Emitter-Coupled Logic (ECL): Evaluate the claim — “In ECL, the logic HIGH and logic LOW levels are determined by which transistor in the differential amplifier pair conducts more current.”

Introduction / Context:
Emitter-Coupled Logic (ECL) is a high-speed bipolar logic family that avoids transistor saturation by operating a differential amplifier stage. The heart of ECL is a long-tailed pair whose two transistors steer current between two collector load paths. This question tests whether you understand that ECL logic states (HIGH/LOW) arise from which side of the differential pair is conducting more, creating distinct voltage levels at the collectors and at the following emitter-follower outputs.

Given Data / Assumptions:

• An ECL gate core is a differential pair referenced to a temperature-compensated emitter-referenced bias (Vref).
• Only one side (more precisely, predominantly one side) of the pair conducts at a time.
• Collector loads translate current steering into voltage differences, then emitter followers shift and buffer the levels.

Concept / Approach:
ECL works by comparing an input to a reference using a differential pair. If the input exceeds the reference, one transistor draws more current; if it is less, the other side conducts. Because neither side saturates, charge storage is minimal and switching is very fast. The output logic levels are, therefore, a direct consequence of which branch of the differential pair is conducting more current through its load resistor(s).

Step-by-Step Solution:

Verification / Alternative check:
Sketch a basic ECL OR/NOR gate: the differential pair drives two collector nodes with load resistors; the higher-conducting side pulls its collector lower. Emitter followers add level shifting to standard ECL logic levels (negative rail referenced), confirming the mechanism.

Why Other Options Are Wrong:

• Incorrect: Contradicts the fundamental differential-pair operation.
• True only if loads are matched: Matching improves symmetry, but the mechanism still holds with typical tolerances.
• Cryogenic temperature requirement: ECL works over normal ranges; no cryogenic operation is required.
• Only for emitter followers: The emitter follower buffers the result; the decision comes from the differential pair.

Common Pitfalls:
Confusing the emitter-follower buffer with the decision element; assuming saturation-based logic like TTL; overlooking the role of the bias reference Vref in setting switching thresholds.

Final Answer:
Correct