Statement (I): Switching speed of ECL logic family is very high.
Statement (II): The transistor used in the family remains unsaturated.

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ESE Electronics 2013 Paper 2: Official Paper

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Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)
Statement (I) is true but Statement (II) is false
Statement (I) is false but Statement (II) is true

Answer 1

Emitter-coupled-logic (ECL):

Emitter-coupled-logic (ECL) is a BJT logic family which is generally considered as the fastest logic available.
ECL achieves its high-speed operation by employing a relatively small voltage swing and preventing the transistors from entering the saturation region. (Reduces the storage delay time).
ECL transistor use differential amplifier configuration as shown:

Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)