Question 1

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

Accepted Answer

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

Question 2

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

Accepted Answer

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

Question 3

Power-supply decoupling uses a radio-frequency capacitor to short out high frequency spikes.

Accepted Answer

True

Question 4

The CMOS series that is pin-compatible with the TTL family is the 4000 series.

Accepted Answer

False

Question 5

The fan-out of CMOS gates is frequency dependent.

Accepted Answer

True

Question 6

The 74XX series TTL operates using saturated switching in which many of the transistors, when conducting, will be in the saturated condition.

Accepted Answer

True

Question 7

The maximum current for a LOW output on a standard TTL gate is 100 µA.

Accepted Answer

False

Question 8

A major drawback in using ECL logic circuits in conjunction with TTL and MOS circuits is its negative supply voltages and logic levels.

Accepted Answer

True

Question 9

Integrated injection logic offers high component density and is easier to fabricate than TTL.

Accepted Answer

True

Question 10

The noise immunity of a logic circuit refers to the circuit's ability to tolerate noise by causing spurious charges in the output voltage.

Accepted Answer

False

Question 11

A current-sourcing transistor may also be referred to as a pull-down transistor.

Accepted Answer

False

Question 12

An unused input of a NAND gate can be left unconnected, pulled high by a pull-up resistor and tied together with another input and not change the logic output.

Accepted Answer

True

Question 13

The major advantage of CMOS logic circuits over TTL is very low power consumption.

Accepted Answer

True

Question 14

The abbreviated designator for a HIGH input voltage is VIH.

Accepted Answer

True

Question 15

CMOS stands for "complementary metal-oxide semiconductors" and the FETs are normally enhancement mode devices.

Accepted Answer

True

Question 16

The TTL HIGH level source current is higher than the LOW level sinking current.

Accepted Answer

False

Question 17

Usually P-MOS and N-MOS circuits are identical with the exception of the voltage polarities.

Accepted Answer

True

Question 18

The time it takes for a square wave to go from 10% to 90% of its voltage level is called propagation delay.

Accepted Answer

False

Question 19

A common means for measuring and comparing the overall performance of an IC family is the speed-power product.

Accepted Answer

True

Question 20

The noise margin for TTL is 0.8 V.

Accepted Answer

False

Integrated-Circuit Logic Families Questions