TTL totem-pole outputs — intended transistor operation In a TTL totem-pole output stage, the upper and lower transistors are designed to operate such that they are:

Introduction / Context:
Totem-pole outputs are a hallmark of TTL logic, providing active pull-up and active pull-down for fast edges and low output impedance. Safe, efficient operation requires careful timing to avoid destructive current spikes. This question focuses on that key design principle.

Given Data / Assumptions:

• Device family: TTL with a push-pull (totem-pole) output structure.
• Two output transistors: an upper pull-up and a lower pull-down transistor.

Concept / Approach:
In a push-pull stage, if both transistors conduct simultaneously, current would shoot directly from VCC to ground (shoot-through), causing large instantaneous power dissipation and noise. TTL design includes cross-conduction prevention using internal biasing and transition shaping so that only one device conducts strongly at a time.

Step-by-Step Solution:
Consider logic HIGH: pull-up turns on, pull-down turns off.Consider logic LOW: pull-down turns on, pull-up turns off.During transitions, internal circuitry minimizes overlap conduction to prevent shoot-through.

Verification / Alternative check:
Examine TTL datasheets and timing diagrams showing output transistor states; they specify maximum transient currents and internal structures (e.g., emitter followers and phase-splitting stages) engineered to avoid simultaneous saturation of both devices.

Why Other Options Are Wrong:

• Always on together: would create a near short between rails.
• Provide phase splitting: an internal earlier stage may provide this, but not the purpose of the output pair acting together.
• Provide voltage regulation: TTL outputs are logic drivers, not regulators.

Common Pitfalls:

• Assuming brief crossover currents are the design goal; they are minimized, not desired.
• Confusing totem-pole with open-collector outputs which use passive pull-up.

Final Answer:
never on together