Do you know what a PNP transistor is, do you know what it is for, what is its importance within electronics? If your answer is no or if you have any questions about the subject, come and learn a little more with us, in this article we will discuss what a PNP transistor without mysteries is.
What is PNP bipolar transistor
Transistors are considered a kind of switch. That’s right, in their most basic configuration the transistors operate as switches, but they can work in different ways and applications, everything depends on the circuit configuration.
PNP transistors operate with positive – negative – positive values, the PNP transistor has two PN junctions, accommodated facing each other, the negative crystals are in contact with each other, so we have the formation of the bipolar junction transistor (TJB ) PNP.
PNP transistors are used in electronic circuits and are often applied as amplifiers of a voltage signal. There are several types of PNP transistors, each of them varies in application and work characteristics, some are used as a simple switch or amplifier others are used in parallel with the loads that demand a high electric current, preventing the current from traveling through more electronic components sensitive. The terminal responsible for activating the transistor is called the base, the current in the base is proportional to the current circulating in the collector and emitter terminals of an NPN transistor.
To understand the operation of the PNP transistor we need to understand how its terminals are arranged, we can find this arrangement in different ways on the market, but generally the transistors are manufactured with three different terminals or “legs”, which are:
- B = Base, main responsible for the activation of the transistor.
- C = Collector, positive terminal of the transistor.
- E = Emitter, Negative terminal of the transistor.
When a PNP transistor receives a negative signal in relation to the emitter it behaves like a closed switch, thus it allows electrical current to flow from the collector terminal to the emitter terminal, this is the basic operating principle of a PNP transistor, it receives a negative supply voltage signal at the base, and allows the load connected to the emitter to be fed, thus the next component of the circuit is supplied. The current that flows from terminals C to E is proportional to the electric current in the Base, this phenomenon is known as gain of the transistor or β.
What are the main types of transistors
The PNP transistor are not the most common in the market, so it is not easy to find all kinds as with NPN transistors, we can find the PNP transistors generally for the following types:
Bipolar Junction Transistor:
The bipolar junction transistor (TBJ) has in its constructive form three terminals, being the base, the collector and the emitter. Unlike field-effect transistors, TBJs are devices controlled by electrical current, that is, a small current at the base causes a much greater current flow in the emitter. As well as the field effect transistor, bipolar junction transistors in the NPN or PNP type can also be found.
These transistors are the only ones that are activated by the current input in their base. The low impedance (or resistance) allows current to flow through the base of the transistor. Due to this low impedance, these transistors also have the highest amplification of all. Their disadvantage is having low input impedance, and being able to draw significant current from a circuit, thus disturbing the power supply.
Force transistors are widely used in high current circuits, where a lot of energy is being used (current and voltage). The transistor collector is connected to a metal base that acts as a heat sink to dissipate excess energy.
Power transistors can be found in NPN and PNP structures. They are widely used in parallel with more sensitive circuits, in order to allow current to pass through the transistor, relieving the electronic circuit.
Small signal transistors:
Small signal transistors are transistors used primarily to amplify low-level signals, but they can also work well as switches. The small transistors can amplify the input signals. Typical hFE (transistor gain) values for small signal transistors range from 10 to 500, with maximum Ic (collector current) values of about 80 to 600 mA. Can be found under construction NPN and PNP. These transistors are excellent for use as a small signal amplifier.
Small switching transistors are transistors used primarily as switches, but which can also be used as amplifiers. Typical hFE values for small switching transistors range from 10 to 200, with maximum Ic values of around 10 to 1000 mA. Because they amplify the signal with less intensity than other models, they are not widely used for signal amplification, because they have low amplification capacity in relation to small signal transistors. They work as switches and switches, and are found as both NPN and PNP
How to use PNP transistor and dimension the circuit
The first step to apply a PNP transistor in a circuit is to dimension the current that flows between the Collector and Emitter terminals of the transistor, for this we need to know what is the load connected to the E terminal.
For the following example, we will supply a small 5V voltage relay and 150 ohmic resistance coil, so we can dimension the current demanded by the load using the ohms law
We found a minimum current for supplying the relay of 33.33mA, it is always advisable to consult the manufacturer’s manual to check what is the minimum current, suggested by the manufacturer so that it can dock the contacts and trigger the load.
Consulting the data of the relay manufacturer, we realize that it is capable of driving a load with a minimum current of 100mA, so we need to dimension the circuit to meet this requirement.
The second step is to dimension the resistor of the base of the transistor, this resistor is responsible for regulating the current in the base in order to allow the exact current, in the base, it is responsible for controlling the gain at the output of the transistor in order to allow the flow correct current between collector and emitter.
For the exemplified circuit, we dimension the transistor using the manufacturer’s data, choosing a gain transistor of 120, ie β 120. To calculate the current required at the base of the transistor, we must use the following equation: