Open collector

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.

Open collector

An open collector output processes an IC's output through the base of an internal bipolar junction transistor (BJT), whose collector is exposed as the external output pin. For NPN open collector outputs, the emitter of the NPN transistor is internally connected to ground, so the NPN open collector internally forms either a short-circuit (technically low impedance or "low-Z") connection to the low voltage (which could be ground) when the transistor is switched on, or an open-circuit (technically high impedance or "hi-Z") when the transistor is off. The output is usually connected to an external pull-up resistor, which pulls the output voltage to the resistor's supply voltage when the transistor is off. For PNP open collector outputs, the emitter of the PNP transistor is internally connected to the positive voltage rail, so the collector outputs a high voltage when the transistor is on or is hi-Z when off. This is sometimes called "open collector, drives high".

Open emitter Open emitter output exposes the emitter as the output. For an NPN open emitter output, the collector is connected to the positive voltage rail, so the emitter outputs a high voltage when the transistor is on and is hi-Z when off. For a PNP open emitter output, the collector is connected to the low voltage supply, so the emitter outputs a low voltage when the transistor is on and is hi-Z when off.

Open drain

Open drain output uses MOS transistor (MOSFET) instead of BJTs, and expose the MOSFET's drain as output. An nMOS open drain output connects to ground when a high voltage is applied to the MOSFET's gate, or presents a high impedance when a low voltage is applied to the gate. The voltage in this high impedance state would be floating (undefined) because the MOSFET is not conducting, which is why nMOS open drain outputs require a pull-up resistor connected to a positive voltage rail for producing a high output voltage. Microelectronic devices using nMOS open drain output may provide a 'weak' (high-resistance, often on the order of 100 kΩ) internal pull-up resistor to connect the terminal in question to the positive power supply of the device so their output voltage doesn't float. Such weak pullups reduce power consumption due to their lower V 2 / R {\displaystyle V^{2}/R} ohmic heating and possibly avoid the need for an external pull-up. External pullups may be 'stronger' (lower resistance, perhaps 3 kΩ) to reduce signal rise times (like with I²C) or to minimize noise (like on system RESET inputs). Modern microcontrollers may allow programming particular output pins to use open drain instead of push–pull output, the strength of the internal pull-up, and allow disabling internal pullups when not desired. For pMOS open drain, the output instead connects to the positive power rail when the transistor is on, and is hi-Z when off. This is sometimes called "open drain, drives high".

Open source Open source output exposes the MOSFET's source as the output. For a nMOS open source output, the drain is internally connected to the positive voltage rail, so the source outputs a high voltage when the transistor is on and is hi-Z when off. For a pMOS open source output, the drain is internally connected to the low voltage rail, so the output instead connects to the low voltage rail when the transistor is on, or is hi-Z when off.

Summary of configurations

Configurations that internally connect to a high voltage are source drivers. Configurations that internally connect to a low voltage are sink drivers.

Schematic symbol

Open output is indicated on schematics with these IEEE symbols:

⎐ – NPN open collector or similar output that can supply a relatively low-impedance low voltage when not turned off. Requires external pullup. Capable of positive-logic wired-AND connection. ⎒ – variant with internal pull-up resistor to provide a high voltage when off. ⎏ – NPN open emitter or similar output that can supply a relatively low-impedance high voltage when not turned off. Requires external pulldown. Capable of positive-logic wired-OR connection. ⎑ – variant with an internal pull-down resistor to provide a low voltage when off.

Applications Note: this section primarily deals with npn open collectors, however nMOS open drain generally applies as well.

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