In this video, we will find out some of differences between NPN and PNP type transistors and why NPN transistors more often seen in circuits and other important things about transistors. Please stay with me in the rest of this video.
BJT transistors fall into two major categories, NPN type and PNP type. But the main question is “Why NPN transistors are most popular?” or “Why NPN transistors more often seen in circuits?” or “Where should I use NPN and where should I use PNP?” these are very important and confusing questions.
Before answering these questions, we need to know what are transistors used for in circuits! This is good question to start with. Transistors have two main uses in electronic circuits.
No1: Transistors are used for amplifying voltage or current, mostly in analog circuits.
No2: Transistors are used for switching voltage or current, mostly in digital circuits.
Since digital circuits like microcontroller based circuits are more popular these days, we often see the second usage of transistors, I mean using transistors as a switch.
For a switch to work properly, it must be placed between the load and the power supply. No matter the switch is placed in positive side or negative side of the power supply. But if you need to use a transistor as a switch, it is important to place it in correct side.
Where to place the transistor? on positive side or on negative side? This is a question that rises from this content!
which side of power supply is better to place a transistor? Answer of this question is depends on transistor type you are going to use. If your transistor is a PNP type then you have to place it on positive side of the power supply and if your transistor is an NPN type, you have to switch negative or ground side of the load.
With a little study about BJT transistors, you can learn about them and about their 4 operation modes; saturation, forward active, cut off and reverse active modes. When the transistor is to be used as a switch in digital circuits, it must be only in two modes of operation, the cut-off mode which the transistor act like open circuit when switch is off or forward active mode which the transistor acts like short circuit when switch is on. Transition between these two modes is very easy and straight forward in an NPN transistor, because of that, NPN transistors are handier and useful and more seen in nowadays circuits.
You may wonder why switching of an NPN transistor is easier than its PNP twin! Let’s discuss about it.
According to our basic knowledge about BJT transistors, base-emitter voltage in NPN type and emitter-base in PNP type should be more than 0.7volts to turn the transistor on and take it into forward active mode otherwise the transistor will fall into cut-off mode and will turned off. In addition the emitter pin must be connected to power supply and collector pin is always connected to load in both types, NPN or PNP. Other fact that we know about BJT transistors is that NPN is for switching ground and PNP is for switching power supply voltage.
Let’s suppose that the load is a 12 volts DC motor. Because of that, we need a 12volt power supply. Emitter pin of NPN transistor must be connected to ground or zero volt, and emitter of PNP type should be connect to 12 volts.
With these assumptions, if you try to switch the motor using NPN transistor, you simply need 0 and a voltage over than 0.7 volts which is available on every IO pin on any microcontroller. But if you try to switch the motor using PNP transistor, I have bad news for you, because the transistor will turn on with 11.3 volts or less and turn off with base voltage higher than 11.3 volts. You can’t turn the transistor off using microcontroller and of course you can’t stop the DC motor, because MCU pins can provide 0 and 3.3 volts or sometimes 5 volts which all of them are less than 11.3 volts and all of these voltages turn the transistor on. This is main reason that NPN transistors are used more than PNP types.