Current sourcing vs. “pull-down” terminology: In digital outputs, a current-sourcing transistor is equivalent to a pull-up device, not a pull-down. Assess the correctness of calling a current-sourcing transistor a “pull-down” transistor.
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ACorrect
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BIncorrect
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CValid only in open-drain MOS outputs
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DValid only for wired-AND buses
Answer
Correct Answer: Incorrect
Explanation
Introduction / Context:In logic outputs, “sourcing” and “sinking” identify the direction of current flow when driving a load. Proper terminology helps avoid wiring mistakes and misinterpretation of fan-out and interfacing requirements.
Given Data / Assumptions:
- A current-sourcing device provides current to the load when output is HIGH (typical of a pull-up path).
- A current-sinking device draws current from the load to ground when output is LOW (typical of a pull-down path).
- We consider standard TTL/CMOS totem-pole or complementary output stages.
Concept / Approach:By definition, “pull-up” raises the node toward the positive rail and sources current; “pull-down” lowers the node toward ground and sinks current. Calling a sourcing transistor a “pull-down” reverses the meaning and is incorrect.
Step-by-Step Solution:
Define current sourcing: output provides positive current to the load at logical HIGH.Define current sinking: output conducts to ground at logical LOW.Map to devices: upper device (p-type in CMOS or transistor tied to Vcc in TTL) = pull-up; lower device (n-type or transistor to ground) = pull-down.Conclude the statement is incorrect.Verification / Alternative check:Review standard output stage diagrams: the top transistor is associated with pull-up (sourcing), the bottom with pull-down (sinking).
Why Other Options Are Wrong:
Correct: Contradicts definitions.Valid only in open-drain MOS outputs / wired-AND buses: These cases still preserve sourcing vs. sinking definitions.Common Pitfalls:Mixing up current direction conventions; assuming LED wiring polarity changes sourcing/sinking meaning—it does not.
Final Answer:Incorrect