TTL NAND gate transistor states — all inputs HIGH Using the standard TTL NAND gate schematic (multi-emitter input, phase splitter, totem-pole output), determine the state of each transistor (ON/OFF) when all inputs are HIGH.

Introduction / Context:
A TTL NAND gate comprises a multi-emitter input transistor (Q1), a phase-splitter transistor (Q2), and a totem-pole output pair (upper transistor Q3 and lower transistor Q4). Understanding which devices conduct for different input conditions explains why the NAND truth table is produced and how the gate sources/sinks current.

Given Data / Assumptions:

• All inputs HIGH (valid TTL HIGH on every emitter of Q1).
• Standard TTL totem-pole output stage.
• Conventional operation without faults or loading anomalies.

Concept / Approach:

With all inputs HIGH, the base-emitter junctions of Q1 are reverse-biased, turning Q1 OFF. This biases Q2 ON via its base network. When Q2 conducts, it drives the output stage such that the upper transistor Q3 is OFF and the lower transistor Q4 is ON, pulling the output LOW—consistent with NAND behavior (output LOW only when all inputs are HIGH).

Step-by-Step Solution:

Verification / Alternative check:

Truth table cross-check: NAND outputs LOW only when all inputs are HIGH. The transistor state set that pulls the output actively LOW is Q4 ON with Q3 OFF, which matches the analysis above.

Why Other Options Are Wrong:

Options with Q3 ON at this input condition would force a HIGH output, contradicting NAND truth table.

Options with Q2 OFF do not propagate the necessary bias to the output pair for a LOW state.

Common Pitfalls:

Confusing TTL with CMOS inverter structures; overlooking the role of the phase-splitter; assuming inputs left floating behave as solid HIGH—proper biasing is required.

Final Answer:

Q1 OFF, Q2 ON, Q3 OFF, Q4 ON