PNP transistor: wiring diagram. What is the difference between PNP and NPN transistors?
A PNP transistor is an electronic device, in a certain sense the opposite of an NPN transistor. In this type of transistor design, its PN junctions are opened by reversed polarity voltages with respect to the NPN type. In the symbol of the device arrow, which also defines the output of the emitter, this time points to the inside of the transistor symbol.
The construction diagram of a PNP-type transistor consists of two regions of p-type semiconductor material on both sides of the region of n-type material, as shown in the figure below.
The arrow identifies the emitter and the generally accepted direction of its current ("inwards" for the PNP transistor).
The PNP transistor has very similar characteristics with its NPN-bipolar counterpart, except that the directions of the currents and the polarity of the voltage in it are reversed for any of the three possible switching schemes: with a common base, with a common emitter and with a common collector.
The main differences between the two types of bipolar transistors
The main difference between them is that the holes are the main current carriers for the PNP transistors, NPN transistors have electrons in this quality. Therefore, the polarities of the voltage supplying the transistor are reversed, and its input current flows out of the base. In contrast, for an NPN transistor, a base current flows into it, as shown below in the wiring diagram of both types of devices with a common base and a common emitter.
The principle of operation of a PNP-type transistor is based on using a small (like NPN-type) base current and a negative (unlike NPN-type) base bias voltage to control a much larger emitter-collector current. In other words, for a PNP transistor, the emitter is more positive with respect to the base, as well as with respect to the collector.
Consider the differences of the PNP type in the inclusion scheme with a common base.
Indeed, it can be seen from it that the collector current IC(in the case of an NPN transistor) flows from the positive pole of battery B2, passes through the collector lead, penetrates into it and must then exit through the base lead to return to the negative pole of the battery. In the same way, considering the emitter circuit, you can see how its current from the positive pole of the battery B1 enters the transistor at the base output and then penetrates the emitter.
According to the output base, thus, passes as a collector current ICand so the emitter current IE. Since they circulate in their contours in opposite directions, the resulting base current is equal to their difference and is very small, since ICslightly less than IE. But since the latter is still greater, the direction of flow of the differential current (base current) coincides with IE, and therefore the PNP-type bipolar transistor has a current flowing from the base, while NPN-type has a current flowing in.
Differences of PNP-type on the example of a circuit with a common emitter
In this new scheme, the PN-base-emitter transition is open to the battery voltage B1, and the collector-base transition is shifted in the opposite direction by the voltage of the battery B2. The emitter output is thus common to the base and collector circuits.
The total emitter current is given by the sum of two currents ICand IB; passing on the output of the emitter in one direction. Thus, we haveE= IC+ IB.
In this circuit, the base current IBjust “branched off” from the emitter current IE, also coinciding with it in direction. In this case, the PNP type transistor still has the current I flowing from the baseB, and NPN-type - flowing.
In the third of the known switching circuits of transistors, with a common collector, the situation is exactly the same.Therefore, we do not present it in order to save readers' time and place.
PNP transistor: connecting voltage sources
Voltage source between base and emitter (VBE)connects the negative pole to the base and positive to the emitter, because the work of the PNP transistor occurs when the base is negatively shifted relative to the emitter.
The emitter supply voltage is also positive with respect to the collector (VCE). Thus, in a PNP-type transistor, the emitter output is always more positive with respect to both the base and the collector.
Voltage sources are connected to a PNP transistor, as shown in the figure below.This time the collector is connected to the supply voltage VCCvia load resistor, RLwhich limits the maximum current flowing through the device. Base Voltage VBwhich shifts it in a negative direction relative to the emitter, is fed to it through a resistor RBwhich is again used to limit the maximum base current.
Work PNP transistor cascade
So, in order to cause the base current to flow in a PNP transistor, the base should be more negative than the emitter (the current should leave the base) by about 0.7 volts for a silicon device or 0.3 volts for germanium.The formulas used to calculate the base resistor, base current, or collector current are the same as those used for the equivalent NPN transistor and are shown below.
We see that the fundamental difference between a NPN and a PNP transistor is the correct offset of the pn junctions, since the directions of the currents and the polarity of the voltages in them are always opposite. Thus, for the above scheme: IC= IE- IB, since the current must flow from the base.
As a rule, the PNP transistor can be replaced by NPN in most electronic circuits, the difference is only in the polarity of the voltage and the direction of the current. Such transistors can also be used as switching devices, and an example of a key on a PNP transistor is shown below.
The output characteristics of a PNP-type transistor are very similar to the corresponding curves of an equivalent NPN transistor, except that they are rotated by 180 °, taking into account the reverse polarity of voltages and currents (base and collector currents, PNP transistors are negative). Similarly, to find the working points of a PNP-type transistor, its dynamic load line can be depicted in the third quarter of the Cartesian coordinate system.
Typical characteristics of the PNP transistor 2N3906 are shown in the figure below.
Transistor pairs in the amplifier stages
You may be wondering, what is the reason for using PNP transistors when there are many NPN transistors available that can be used as amplifiers or solid state switches? However, the presence of two different types of transistors - NPN and PNP - gives great advantages in designing power amplifier circuits. Such amplifiers use “complementary”, or “matched” pairs of transistors (representing one PNP transistor and one NPN, connected together, as shown in the figure below) in the output stage.
Two corresponding NPN and PNP transistors with similar characteristics that are identical to each other are called complementary. For example, TIP3055 (NPN-type) and TIP2955 (PNP-type) are a good example of complementary silicon power transistors. They both have a constant current gain of β = IC/ IBA high collector current of around 10A is consistent within 10%, which makes them ideal for motor control devices or robotic applications.
In addition, class B amplifiers use matched pairs of transistors and in their output high-power stages.In them, the NPN transistor conducts only the positive half-wave of the signal, and the PNP transistor only conducts its negative half.
This allows the amplifier to conduct the required power through the loudspeaker in both directions for a given nominal power and impedance. As a result, the output current, which is usually of the order of several amperes, is evenly distributed between two complementary transistors.
Transistor pairs in motor control circuits
They are also used in H-bridge control circuits for reversible DC motors, which allow the current to be regulated through the motor evenly in both directions of its rotation.
The H-bridge circuit above is so called because the basic configuration of its four switches on transistors resembles the letter “H” with the motor located on the transverse line. The transistor H-bridge is probably one of the most commonly used types of control circuit of a reversible DC motor. It uses “complementary” pairs of NPN- and PNP-type transistors in each branch, which act as keys for motor control.
Control input A provides motor operation in one direction, while input B is used for reverse rotation.
For example, when transistor TR1 is on and TR2 is off, input A is connected to the supply voltage (+ Vcc), and if transistor TR3 is turned off and TR4 is on, then input B is connected to 0 volts (GND). Therefore, the motor will rotate in one direction, corresponding to the positive potential of input A and negative input B.
If the key states are changed so that TR1 is turned off, TR2 is on, TR3 is on, and TR4 is off, the motor current will flow in the opposite direction, which will cause its reversal.
Using opposite levels of logic "1" or "0" at the inputs A and B, you can control the direction of rotation of the motor.
Determining the type of transistors
Any bipolar transistors can be represented consisting mainly of two diodes connected together back to back.
We can use this analogy to determine if a transistor is PNP or NPN by testing its resistance between its three terminals. Testing each pair of them in both directions with a multimeter, after six measurements, we get the following result:
1.Emitter - Base.These pins should act like a normal diode and conduct current only in one direction.
2.Collector - Base.These pins should also act as a normal diode and conduct current only in one direction.
3.Emitter - Collector.These findings should not lead in any direction.
The values of the resistance of the transitions of the transistors of both types
|A pair of transistor leads||PNP||NPN|
Then we can determine the PNP transistor as serviceable and closed. A small output current and a negative voltage on its base (B) relative to its emitter (E) will open it and allow a much larger emitter-collector current to flow. PNP transistors are conducted at a positive potential emitter.