Modern electrical engineering, in fact, the entire modern world is inextricably linked to the device of transistors. These components also have these two functions and switchs as amplifiers. Although field -effect transistor is currently dominated by the electronics field, the original transistor is bipolar crystal pipes. This device is quickly replaced by the first bipolar transistor to connect the transistor or BJT.
BJT has two basic types: npn and PNP. These letters indicate the arrangement of positive and negative doped semiconductor layers.
NPN and PNP: Why is PNP transistor very important
According to my experience, NPN transistors are more prone to attention than PNP transistors. I thought of a few reasons:
The voltage and current behavior of the NPN transistor (at least in my opinion) is much more intuitive.
When a switch or drive circuit is required, the NPN provides a more direct interface for the digital output signal (such as the control signal generated by the microcontroller).
In fact, NPN is better than PNP in many important aspects. This has led to a special dominant position of NPN, because BJT must compete with MOSFETS. When the BJT team sent NPN to participate in the competition, it is easier to win. The author of the author of the University of California Berkeley in 2009, Hu Zhengming said that due to this, the higher NPN performance and the general preference for MOSFETs, BJT "almost entirely NPN" type "
Therefore, we cannot deny that PNP is not very common, and generally not very popular -but this does not mean that we should ignore them. The rest of the article will discuss the characteristics and applications of PNP.
Charge carrier: electronics and holes
As shown above, the launch pole and the set electrode of the PNP transistor are formed by the P -type doping. This means that most of the charged carriers in PNP are groundless.
This fact seems to have nothing to do with actual projects, because as long as the circuit works, we really don't care what type of charge carrier to use. But it turns out that we cannot simply ignore the problem of the acupuncture point on electronics, because the acupuncture points are "slower" than the electronics. More specifically, their liquidity is low.
The high electronic migration rate makes the NPN transistor has a speed advantage than the PNP transistor. The data cited on the document of the University of California Berkeley indicate that the higher migration rate also leads to higher cross -guidance, and higher cross -guidance means higher small signal gain. However, I am not sure. As far as I know, the migration rate only has a significant impact on the cross -guidance of MOSFET, and has no effect on BJT cross -guidance. If I was wrong, please tell me in the comment area.
Made in NPN and PNP IC
There is a reason for PNP as well as NPN's popularity. It is related to things that many electrical engineers have never needed: the actual process of manufacturing integrated circuits. I have seen various signs that NPN is easier and/or cheaper than PNP, although it is difficult to find detailed (and authoritative) information about this theme.
However, I did find a reliable explanation, which is especially related to Bicmos technology. My old SEDRA and Smith textbooks (microelectronics circuits) said that "most Bicmos technology" cannot produce optimized PNP transistors. IC designers who cooperate with Bicmos obviously have to be satisfied with non -optimized devices, or "complete mediocre" may be a better way to describe them. The book pointed out that β is around 10, and the high -frequency performance is not impressive; in contrast, the β of the BICMOS NPN device is 50 to 100, which can be used for the frequency range of up to up to gigabit.
Realization of PNP transistor
The basic working principle of PNP is the same as NPN, but the opposite is the opposite, and sometimes the circuit configuration is embarrassing.
The current is from the emission pole to the base. In order to positive bias base-launch polar knot, the emission pole must be about 0.6 V.
The current flows out of the collector, and the collector voltage is lower than the transmitting polar voltage.
For NPNS, the total launch is very intuitive, but it is a bit strange for PNPS, because the "common" launch is extremely unscrupulous, but is connected to the power rail.
Application of PNP transistor circuit
My goal here is not to list all circuits that can use PNP transistors. In fact, this is impossible, because PNP can have countless uses, although NPN may be better in many cases. On the contrary, I will focus on several circuits or applications, and I noticed that they are common applications for PNP transistors.
Low pressure differential regulator. Using PNP instead of NPN as the transmission element can significantly reduce the pressure difference of the regulator, but it will also increase static current (see this application notes to learn more information).
Driver application with a load on the ground. The transmission pole of the PNP is connected to the driving voltage, and the other end of the load is connected to the collector.
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