What can an open-collector output do? Choose the statement that correctly describes the current capability and limitation of an open-collector digital output stage.

Introduction / Context:
Open-collector outputs are ubiquitous for wired-OR logic, level shifting, and driving external loads like LEDs or relays (through appropriate interfaces). Knowing whether they can sink or source current determines how you connect pull-ups and loads.

Given Data / Assumptions:

• An open-collector stage exposes the collector of an NPN transistor (or open-drain of an NMOS) to the pin.
• When the transistor turns ON, it pulls the node toward ground (sinks current).
• When OFF, the node is high-impedance and must be pulled up externally to a defined HIGH level.

Concept / Approach:
Because the internal device only provides a path to ground, it cannot source current up to the node. It can only sink current. A pull-up resistor (or active pull-up) provides the current for the HIGH state; the transistor shunts it to ground for the LOW state. Thus, the correct phrasing is that an open-collector can sink current but cannot source current.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets specify I_OL (sink) but not I_OH sourcing in open-collector mode; application circuits always include a pull-up resistor.

Why Other Options Are Wrong:

Common Pitfalls:
Omitting the pull-up resistor and concluding the output is stuck LOW or undefined; forgetting level shifting limits of the pull-up voltage vs. device ratings.

Final Answer:
sink, source current