![]() When designing a circuit, it’s extremely helpful to have this second type of switching option available. 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. ![]() 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. Adding on to this advantage is the fact that NPN transistors are also easier, and thus cheaper, to manufacture than PNP transistors. This presents a huge advantage in high-speed switching and amplifier circuits applications. This is due to the fact that the “N” substrate can transfer electrons significantly faster than “P” type substrates can transport positive electron holes. There are, in fact, some differences, and in most circuit design applications NPN transistors are preferred. ![]() While PNP and NPN sensors do the same basic job, you might wonder why one would be used over another. Whereas in a PNP configuration, the load is wired between the collector and ground. Also, the load you’re controlling is wired between a positive voltage and the controlling transistor’s collector in an NPN configuration. 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. Above a certain point a transistor reaches saturation, allowing electrons to flow freely. This proportional current flow occurs in an active range, but below a certain cutoff voltage no current flows. NPNsįrom 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. 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. The remaining holes diffuse across the base region, reaching the collector.Difference Between NPN and PNPs Transistors Some of these holes recombine with the electrons injected into the base region. Therefore, holes are attracted from the emitter into the base region. In the case of the pnp transistor, electrons are injected into the n-doped base region. Suppose that the collector potential is lower than the emitter potential and that the base potential is roughly 0.7 V lower than the emitter potential. Most of the remaining electrons are swept across the very thin base region and contribute to the collector current. Since the emitter region is very heavily doped, many more electrons enter the base region than holes, and some of the electrons recombine with holes. As a result, these holes attract electrons from the emitter into the base region across the forward-biased base-emitter junction. When the base-emitter junction is forward-biased, a small current flows into the base, injecting holes into the p-doped base region. In other words, the base-emitter junction is forward-biased whereas the base-collector junction is reverse-biased. Let’s consider the npn transistor where the collector potential is higher than the emitter potential and the base potential is roughly 0.7 V higher than the emitter potential. When each terminal is at the specified voltage, the collector draws a current that is h FE times higher than the current applied to the base. In this way, a bipolar transistor is formed by two back-to-back diodes. To obtain a high current gain, the emitter region is orders of magnitude more heavily doped than the base region. The base region shares two pn junctions, each with collector and emitter. A bipolar transistor (bipolar junction transistor: BJT) consists of the collector, base, and emitter regions, with the very thin base region located between the collector and emitter regions.
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