If you turn it ON, current can flow through it from the collector to the emitter. In the example circuit below, the transistor is OFF. If you had a 0. The base-to-emitter part of a transistor works like a diode. If you add a resistor in series, the rest of the voltage drops across the resistor.
When a current flows from the base to the emitter, the transistor turns on so that a larger current can flow from the collector to the emitter. There is a connection between the sizes of the two currents. This is called the gain of the transistor.
That means that if you have 0. If the battery is 9V, and the base-to-emitter of the transistor grabs 0. R2 is there to limit the current to the LED. You can choose the value you would have chosen if you were to connect the LED and resistor directly to the 9V battery, without the transistor. Check out the video explanation I made on the transistor a few years back forgive the old-school quality :. But there is another one called a PNP transistor that works the same way, just that all the currents are in the opposite direction.
When choosing a transistor, the most important thing to keep in mind is how much current the transistor can support. This is called the Collector Current I C.
In the BJT transistor , the current from base to emitter decides how much current can flow from collector to emitter. In the MOSFET transistor , the voltage between gate and source decides how much current can flow from drain to source. Therefore, if we input a signal to the base pin, the transistor acts as an amplifier.
We could connect a microphone which varies the voltage signal on the base pin, and this will amplify a speaker in the main circuit to form a very basic amplifier. Typically, there is a very small current in the base pin, perhaps just 1 milliamps, or even less. The collector has a much higher current for example milliamps. In this example the collector current is milliamps and the base current is 1milliamps so the ratio is divided by 1 which gives us We can rearrange this formula to find the currents also.
The two transistors look nearly identical so we need to check the part number to tell which is which. With a NPN transistor, we have the main circuit and the control circuit. Both are connected to the positive of the battery.
The main circuit is off, until we press the switch on the control circuit. We can see that current is flowing through both wires to the transistor. We can remove the main circuit and the control circuit LED will still turn on when the switch is pressed as the current is returning to the battery, through the transistor. In this simplified example when the switch is pressed, there are 5 milliamps flowing into the base pin.
There are 20 milliamps flowing into the collector pin and 25 milliamps flowing out of the emitter. The current therefore combines in the transistor. With a PNP transistor, we again have the main circuit and the control circuit. But now, the emitter is connected to the positive of the battery. We can see with this type, that some of the current flows out of the base pin and returns to the battery, the rest of the current flows through the transistor and through the main LED and back to the battery.
If we remove the main circuit, the control circuit led will still turn on. In this example, when the switch is pressed, there are 25 milliamps flowing into the emitter, 20 milliamps flowing out of the collector and 5 milliamps flowing out of the base.
The current therefore divides in the transistor. Transistors are shown on electrical drawings with symbols like these. The arrow is placed on the emitter lead. The arrow points in the direction of conventional current so that we know how to connect them into our circuits. To understand how a transistor works, we want you to first imagine water flowing through a pipe. It flows freely through the pipe, until we block it with a disc. Now, if we connect a smaller pipe into the main one and place a swing gate within this small pipe- We can move the disc using a pulley.
The further the swing gate opens; the more water is allowed to flow in the main pipe. A certain amount of water is required to force the gate to open. The more water we have flowing in this small pipe, the further the valve opens and allows more and more water to flow in the main pipe.
This is essentially how a NPN transistor works. You might already know that when we design electronic circuits, we use conventional current. So in this NPN transistor circuit we assume that the current flows from the batteries positive, into both the collector and base pins and then out of the emitter pin. We always use this direction to design our circuits. In reality the electrons are flowing from the negative to the positive of a battery. This was proved by Joseph Thompson who carried out some experiments to discover the electron and also prove they flowed in the opposite direction.
So, in reality, electrons flow from the negative into the emitter and then out of the collector and the base pins. We call this electron flow. Remember, we always design circuits using the conventional current method. But, scientists and engineers know that electron flow is how it actually works. By the way we have also covered how a battery works in detail in our previous article, do check that out HERE.
Ok, so we know that electricity is the flow of electrons through a wire. The copper wire is the conductor and the rubber is the insulator. If we look at the basic model of an atom for a metal conductor, we have the nucleus at the centre and this is surrounded by a number of orbital shells which hold the electrons. Each shell holds a maximum number of electrons, and an electron needs to have a certain amount of energy to be accepted into each shell. The electrons located furthest away from the nucleus hold the most energy.
The outer most shell is known as the valance shell, a conductor has between 1 and 3 electrons in its valance shell. The electrons are held in place by the nucleus, but there is another shell known as the conduction band. If an electron can reach this, then it can break free from the atom and move to other atoms. With an insulator, the outer most shell is packed. Therefore, electricity cannot flow through this material. Silicon is an example of a semiconductor. But, as the conduction band is quite close, if we provide some external energy, some electrons will gain enough energy to make the jump into the conduction band and become free.
Therefore, this material can act as both an insulator and a conductor. Pure silicon has almost no free electrons, so what engineers do is dope the silicon with a small amount of another material, which changes its electrical properties.
We call this P-Type and N-type doping. We combine these materials to form the P-N junction. Inside the transistor we have the collector pin and the emitter pin. The base wire is connected to the P type layer. Read more about Bipolar Transistor Circuit Design.
The transistor is a three terminal device and consists of three distinct layers. Two of them are doped to give one type of semiconductor and the there is the opposite type, i. They are arranged so that the two similar layers of the transistor sandwich the layer of the opposite type. As a result these semiconductor devices are designated as either PNP transistors or NPN transistors according to the way they are made up. The names for the three electrodes widely used but their meanings are not always understood: Base: The base of the transistor gains its name from the fact that in early transistors, this electrode formed the base for the whole device.
The earliest point contact transistors had two point contacts placed onto the base material. This base material formed the base connection. Emitter: The emitter gains its name from the fact that it emits the charge carriers. Collector: The collector gains its name from the fact that it collects the charge carriers. For the operation of the transistor, it is essential that the base region is very thin.
It is the fact that the base region of the transistor is thin that is the key to the operation of the device. A transistor can be considered as two P-N junctions placed back to back. One of these, namely the base emitter junction is forward biased, whilst the other, the base collector junction is reverse biased.
It is found that when a current is made to flow in the base emitter junction a larger current flows in the collector circuit even though the base collector junction is reverse biased.
For clarity the example of an NPN transistor is taken. The same reasoning can be used for a PNP device, except that holes are the majority carriers instead of electrons. When current flows through the base emitter junction, electrons leave the emitter and flow into the base.
However the doping in this region is kept low and there are comparatively few holes available for recombination. As a result most of the electrons are able to flow right through the base region and on into the collector region, attracted by the positive potential.
Only a small proportion of the electrons from the emitter combine with holes in the base region giving rise to a current in the base-emitter circuit. This means that the collector current is much higher.
For most small signal transistors this may be in the region 50 to
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