Logic family performance comparison: which logic family is generally recognized for the highest switching speed in classic digital design and is therefore used for high-speed applications?

Introduction / Context:
Digital logic families differ in speed, power consumption, and noise margins. Designers select families appropriate to application needs. Historically, emitter-coupled logic (ECL) is known for top-tier speed due to its non-saturating operation.

Given Data / Assumptions:

• We compare classic logic families: DTL, TTL, ECL, and MOS.
• Metric of interest: propagation delay (lower is faster).
• Power and complexity trade-offs are acknowledged but secondary here.

Concept / Approach:
ECL transistors are biased so they do not enter deep saturation. Avoiding saturation minimizes storage delay (the time to remove excess charge), which results in very low propagation delays and high speed.

Step-by-Step Solution:

Verification / Alternative check:
Data books list sub-nanosecond to a few-nanoseconds delays for ECL families, beating typical TTL and early MOS families under similar conditions.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing modern high-speed CMOS (very fast today) with historical “MOS” category in the options; the question targets classic families.

Final Answer:
ECL