The particularity of open collector amplifier circuit (OC gate) of NPN and PNP: the switching output of saturation amplification.

Open collector is a special application of triode amplifier circuit. The collector of C is set at high level, and the emitter of E is set at low level. When the base of B sends a small signal, the saturated amplified switching signal is output at the collector, which is "1" in digital circuit. Open collector amplifier circuit is sometimes referred to as OC gate.

In the automation industry, "PNP" and "NPN" have been directly expressed as the signal output of PNP open collector amplifier circuit and the output of NPN open collector amplifier circuit.

The common terminal of power supply and signal of PNP open collector is at low level, which is also called common cathode.

The power supply and signal common terminal of NPN open collector are at high level, which is also called common anode.

Due to the difference of signal flow direction and common terminal connection mode, PNP signal and NPN signal should also match the polarity of the corresponding receiving circuit, that is, PNP receiving circuit receives PNP signal and NPN receiving circuit receives NPN signal.

Source type and drain type are the abbreviations of "source amplifier circuit" and "drain amplifier circuit", which are visual relative to the current flow direction of signal amplifier.

The triode open collector circuit sends a small signal, which is amplified to a switching signal to be output at the collector, which is consistent with the sensor sensing a physical signal and then outputting a switching signal. Therefore, many sensors choose this kind of NPN and PNP open collector output type. The pulse output of incremental encoder is also a change of high and low level pulse, so many early incremental encoders also chose this open collector output type of NPN and PNP.