TTL output asymmetry: Is the HIGH-level source current capability of TTL typically smaller than the LOW-level sink current (i.e., TTL sinks more than it sources)?
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ACorrect — TTL can usually sink much more at LOW than it can source at HIGH.
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BIncorrect — TTL sources more current at HIGH than it sinks at LOW.
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CCorrect only for open-collector outputs.
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DIncorrect — TTL source and sink are always equal.
Answer
Correct Answer: Correct — TTL can usually sink much more at LOW than it can source at HIGH.
Explanation
Introduction / Context:TTL outputs are asymmetric. Designers exploit this when selecting pull-ups, driving LEDs, or budgeting fan-out. Knowing which state offers stronger drive prevents logic-level violations and slow edges.
Given Data / Assumptions:
- Standard 74xx/74LS TTL family behavior.
- Typical IOH is small in magnitude; IOL is significantly larger.
- 5 V nominal supply.
Concept / Approach:In TTL, the output transistor network sinks current efficiently at a logical LOW and sources less at a logical HIGH. Datasheets often show IOL in the range of 8–16 mA (or higher for drivers), while IOH is around -0.4 mA. This asymmetry shapes interface design.
Step-by-Step Solution:
Check datasheet: note IOH (source when HIGH) is small in magnitude.Note IOL (sink when LOW) is much larger.Conclude that TTL is better at sinking loads; prefer active-low driving for LEDs or similar loads.Verification / Alternative check:Practical lab tests show LEDs glow brighter with active-low connection through TTL because sinking current is stronger and VOH degrades with heavy sourcing.
Why Other Options Are Wrong:
- “Sources more than sinks” contradicts specifications.
- “Only for open-collector” is incorrect; asymmetry also exists in totem-pole outputs.
- “Always equal” is not true for TTL families.
Common Pitfalls:Overloading a TTL output at HIGH, causing VOH to sag and violating VIH of the next stage.
Final Answer:Correct — TTL typically sinks more current than it can source.