Bipolar Junction Transistors come in two different flavors: NPN and PNP. These abbreviations note that they’re formed with either a positively-doped semiconducting material sandwiched between two negatively-doped materials in the case of an NPN transistor, or a negatively doped material sandwiched between two positive layers in the case of PNP devices.
Difference Between NPN and PNPs Transistors
While both have the ability to amplify signals or even act to switch larger currents on and off, they accomplish this signal boosting ability in different ways.
NPNs
From a practical standpoint, NPN transistors (sometimes called "sinking sensors") amplify a positive signal applied to the base by allowing a larger current to flow from the collector to emitter pins of the device, proportional to the base voltage. This proportional current flow occurs in an active range, but below a certain cutoff voltage no current flows. Above a certain point a transistor reaches saturation, allowing electrons to flow freely.
PNPs
PNP transistors (sometimes called "sourcing sensors") performs the same job, but is set up so that a negative voltage must be applied to the base pin to allow current to flow from the emitter to the collector lead. Also, the load you’re controlling is wired between a positive voltage and the controlling transistor’s collector in an NPN configuration. Whereas in a PNP configuration, the load is wired between the collector and ground.
NPN Transistor Applications
While PNP and NPN sensors do the same basic job, you might wonder why one would be used over another. There are, in fact, some differences, and in most circuit design applications NPN transistors are preferred. This is due to the fact that the “N” substrate can transfer electrons significantly faster than “P” type substrates can transport positive electron holes. This presents a huge advantage in high-speed switching and amplifier circuits applications. Adding on to this advantage is the fact that NPN transistors are also easier, and thus cheaper, to manufacture than PNP transistors.
There are, however, certain circuits that benefit from PNP-type transistors, that would be difficult if not impossible to implement without this second type of transistor. One such application is a class B amplifier, where a matched pair of PNP and NPN transistors work in tandem in order to efficiently amplify oscillating signals. When designing a circuit, it’s extremely helpful to have this second type of switching option available.
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PNP Transistor Applications
If you’re just learning about these components, industrial sensors can throw another wrench into one’s understanding of the concept if you’re not careful. As they are normally referred, PNP and NPN sensors are both supplied with positive and negative power leads, then produce a signal to indicate an “on” state. PNP sensors produce a positive output to your industrial controls input, while NPN sensors produce a negative signal during an “on” state. If you learned to use sensors before gaining an understanding of transistors themselves, it would be easy to think that a PNP transistor is controlled with positive voltage.
Of course, things don’t work that way—quite the opposite, actually—as the PNP and NPN sensor designation refers to the type of transistor (or equivalent for more involved devices—some can even be wired in either configuration) used inside of it. The sensed stimulus acts as the base signal, and in the case of a PNP sensor—which is most common in this type of usage—the positive collector output is then connected to a PLC input signal. NPN, or “sinking” output sensors, work in the opposite way, sinking ground voltage to an input when it’s on. The base switching voltage (+ for NPN and – for PNP) is never seen by the controls engineer, making these terms confusing from a fundamental standpoint.
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Conclusion
So while NPN transistors as bare components are more common for the reasons listed earlier, the PNP paradigm has found a home in the industrial control world for at least two reasons. First, while NPN usage might make the most sense to someone with an electrical engineering degree, PNP sensors—where a positive output indicates an “on” state—can be more approachable for the technicians and engineers of other disciplines that often have to work with them. Another reason is that if an output wire from an NPN sensor did fray and somehow grounded, this would read to a controller as an “on” signal, a potential hazard in some situations.
NPN sensors do have their applications in industry, and they are more common in Asian manufacturing than in Europe and North America. As with circuit design, while one style might be more appropriate most of the time, having both NPN and PNP options gives you a certain amount of flexibility controls-wise.
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