U+2391 Passive-Pull-Down-Output Symbol

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Open collector, open drain, open emitter, and open source refer to integrated circuit (IC) output pin configurations that process the IC's internal function through a transistor with an exposed terminal that is internally unconnected (i.e. "open"). One of the IC's internal high or low voltage rails typically connects to another terminal of that transistor. When the transistor is off, the output is internally disconnected from any internal power rail, a state called "high-impedance" (Hi-Z). Open outputs configurations thus differ from push–pull outputs, which use a pair of transistors to output a specific voltage or current.

These open outputs configurations are often used for digital applications when the transistor acts as a switch, to allow for logic-level conversion, wired-logic connections, and line sharing. External pull-up/down resistors are typically required to set the output during the Hi-Z state to a specific voltage. Analog applications include analog weighting, summing, limiting, and digital-to-analog converters.

The NPN BJT (n-type bipolar junction transistor) and nMOS (n-type metal oxide semiconductor field effect transistor) have greater conductance than their PNP and pMOS relatives, so may be more commonly used for these outputs. Open outputs using PNP and pMOS transistors will use the opposite internal voltage rail used by NPN and nMOS transistors.