Totem-pole TTL output behavior: The lower transistor of a totem-pole output stage is driven into saturation when the logic gate's output is in which state?

Introduction / Context:
Classic TTL gates commonly use a totem-pole output stage consisting of a “pull-up” transistor on top and a “pull-down” transistor on the bottom. Understanding which device conducts (and when) is essential for predicting output drive, current flow, and valid logic levels during operation.

Given Data / Assumptions:

• Totem-pole output uses two BJTs arranged vertically between Vcc and ground.
• Only one transistor (ideally) conducts strongly at a time to avoid shoot-through.
• “Saturation” here refers to the pull-down transistor entering a low-voltage, high-current state to produce a firm logic LOW.

Concept / Approach:
When the output must be LOW, the pull-down (lower) transistor switches on hard to sink current from the load to ground. In classic saturated TTL, this device often enters saturation to achieve a low VOL. Conversely, for logic HIGH, the upper transistor sources current while the lower device is turned off. Therefore, the lower transistor is associated with the LOW state, not HIGH.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets and TTL internal schematics show the pull-down path active for LOW with specified IOL and VOL limits. This behavior is consistent across standard TTL families, though Schottky variants reduce deep saturation to improve speed.

Why Other Options Are Wrong:

• Overdriven/Malfunctioning: These are not normal logic states and do not describe the standard operating condition.
• HIGH: For HIGH output, the upper transistor sources current; the lower is off.
• High-impedance: Totem-pole TTL is not three-stated unless a tri-state variant is used.

Common Pitfalls:
Confusing source vs. sink paths; assuming both transistors can be on strongly at once (designs prevent that to avoid shoot-through); ignoring differences between standard TTL and Schottky TTL.

Final Answer:
LOW