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Open-collector TTL output behavior: In transistor–transistor logic (TTL), an open-collector gate uses an output transistor without an internal pull-up resistor. Considering load driving and current direction, which statement best describes an open-collector TTL gate?

Difficulty: Easy

Correct Answer: can sink current but cannot source current

Explanation:


Introduction / Context:
Open-collector TTL outputs are widely used whenever wired-AND connections, level shifting, or driving loads beyond the capability of a standard totem-pole TTL output are needed. Understanding exactly what an open-collector output can and cannot do is essential for correct interfacing and for avoiding damage to components.



Given Data / Assumptions:

  • The output transistor's collector is left unconnected internally (open), hence the name open-collector.
  • No internal pull-up device is provided; the external circuit must supply a pull-up resistor to Vcc or to some other appropriate rail.
  • The output transistor is an NPN device that conducts to ground when turned on.
  • Logic levels are defined by the presence of the external pull-up (logic HIGH) or by the transistor pulling down (logic LOW).


Concept / Approach:
An open-collector TTL output can actively pull the line LOW by sinking current through the NPN transistor to ground. It cannot actively drive the line HIGH, because there is no internal path to Vcc. The line goes HIGH only through the external pull-up. Therefore, the correct functional description centers on “can sink current but cannot source current.”



Step-by-Step Solution:

Identify the available device: an NPN transistor with its collector exposed and emitter at ground.If the transistor is ON, current flows from Vcc through the pull-up resistor, through the transistor to ground: the output is a valid LOW (sinking current).If the transistor is OFF, no sourcing path exists inside the IC; the pull-up resistor raises the node to HIGH.Therefore, the output can sink but not source current.


Verification / Alternative check:
Datasheets show open-collector outputs require a pull-up resistor and specify maximum sink current but no source current rating. Timing diagrams also show longer rising edges due to the RC formed by the pull-up and bus capacitance.



Why Other Options Are Wrong:

can source current but cannot sink current: Reversed; an open-collector device lacks an internal source path.cannot source or sink current: It definitely sinks current when active.can sink more current than a standard TTL gate: Not universally true; sink capability depends on the device rating and resistor value, not an inherent “more than standard” rule.


Common Pitfalls:
Forgetting the external pull-up, miscalculating the pull-up resistor so the rise time is too slow, or attempting to drive a load that requires sourcing current directly from the output pin.


Final Answer:
can sink current but cannot source current

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