Totem-pole TTL output behavior Consider a TTL gate with a totem-pole output stage. The statement claims: “The upper (pull-up) transistor is OFF when the gate output is LOW.”

Introduction / Context: TTL gates often use a totem-pole (push-pull) output stage: an upper transistor sources current to drive a HIGH, and a lower transistor sinks current to drive a LOW. Proper operation avoids both devices conducting heavily at the same time, which would waste power and stress the devices.

Given Data / Assumptions:

• In a LOW output state, the output node is pulled toward ground.
• To achieve a strong LOW, the lower (pull-down) transistor turns ON to sink current.
• To prevent shoot-through, the upper (pull-up) transistor must be OFF (or minimally conducting during transitions) when the output is LOW.

Concept / Approach: The totem-pole arrangement alternately sources or sinks current. Logic design ensures complementary action: pull-up drives HIGH, pull-down drives LOW. When the output is LOW, keeping the pull-up off prevents contention and reduces static power dissipation.

Step-by-Step Solution:

Verification / Alternative check: Timing diagrams show brief crossover currents during transitions, but in steady LOW the upper device is OFF and the lower device holds the node LOW.

Why Other Options Are Wrong: “Incorrect” contradicts standard totem-pole operation. Open-collector outputs lack the upper transistor entirely, making option “True only for open-collector” inapplicable. Floating inputs are unrelated to the output topology’s intended steady state.

Common Pitfalls: Confusing transient overlap during switching with steady-state conditions. The question concerns the steady LOW state.

Final Answer: Correct