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 swing, speed, noise margins, and power?

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