PNP VS NPN: Working principle

What are the NPN transistor and PNP transistor?

The triode is the most important device in an electronic circuit, and its main function is current amplification and switching.

There are two types of triodes: germanium tubes and silicon tubes. Each of them has two structural forms, NPN and PNP, but the most commonly used transistors are silicon NPN and PNP. Both of them have the same working principle except for the polarity of the power supply.

The common triodes are 9012, s8550, 9013,?bd140?and s8050. The main function of the triode in the single-chip microcomputer application circuit is the switching function. Among them, 9012 and 8550 are pnp transistors, which can be used universally. Among them, 9013 and 8050 are npn type transistors, which can be used universally.

The NPN transistor consists of two N-type semiconductors and one P-type semiconductor, with the P-type semiconductor in the middle and two N-type semiconductors on both sides.

The PNP-type transistor is a transistor composed of two P-type semiconductors sandwiching an N-type semiconductor, so it is called a PNP-type transistor. It can also be described as a triode where current flows from the emitter E.

The working principle of the triode

The principle of the triode has three working states: cut-off, amplification, and saturation. The enlarged state is mainly used in analog circuits, and the usage and calculation methods are relatively complicated, so we will not use it temporarily. The digital circuit mainly uses the switching characteristics of the triode, and only uses the two states of cut-off and saturation.

What we generally call an ordinary triode is a device with current amplification. Other triodes also extend their functions based on this principle. The structure and symbol of the triode are shown in the figure below.

NPN and PNP are mainly different in the direction of current and the positive and negative voltage.

Here we take the NPN triode as an example to explain its working principle.

It is a device that drives the current Ic flowing through ce with b (base) current Ib, and its working principle is very similar to a controllable valve.

The small blue water flow impulse lever in the thin tube on the left widens the valve of the large water pipe, allowing a larger flow of red water to pass through the valve. When the blue water flow is larger, the red water flow in the large pipe is also larger. If the magnification is 100, then when the small blue water flow is 1 kg/hr, then the big pipe is allowed to flow 100 kg/hr of water. Similarly, when the magnification of the triode is 100 and Ib (base current) is 1mA, a current of 100mA is allowed to pass through Ice. The working principles of the two triodes are summarized as follows: the emitter (e) of the NPN is grounded, the collector (c) is connected to a high level, the base (b) is connected to the control signal, and the current (Ib) of b-e is used to control the current of c-e (Ic ), the potential of the e pole is the lowest, and the potential of the c pole is usually the highest during normal amplification, that is, Vc > Vb > Ve. The triode is turned on, and the current flows from the c pole to the e pole. The emitter (e) of PNP is connected to the high level, the collector (c) is connected to a low level, the base (b) is connected to the control signal, and the current (Ib) of e-b is used to control the current (Ic) of e-c, and the potential of e-pole is the highest, and the c-pole potential is usually the lowest during normal amplification, that is, Vc < Vb < Ve. The triode is turned on, that is, the current flows from the e pole to the c pole.

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2.?How to use triode?

The usage characteristics of the triode, the key point is the voltage between the b pole (base) and the e level (emitter). For PNP, the e pole voltage is only 0.7V higher than the b level. There is smooth conduction between stages. That is, the controlling end is between b and e, and the controlled end is between e and c. Similarly, the turn-on voltage of the NPN transistor is 0.7V higher than the b pole than the e pole. This is the explanation of "the conduction voltage passes along the arrow, and the voltage conducts". Let's take a common control LED circuit as an example to illustrate the working state of cut-off and saturation. As shown in the figure below, the base of the triode is connected to an IO port of the microcontroller through a 10K resistor, assuming it is P1, the emitter is directly connected to the 5V power supply, the collector is connected to an LED, and a 1K current limiter is connected in series The resistor is finally connected to the negative GND of the power supply. If P1 is given a high level 1 by our program, then the base b and the emitter e are both 5V, that is to say, there will be no 0.7V voltage drop from e to b. At this time, the emitter and the collector will also It will not be turned on, so the circuit is disconnected at the triode when viewed vertically. If there is no current passing through, the LED will not light up. If the program gives P1 a low level of 0, then the e pole is still 5V, so there is a voltage difference between e and b, and the transistors e and b are also turned on, and there is about 0.7V between the transistor e and b The voltage drop, then there is a voltage of (5-0.7) V across the resistor R47. [Note] The output high level of the P1 port here is 5V, and the high-level output voltage of the IO port of different single-chip microcomputers is different. The IO output of some single-chip microcomputers is 1.2V, which requires triode amplification to drive LEDs, etc Work.

At this time, the connection between e and c will also be turned on, so the LED itself has a voltage drop of 2V, and the triode itself has a voltage drop of about 0.2V between e and c, which we ignore. Then there will be a voltage drop of about 3V on R41, and it can be calculated that the current of this branch is about 3mA, which can successfully light up the LED. As mentioned earlier, the triode has three states: cut-off, amplification, and saturation. Needless to say, the cut-off is as long as there is no conduction between e and b. We want this triode to be in a saturated state, which is what we call switching characteristics, and a condition must be met. The triode has an amplification factor β. To be in saturation, the b-pole current must be greater than the current value between e and c divided by β. This β can be considered to be about 100 for commonly used triodes. Then we have to calculate the resistance value of R47 above. Just now we calculated that the current between e and c is 3mA, then the minimum current of the b pole is 3mA divided by 100 equals 30uA, about 4.3V voltage will fall on the base resistor, then the maximum value of the base resistor is 4.3V/ 30uA = 143K. As long as the resistance value is smaller than this value, it is fine. Of course, it can’t be too small. Too small will cause the IO port current of the microcontroller to burn out the triode or the microcontroller. The maximum theoretical value of the input current of the IO port is 25mA. I recommend not exceeding 6mA. By calculating the voltage and current, the minimum resistance value can be calculated.

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