Logic family identifier — which uses a multiemitter transistor at its input? Select the logic family that is historically characterized by a multiemitter NPN transistor on the input stage of its basic gate.

Introduction / Context:
Different logic families use distinctive input structures that influence speed, power, noise margins, and fan-in behavior. Recognizing these structures helps in understanding legacy circuits and in troubleshooting boards that mix families.

Given Data / Assumptions:

• Transistor–Transistor Logic (TTL) classic NAND gate uses a multiemitter input transistor.
• Emitter-Coupled Logic (ECL) uses differential emitter-coupled pairs.
• CMOS logic uses MOSFET pairs with insulated gates and very high input impedance.

Concept / Approach:
In TTL, the multiemitter NPN replaces individual input diodes of a diode–transistor logic (DTL) gate. Each emitter acts as an input node, enabling compact multi-input NAND functionality while maintaining fast switching with Schottky variants. This multiemitter element is iconic of TTL input stages.

Step-by-Step Reasoning:

Verification / Alternative check:
Block diagrams and transistor-level schematics of 74XX NAND gates show a single NPN transistor with multiple emitters tied to separate input pins, confirming the characteristic.

Why Other Options Are Wrong:

• ECL relies on emitter-coupled differential pairs and resistive loads, not multiemitters.
• CMOS uses MOSFET complementary pairs, not bipolar multiemitters.
• I^2L uses current injection into lateral transistors and does not characterize standard TTL inputs.

Common Pitfalls:

• Assuming modern CMOS-compatible TTL (HCT) changes the internal TTL signature; HCT is CMOS internally, TTL-compatible only at the input thresholds.

Final Answer:
TTL