Functioning of the Transistor

To interpret the schemes it is very important to know in detail the functioning of the transistor. For it it is suitable to see as he behaves in accordance with the base current, which is the principal peculiarity of this electronic device. We will analyze it better by means of images.
In the image we continue with a transistor of type NPN, but it would be the same to do the test with another type of transistor, the PNP, but it would be necessary to do it with the connections invested for this case. In this image there goes us to be of big utility the potenciómetro (P) that is appreciated in the low part and also the miliamperímetro (A) that it will indicate us the value of the current that will circulate along the collector. We make sure that we have been right in the connections, that is to say, the negative of the battery to the crystal N issuing, the positive the collector; and as for the base with his positive connection for being a crystal P. In this image that we saw we have the potenciómetro to zero, so that his high resistance blocks the way of the current to the base and the transistor does not lead current.
When we drive the cursor of the potenciómetro and diminish the resistance of the circuit, as one sees in the following image; passing to an intensity of current (IB) of, for example 0,1 mA, the current happens to feed the base and we observe that the miliamperímetro connected in series with the collector moves his needle and causes a step of current of 10 mA. If we drive the potenciómetro so that the possible current maxim happens, the needle of the miliamperímetro also denounces the increase of the step of collector's current. Then we deduce that the base current, the more intense it is, the more intense it allows to be the current of the collector. Hence we extract an important characteristic of the transistor, and the fact is that it is possible to regulate the current of step along the same one, along the fact of establishing a certain current of base. In the previous example we saw that with a current of 0,1 mA a current can control another current of 10 mA, that is to say, 10/0,1 = 100 times top.
Another condition of the biggest importance to know to know the functioning of the transistor there are the following rules that we have always to consider when it is a question of interpreting his functioning. In these cases:

- There will have to be applied to the issuer a polarity of the same sign as the crystal that constitutes them. If the crystal is of the type P it will have to apply positive polarity to him; and if it is of the type N it will have to apply negative polarity to him.
- There will be applied to him to the base the same way a polarity of the same sign as the crystal that constitutes it. If it is a crystal N negative polarity will be applied to him; and if it is a crystal P it will have to be a print.
- There will be applied to the collector a polarity opposite to the crystal that constitutes it. If it is a crystal P it will have to apply the negative polarity to him; and if it is of crystal N the positive polarity will have to be applied.

These conditions it is necessary to bear them much in mind whenever it has to connect a transistor in a circuit.

 

Keep on reading on transistors:

Principal functions of the Transistor.
Code of designation of Transistors.