidirectional and not extremely high-speed or high fan-out. Concept / Approach: Adding a pull-up resistor from the TTL output to VCC raises the output high level by providing additional current to the output node when the TTL transistor is off. This helps the node reach closer to VCC so the CMOS input sees a valid logic HIGH. Care must be taken regarding rise time and current draw. Step-by-Step Solution: Identify mismatch: TTL VOH(min) < CMOS VIH(min).Add pull-up to VCC to raise VOH toward VCC.Choose resistor value balancing speed (rise time) and current (power).Verify with oscilloscope under worst-case load conditions. Verification / Alternative check: Some systems instead use buffers/level translators; for simple cases, the pull-up is sufficient and documented in interfacing app notes. Why Other Options Are Wrong: A: A divider reduces voltage; it does not raise VOH. B: Not a solution—just avoidance. C: Pull-down makes the issue worse by lowering the high level. Common Pitfalls: Using too strong a pull-up causing excess current when low; using too weak a pull-up causing slow edges and timing failures. Final Answer: adding an external pull-up resistor to VCC

Question 6

Non-saturating logic family — avoiding transistor saturation Which logic family is characterized by operation that prevents transistor saturation during switching?

Accepted Answer

Introduction / Context: Switching speed is heavily influenced by whether devices enter saturation. When transistors saturate, stored charge must be removed before the device turns off, increasing delay. A logic family that avoids saturation can therefore switch faster at the cost of higher power. Given Data / Assumptions: ECL (Emitter-Coupled Logic) uses differential transistor pairs biased in the active region. TTL and IIL are bipolar families that can involve saturation or current-steering differently. MOS is a field-effect technology, not a single “family.” Concept / Approach: ECL keeps BJTs from saturating by using constant current sources and emitter-coupled pairs. Voltages swing over a small range and never drive the transistors into saturation, minimizing storage delay and enabling very short propagation times. TTL variants (even Schottky TTL) reduce but do not fully eliminate saturation effects; IIL is current-mode but distinct in behavior. MOS refers broadly to PMOS/NMOS/CMOS and does not answer the saturation-specific question for bipolar BJTs. Step-by-Step Solution: Identify family known for non-saturating operation: ECL.Recall ECL's differential, constant-current design.Conclude ECL avoids saturation and achieves fast switching.Select ECL from options. Verification / Alternative check: Classic 10K/100K ECL datasheets highlight small-swing, non-saturating characteristics as the source of speed. Why Other Options Are Wrong: TTL: Schottky clamps reduce saturation depth but do not fully avoid it. MOS: Not a bipolar family; the question targets bipolar saturation behavior. IIL: Uses current injection; not characterized primarily by “no saturation” like ECL. Common Pitfalls: Assuming “MOS” implies non-saturation; the concept here is about BJT saturation avoidance. Final Answer: ECL

Question 7

Emitter-Coupled Logic (ECL) — Identify the key characteristics Which of the following choices best summarizes the important features of emitter-coupled logic (ECL) families, considering typical operating practice (negative supply rails), voltage swi…

Accepted Answer

Introduction / Context: ECL (Emitter-Coupled Logic) is a bipolar logic family known for very high speed. Unlike saturated logic such as TTL, ECL keeps transistors out of saturation by steering current between differential pairs. This design yields extremely short propagation delays but with trade-offs in voltage swing, noise margin, and power consumption. Given Data / Assumptions: ECL commonly uses a negative supply rail (for example, VEE around –5.2 V) with ground near logic reference. ECL output swing is small (hundreds of millivolts), which reduces storage and charging delays. High speed operation is achieved at the expense of static current draw and thus power. Noise margins are modest because of the small swings. Concept / Approach: The signature of ECL is continuous bias current and differential steering. This minimizes saturation and charge storage, drastically reducing propagation time. However, because the signal swing is intentionally small, the noise margin is also smaller than families with larger swings, and steady bias current implies relatively high power consumption. Step-by-Step Solution: Identify speed attribute → ECL is very fast (short propagation time).Identify voltage swing → small swing by design.Identify noise margin → typically lower than CMOS/TTL because swing is small.Identify supply convention → negative rail operation is common.Identify power → continuous current leads to higher power dissipation.Therefore, the correct summary is “low noise margin, low output voltage swing, negative voltage operation, fast, and high power consumption.” Verification / Alternative check: Datasheets for classic ECL (e.g., 10K/100K families) show typical swings around a few hundred millivolts, VEE negative rails, and propagation delays well below comparable TTL generations. Why Other Options Are Wrong: Option b: Claims low power dissipation and “good” noise immunity, which do not match ECL. Option c: Pairs “low power” with “high swing,” inconsistent with ECL. Option d: Suggests positive supply operation and “low power,” again inconsistent. Common Pitfalls: Confusing “negative logic” with “negative supply.” ECL may use negative rails but logic polarity is independent of rail polarity. Final Answer: low noise margin, low output voltage swing, negative voltage operation, fast, and high power consumption

Question 8

TTL Best Practice — Why a decoupling capacitor is needed and where to connect it In transistor–transistor logic (TTL) circuits, what is the purpose of a local decoupling (bypass) capacitor and where should it be connected for best results?

Accepted Answer

Introduction / Context: Every TTL IC switches current quickly, creating sharp edges that momentarily draw high transient currents. Without local decoupling, these transients can cause ground bounce, supply droop, and unintended logic errors. Decoupling capacitors provide a low-impedance local energy reservoir near each IC. Given Data / Assumptions: IC supply is typically VCC = +5 V with ground reference. Fast edges cause di/dt and voltage spikes on supply traces. A small ceramic capacitor (for example 0.1 µF) is physically placed close to the VCC and GND pins. Concept / Approach: The decoupling capacitor shunts high-frequency noise to ground and locally supplies transient current, stabilizing the IC’s supply voltage. The correct connection is directly between the power pin and ground pin, with the shortest possible loop (minimal lead/trace inductance). Step-by-Step Solution: Identify noise source → switching currents and trace inductance.Provide local energy → place a capacitor near the IC.Define connection → from VCC to GND at the IC pins.Result → reduced supply ripple, fewer false toggles, improved signal integrity. Verification / Alternative check: Scope measurements with and without local 0.1 µF ceramics show visibly reduced supply ringing and ground bounce at the IC; adding bulk capacitance (for example 10 µF per board region) further helps low-frequency transients. Why Other Options Are Wrong: a, b: Connecting to inputs does not address supply transients and can even degrade signal integrity. d: Not applicable; the meaningful, standard practice is VCC-to-GND decoupling. Common Pitfalls: Using long traces to the capacitor or omitting one per IC; both undermine the capacitor’s ability to handle high-frequency spikes. Final Answer: to reduce the effects of noise, connect between power supply and ground

Question 9

Open-Collector Outputs — Source vs. sink capability Complete the statement correctly: An open-collector output can ________ current, but it cannot ________.

Accepted Answer

Introduction / Context: Open-collector (OC) outputs are widely used for wired-AND interconnections, level shifting, and driving loads that require voltages different from the logic family’s supply. Understanding what an OC stage can and cannot do is essential for correct pull-up selection and load interfacing. Given Data / Assumptions: An OC output is the collector of an NPN transistor left “open” (no internal pull-up). The external circuit must provide a pull-up resistor to a suitable positive supply for a logic HIGH level. The transistor can pull the output LOW by conducting to ground (sinking current). Concept / Approach: An OC stage works by sinking current through the transistor to ground when ON. When the transistor is OFF, it does not source current; the pull-up resistor sources current to raise the output HIGH. Thus, OC outputs can sink current but cannot source it. Step-by-Step Solution: Transistor ON → current flows from pull-up through load into collector → to emitter (ground): sink operation.Transistor OFF → output is “released”; pull-up resistor provides the sourcing path, not the IC.Therefore, fill blanks as “sink, source current.” Verification / Alternative check: Datasheets specify IOL (sink current) ratings for OC outputs and do not give IOH (source) ratings; the HIGH level depends on the external pull-up. Why Other Options Are Wrong: b: Reverses the roles; OC cannot source. c, d: Phrase “source/sink voltage” is incorrect usage; devices source or sink current. Common Pitfalls: Choosing too weak a pull-up (slow rise, susceptibility to noise) or too strong (excessive sink current and power when LOW). Final Answer: sink, source current

Question 10

TTL Output Drive — Compare sourcing at HIGH vs. sinking at LOW Evaluate the statement: “The TTL HIGH-level source current is higher than the LOW-level sinking current.” Is it true or false in standard TTL practice?

Accepted Answer

Introduction / Context: TTL totem-pole outputs are asymmetric: they typically can sink substantially more current when LOW than they can source when HIGH. Knowing this asymmetry guides correct fan-out calculations and pull-up/pull-down choices. Given Data / Assumptions: Standard TTL logic HIGH source current capability is modest (for example, around –0.4 mA). Standard TTL logic LOW sink current capability is much larger (for example, around 16 mA for LS-TTL families). Exact numbers vary by subfamily, but the asymmetry persists. Concept / Approach: Compare typical IOH (source at HIGH) to IOL (sink at LOW). Since IOH is much smaller in magnitude than IOL, the statement claiming HIGH-level source current is higher is incorrect. Step-by-Step Solution: Identify IOH (small) vs. IOL (large).Evaluate statement: “HIGH-level source current higher than LOW sink” → contradicts specs.Therefore, the statement is False. Verification / Alternative check: Check a 74LSxx datasheet: IOH around –0.4 mA; IOL around 8–16 mA. The asymmetry is clear and consistent. Why Other Options Are Wrong: “True” would only apply to a hypothetical family with symmetric or reversed capabilities, not standard TTL. Common Pitfalls: Designers sometimes assume symmetric drive strength; ensure loads are oriented so the logic level that sinks current handles the heavier load when possible. Final Answer: False

Question 11

Common TTL Series Identification — Which family code is most prevalent? Among the listed options, select the designation that refers to the classic, widely used TTL logic IC series.

Accepted Answer

Introduction / Context: The “7400” designation identifies the ubiquitous TTL logic family that defined small-scale and medium-scale integration for decades. Many subfamilies (LS, ALS, S, AS, H, F, etc.) expand on the basic 7400 lineage, each optimizing for speed, power, or drive. Given Data / Assumptions: Standard TTL parts are commonly denoted 74xx for commercial temperature ranges (and 54xx for military). Examples: 7400 (quad NAND), 74138 (3-to-8 decoder), 7474 (dual D flip-flop). Other strings in the options are not family designations. Concept / Approach: Recognize that “7400” is a canonical family marker for TTL logic ICs. Terms like “E-MOSFET” describe device physics, not a TTL series. “AC00” refers to an advanced CMOS (74AC) variant—not TTL. “quad” describes a package function count, not a series. Step-by-Step Solution: Identify a family indicator among options.Match to standard TTL naming → “7400.”Exclude descriptive or CMOS-family terms. Verification / Alternative check: Datasheets and catalogs list 74xx series parts as TTL; 74HC/AC families are CMOS with TTL-compatible inputs but are distinct from original TTL. Why Other Options Are Wrong: E-MOSFET: Device type, not a TTL family. quad: Merely indicates four gates/units; not a series. AC00: Part of 74AC CMOS family, not TTL. Common Pitfalls: Assuming all “74xx” parts are TTL; some modern “74xx” are CMOS (HC/HCT/AC/LCX). Check family suffixes. Final Answer: 7400

Question 12

Pulse Edges — Standard definitions of rise time (tr) and fall time (tf) Fill in the standard reference levels used to define a pulse’s rise and fall times: tr is measured from ________ to ________; tf is measured from ________ to ________.

Accepted Answer

Introduction / Context: Rise and fall times provide standardized measures of how quickly a digital signal transitions. The industry-standard convention uses percentage thresholds of the final amplitude to avoid ambiguity caused by ringing and overshoot. Given Data / Assumptions: We assume a monotonic transition between well-defined LOW and HIGH levels. Measurement thresholds are taken as fractions of the steady-state amplitude. Oscilloscope rise/fall measurements typically default to these standard percentages. Concept / Approach: By convention, rise time tr is the time required for a waveform to transition from 10% to 90% of its final amplitude; fall time tf is measured from 90% down to 10%. This symmetric convention captures the main transition interval and minimizes sensitivity to endpoint aberrations. Step-by-Step Solution: Identify rise time reference → lower threshold 10% to upper threshold 90%.Identify fall time reference → upper threshold 90% to lower threshold 10%.Map to choices → “10%, 90%, 90%, 10%.” Verification / Alternative check: Oscilloscope manuals and digital design texts consistently define tr and tf using 10–90% thresholds. Some RF specs use 20–80% for bandwidth correlation, but that is a different (non-default) standard. Why Other Options Are Wrong: b: Reverses rise/fall definitions. c: 20–80% is a nonstandard alternative, not the common digital default. d: 70.7% is a special amplitude (1/√2) used for bandwidth, not edge timing. Common Pitfalls: Measuring from 0% to 100% leads to inconsistent results due to overshoot, undershoot, and noise at endpoints. Final Answer: 10%, 90%, 90%, 10%

Question 13

Fan-Out in TTL — Define “ten TTL loads per TTL driver” In standard TTL terminology, the phrase “ten TTL loads per TTL driver” refers to which specification?

Accepted Answer

Introduction / Context: Fan-out quantifies how many standard logic inputs a single logic output can reliably drive while still meeting specified voltage thresholds for HIGH and LOW. It is a key system-level parameter for building combinational networks. Given Data / Assumptions: For classic TTL, a “standard load” is defined by input current requirements at the recognized logic thresholds. Fan-out is often different for LOW vs. HIGH due to asymmetric sink/source capabilities. “Ten loads” implies the output can support ten standard inputs without violating VOH/VOL specs. Concept / Approach: Fan-out reflects current-drive capability. A TTL output that can sink the sum of input LOW currents (and source the sum of input HIGH currents) for ten standard inputs has a fan-out of 10. Designers must check both LOW and HIGH cases, using the lower of the two limits as effective fan-out. Step-by-Step Solution: Identify metric → number of input loads per output driver.Ensure compliance → sum of currents must not exceed driver limits.Interpret statement → “ten loads” is fan-out = 10. Verification / Alternative check: Datasheets list IOH/IOL and IIH/IIL values; dividing driver capability by input demand yields the permissible fan-out for each logic level. Why Other Options Are Wrong: noise immunity: A voltage margin metric, not a count. power dissipation: Measured in mW; unrelated to the count of loads. propagation delay: Time measure (ns), not a count. Common Pitfalls: For mixed families, ensure the driven inputs truly match the “standard load” definition; otherwise, the real fan-out may be smaller. Final Answer: fan-out

Question 14

Open-Collector Gates — Purpose of the pull-up resistor Why is an external pull-up resistor required when using an open-collector logic gate output, and what does it provide to the node?

Accepted Answer

Introduction / Context: Open-collector outputs act like a switch to ground but cannot drive a logic HIGH level on their own. An external pull-up resistor is therefore mandatory to create a defined HIGH state when the transistor is OFF. Given Data / Assumptions: OC outputs can only sink current (pull LOW) when ON. When OFF, the output floats unless externally biased. A resistor to a positive rail is used to generate the HIGH logic level. Concept / Approach: The pull-up resistor sources current into the node, raising it to the desired HIGH voltage when the OC transistor is OFF. When the transistor turns ON, it sinks current through the resistor to ground, pulling the node LOW. Step-by-Step Solution: OFF state → no internal source → external pull-up sets node HIGH.ON state → transistor conducts → node pulled LOW by sinking current.Therefore, the resistor provides the HIGH voltage level. Verification / Alternative check: Design equations select Rpull-up to balance rise time and sink current limits: I_sink = (VCC − VOL) / Rpull-up and tr ≈ Rpull-up * Cload. Why Other Options Are Wrong: a, b: VCC and GND pins are supplied directly; the resistor does not power the IC. d: The LOW level is provided by the transistor’s sinking action, not the resistor. Common Pitfalls: Using an excessively weak pull-up (slow edges) or too strong (over-current when LOW). Also ensure voltage compatibility if level-shifting. Final Answer: to provide the HIGH voltage

Question 15

TTL NAND Gate (All Inputs LOW) — Transistor conduction states In a standard TTL NAND gate with a totem-pole output, if all inputs are at LOW level, which ON/OFF combination for the internal transistors (Q1 through Q4) is correct?

Accepted Answer

Introduction / Context: Understanding the internal operation of a TTL NAND gate clarifies why the output is HIGH when any input is LOW. The gate employs a multi-emitter input transistor (Q1), a phase-splitter (Q2), and a totem-pole pair (Q3 upper pull-up, Q4 lower pull-down). Given Data / Assumptions: All inputs are LOW, so at least one emitter-base junction in Q1 is forward-biased to ground. The gate is a conventional TTL NAND with a totem-pole output stage. We are identifying conduction states that produce a logic HIGH output. Concept / Approach: With any input LOW, the internal biasing causes the phase-splitter to turn OFF, which in turn drives the upper transistor ON and the lower transistor OFF in the totem pole. This results in a HIGH logic level at the output. Step-by-Step Solution: All inputs LOW → Q1 conducts (ON) via the grounded emitter path.Q1 conduction biases Q2 (phase-splitter) OFF.With Q2 OFF → drive to Q3/Q4 results in Q3 (pull-up) ON and Q4 (pull-down) OFF.Output node is driven HIGH by Q3; Q4 is non-conducting.Thus, the correct pattern is Q1-ON, Q2-OFF, Q3-ON, Q4-OFF. Verification / Alternative check: Logic truth: NAND with any LOW input → HIGH output. The transistor state sequence above is the canonical internal path producing that HIGH. Why Other Options Are Wrong: b: Q2 cannot be ON with all inputs LOW if the output is HIGH. c: Both Q3 and Q4 ON would create contention; also contradicts standard operation. d: Q4 ON would force a LOW output, contradicting NAND truth for a LOW input. Common Pitfalls: Confusing the roles of Q3/Q4 or assuming both can be ON simultaneously; proper TTL design avoids extended shoot-through by steering control. Final Answer: Q1-ON, Q2-OFF, Q3-ON, Q4-OFF

Question 16

IC Power Dissipation from Supply Current — Average ICC method An IC operates from VCC = +5 V with ICC in HIGH state (ICCH) = 10 mA and ICC in LOW state (ICCL) = 23 mA. Assuming a 50% duty cycle, what is the chip’s average power dissipation?

Accepted Answer

Introduction / Context: Digital ICs often specify supply current in two states: when outputs are HIGH (ICCH) and when outputs are LOW (ICCL). To estimate average power, designers compute the duty-cycle-weighted average current and multiply by the supply voltage. Given Data / Assumptions: VCC = 5 V (constant supply). ICCH = 10 mA (average current when outputs are HIGH). ICCL = 23 mA (average current when outputs are LOW). Outputs spend equal time HIGH and LOW (50% duty). Concept / Approach: Average current for a two-state system with equal time in each state is: I_avg = (ICCH + ICCL) / 2. The average power is then P_avg = VCC * I_avg. Step-by-Step Solution: Compute I_avg = (10 mA + 23 mA) / 2 = 33 mA / 2 = 16.5 mA.Compute P_avg = 5 V * 16.5 mA = 82.5 mW.Thus, the average power dissipation is 82.5 mW. Verification / Alternative check: If duty cycle were not 50%, use I_avg = D * ICCH + (1 − D) * ICCL. For D = 0.5, the calculation above is confirmed. Why Other Options Are Wrong: 50 mW: Corresponds to I_avg = 10 mA, ignoring the higher ICCL contribution. 115 mW: Would imply I_avg = 23 mA; that assumes 100% LOW time. 165 mW: Would require 33 mA average at 5 V, which is the sum, not the average. Common Pitfalls: Confusing “sum” with “average,” or forgetting to multiply by VCC to convert current to power. Ensure units are consistent: volts * amperes = watts. Final Answer: 82.5 mW

Question 17

Fan-out calculation for logic families In digital design, fan-out is determined by calculating how many standard inputs an output can drive in both the HIGH and LOW states, and then taking which result and from which ratios?

Accepted Answer

Introduction / Context: Fan-out tells you the maximum number of identical logic inputs that a single logic output can drive without violating guaranteed logic-level specifications. It is a fundamental specification for interfacing devices across TTL, CMOS, and mixed-logic systems. Correctly computing fan-out prevents marginal logic levels, excessive current draw, and reliability issues in production designs. Given Data / Assumptions: Definitions: IOH = output current available in the HIGH state, IOL = output current available in the LOW state. IIH = input current required by one input when HIGH; IIL = input current required by one input when LOW. Logic family input thresholds and currents are per datasheet “worst-case” values. Concept / Approach: You must satisfy output drive requirements for both logic states. Compute two separate limits: HIGH-state fan-out = IOH / IIH and LOW-state fan-out = IOL / IIL. The system must meet both simultaneously, so the overall fan-out is the smaller (i.e., the more restrictive) of the two values. Using worst-case currents ensures sufficient margin under temperature, voltage, and process variations. Step-by-Step Solution: Compute HIGH-state limit: N_H = IOH / IIH.Compute LOW-state limit: N_L = IOL / IIL.Overall fan-out N = min(N_H, N_L) (take the smaller result).Round down to the nearest whole input count for safe design. Verification / Alternative check: Check with datasheet example values; the calculated minimum of the two ratios will match the manufacturer’s specified fan-out figures for the family standard load. Why Other Options Are Wrong: Taking the larger ratio or the average can overestimate capability and cause failures. Using voltage ratios (VOH/VIH, VOL/VIL) ignores current loading limits. Summing ratios has no physical meaning for drive capacity. Common Pitfalls: Using typical instead of worst-case currents; ignoring that IIH in CMOS is tiny but VIH may be higher, requiring level considerations; forgetting temperature derating. Final Answer: The smaller of (IOH / IIH) and (IOL / IIL)

Question 18

TTL-to-CMOS interfacing — reason for a pull-up resistor When connecting a TTL output to a CMOS input (same nominal VCC), why is a pull-up resistor commonly added on the TTL output line?

Accepted Answer

Introduction / Context: TTL and CMOS families use different guaranteed logic-level thresholds. Standard TTL guarantees VOH(min) around 2.4 V at load, whereas many CMOS families (at 5 V) require VIH(min) near 0.7 * VCC ≈ 3.5 V. Without proper level conditioning, a TTL HIGH might not be recognized as a valid HIGH by CMOS inputs, risking indeterminate logic states. Given Data / Assumptions: TTL VOH(min) is lower than typical CMOS VIH(min). Systems share the same 5 V supply in this scenario. Pull-up resistor connects the TTL output to VCC, assisting the HIGH level. Concept / Approach: A pull-up resistor sources additional current so the line rises closer to VCC when the TTL output goes HIGH (especially if the TTL device is an open-collector or has limited sourcing). This ensures the node exceeds the CMOS VIH(min) level reliably. When the TTL output is LOW, it sinks current through the pull-up to maintain VOL constraints, which the TTL output can handle by design if the resistor value is chosen appropriately. Step-by-Step Solution: Identify mismatch: VOH(min)_TTL < VIH(min)_CMOS.Add pull-up so node charges toward VCC, raising the HIGH level.Choose resistor to satisfy both VOL(max) (sink current) and rise-time needs (RC with input capacitance).Verify margins against datasheet limits and worst-case conditions. Verification / Alternative check: Use a level-shifter or TTL-compatible CMOS (HCT) as an alternative; both aim to ensure VIH thresholds are met without relying solely on a resistor. Why Other Options Are Wrong: Changing LOW level is not the goal; the problem is ensuring a sufficiently high HIGH level. Reducing CMOS input capacitance is unrelated to a pull-up resistor. Common Pitfalls: Using too strong a pull-up (excess current at LOW) or too weak (slow edges); forgetting that HCT inputs accept TTL levels without pull-ups. Final Answer: to increase the output HIGH voltage

Question 19

Naming the HIGH-state output current for a 7400 NAND gate The output current capability of a single 7400 NAND gate in the HIGH state is referred to as:

Accepted Answer

Introduction / Context: Datasheets specify output current capabilities separately for HIGH and LOW states. The conventional symbols are IOH for the HIGH-state output current (positive current sourced by the output) and IOL for the LOW-state output current (current sunk into the output). Knowing these terms is essential when computing fan-out and selecting pull-ups or loads. Given Data / Assumptions: Part family: TTL 7400 NAND gate. HIGH-state current naming convention: IOH. LOW-state current naming convention: IOL. Concept / Approach: When the output is HIGH, a TTL gate can typically source only a small current (IOH), while it can sink a larger current when LOW (IOL). This asymmetry impacts interfacing and pull-up sizing in mixed-technology systems. Step-by-Step Solution: Identify state: output HIGH.Recall symbol: IOH denotes HIGH-state output current.Select term accordingly: IOH. Verification / Alternative check: Consult any standard TTL datasheet: the tables list VOH with IOH and VOL with IOL constraints. Why Other Options Are Wrong: “Source current” is descriptive but not the exact symbol; “sink current”/IOL correspond to the LOW state. “Source current of IOH” is redundant phrasing; the standard symbol alone is correct. Common Pitfalls: Confusing IOH and IOL; assuming symmetric sourcing and sinking abilities; overlooking worst-case values for fan-out calculations. Final Answer: IOH

Question 20

Best practice for unused TTL gate inputs What should be done with unused inputs on TTL logic gates to ensure reliable operation and prevent noise or extra power draw?

Accepted Answer

Introduction / Context: Floating TTL inputs are susceptible to noise pickup and may bias internal transistors in their linear region, increasing power consumption and causing spurious switching. Properly terminating unused inputs ensures defined logic levels and improves noise immunity and reliability in the field. Given Data / Assumptions: Technology: TTL (not CMOS). TTL inputs source small currents and should not be left floating. Goal: Force a solid logic HIGH for AND/NAND unused inputs and a solid logic LOW for OR/NOR unused inputs. Use modest resistors (around 1 kΩ–10 kΩ) if direct tying is undesirable. Concept / Approach: For functions that are active when inputs are HIGH (e.g., NAND/AND), tying unused inputs HIGH prevents unintended activation; for NOR/OR, tying unused inputs LOW prevents spurious outputs. Although TTL inputs often default “HIGH-ish,” relying on that behavior invites noise and extra current consumption. Step-by-Step Solution: Identify gate family and unused input type.For AND/NAND: tie to VCC (often via ~1 kΩ to limit fault current).For OR/NOR: tie to ground to hold them inactive.Verify that tying does not conflict with any intended future re-use or test points. Verification / Alternative check: Datasheets and logic design guides consistently recommend defined biasing of unused inputs. Some TTL series allow direct ties; using a small resistor adds protection against shorts during rework. Why Other Options Are Wrong: Leaving disconnected or floating is unsafe; tying to an unused output is undefined and may drive illegal states; tying everything to VCC ignores gate function differences. Common Pitfalls: Assuming TTL’s default bias is sufficient; mixing TTL and CMOS rules (CMOS gates should not be left floating either but have different input structures). Final Answer: Tie unused AND/NAND inputs to VCC (via about 1 kΩ); tie unused OR/NOR inputs to ground.

Logic Families and Their Characteristics Questions