Output stage comparison: Do TTL totem-pole output stages typically switch faster than open-collector outputs that rely on external pull-up resistors?
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AYes — totem-pole stages provide active pull-up and pull-down for faster edges.
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BNo — open-collector outputs are always faster because of larger pull-up resistors.
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COnly at supply voltages above 12 V.
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DOnly when multiple outputs are wire-ORed together.
Answer
Correct Answer: Yes — totem-pole stages provide active pull-up and pull-down for faster edges.
Explanation
Introduction / Context:TTL outputs appear in two common forms: totem-pole and open-collector. Designers must know which style offers better transition times for high-speed logic and which is suited for bus sharing or level translation.
Given Data / Assumptions:
- Totem-pole outputs include an active pull-up transistor.
- Open-collector outputs require an external resistor to VCC for the HIGH level.
- Standard 5 V TTL logic families.
Concept / Approach:Rise time at an open-collector output is set by the RC time constant of the pull-up resistor and total load capacitance (board traces, inputs). In contrast, a totem-pole actively sources current to charge capacitance rapidly, and actively sinks to discharge, producing faster edges and lower propagation delay.
Step-by-Step Solution:
Model open-collector rise as tr ≈ Rpullup * Cload.Realize totem-pole rise is limited by transistor source current, not a large resistor.Compare delays: totem-pole shows lower tr/tf and lower tp.Verification / Alternative check:Datasheets show significantly shorter rise times for totem-pole outputs than for the same device configured as open-collector with typical pull-ups.
Why Other Options Are Wrong:
- “Open-collector always faster” contradicts RC limitations.
- Voltage threshold (12 V) is irrelevant to TTL.
- Wire-ORing applies to open-collector buses, not to speed advantages.
Common Pitfalls:Choosing too-weak pull-ups on open-collector lines, leading to slow edges and potential signal integrity problems at higher speeds.
Final Answer:Yes — totem-pole outputs switch faster than open-collector outputs.