Open-Collector Outputs — Source vs. sink capability Complete the statement correctly: An open-collector output can ________ current, but it cannot ________.

Introduction / Context:
Open-collector (OC) outputs are widely used for wired-AND interconnections, level shifting, and driving loads that require voltages different from the logic family’s supply. Understanding what an OC stage can and cannot do is essential for correct pull-up selection and load interfacing.

Given Data / Assumptions:

• An OC output is the collector of an NPN transistor left “open” (no internal pull-up).
• The external circuit must provide a pull-up resistor to a suitable positive supply for a logic HIGH level.
• The transistor can pull the output LOW by conducting to ground (sinking current).

Concept / Approach:
An OC stage works by sinking current through the transistor to ground when ON. When the transistor is OFF, it does not source current; the pull-up resistor sources current to raise the output HIGH. Thus, OC outputs can sink current but cannot source it.

Step-by-Step Solution:
Transistor ON → current flows from pull-up through load into collector → to emitter (ground): sink operation.Transistor OFF → output is “released”; pull-up resistor provides the sourcing path, not the IC.Therefore, fill blanks as “sink, source current.”

Verification / Alternative check:
Datasheets specify IOL (sink current) ratings for OC outputs and do not give IOH (source) ratings; the HIGH level depends on the external pull-up.

Why Other Options Are Wrong:

• b: Reverses the roles; OC cannot source.
• c, d: Phrase “source/sink voltage” is incorrect usage; devices source or sink current.

Common Pitfalls:
Choosing too weak a pull-up (slow rise, susceptibility to noise) or too strong (excessive sink current and power when LOW).

Final Answer:
sink, source current