Bipolar transistor BJT

Bipolar transistor.

The transistor is probably the most important electronic device known, from inductive detectors normally used in industry to microprocessors used in home computers, in all of them we can find electronic devices composed of one or more transistors.

The transistor was invented in 1947 and came to replace the three-electrode thermionic valve or also called triodium, basically composed of silicon and allows the passage of current through N-type and P-type semiconductor crystals.

How does a transistor work?

The transistor has a current input (can be the emitter – E), a current output (can be the collector – C) and a signal input (the base – B) that when it acts facilitates the transmission of electrons between the emitter and the collector.

In other words, transistor acts as a switch, only current will flow between E and C when acting B.

The three working regions that habe a transistor are: Cut-off, saturation and active.

We call transistor in cut when between collector and emitter does not pass current, for example if we used a transistor to light a light bulb, when we speak of the region of cut means that the light bulb will not pass any current therefore it will be off.

The other option we have is to use the saturation region of the transistor, in this case using the example of the light bulb, the current would be passing through the bulb and it would be on.

The last case would be to use the transistor in its active zone, this means that if we use the example of the light bulb it would not be off or fully on, but we would be in a position between off and on, for example the same result we get when we regulate with a potentiometer the brightness of the bulb.

Depending on the polarization, the active region of the transistor can be: direct active region or reverse active region.

Working modes of a transistor

Normally the transistors work in the direct active region, this region is the one used to amplify the signal that enters through the transistor.

Depending on whether the transistor is PNP or NPN, the passage of the current with respect to the base, can go in different directions:

What is the saturation region?

A transistor is in the saturation region when the electric current flows between E and C as if it were a closed circuit. For example all the current that enters through «C», the collector (), comes out by «E», the emitter ().

How an npn transistor behaves in saturation.

Let’s see how the transistor of the above image works when it is in saturation. The first thing is to know the type, that is, if it is npn or pnp. The one in the image is a BJT model and it is of type npn (to remember that it is npn we can look at the direction of the arrow, it is said to be npn because the arrow n (o) p (enters) n, that is, the arrow does not enter the transistor, but comes out of the transistor).

In the image below we can see the symbol of an npn transistor (to emember which of the two is the transistor npn say that the first two letters are np: does not penetrate) and a transistor pnp (where the first letter is the p of penetrates the arrow on the transistor).

How does the intensity in an npn transistor behave when it is in saturation? For that we have the image below.

In the image above we have a model BJT and type npn. The arrows indicate the direction of the current in this type of transistors. The equation that relates the intensities is:

With this equation we remember Kirchhoff and the expression «The sum of the input currents is equal to the output intensity».

The letter «» is equal to the gain and represents the relation between the intensity of the collector and the intensity of the emitter, it is the gain or the increase of intensity that comes out of the emitter.

Regarding the tension we have:

The relationship between the tensions is defined in the equation:

This equation tells us that the voltage between the emitter and the collector is equal to the voltage between the collector and the base plus the voltage between the base and the emitter.

How a pnp transistor behaves in saturation

A pnp transistor in saturation behaves in the same way as an npn transistor in saturation except for the direction of the electric current and the voltages between emitter, base and collector.

In the image below we see that the arrow in the part of the emitter, enters the transistor, while the collector is located in the bottom.

The electric current moves from the part of the emitter until it exits through the part of the collector, and also through the part of the base.

In the following equations we see how the intensities behave and the relationship that exists between the gain and the intensities.

The difference between the voltages of the terminals (emitter, base and collector) of the transistor is very important since it changes the polarization between the terminals of the transistor.

How does the cutoff region work with npn and pnp?

It is called the cutoff region when there is no voltage at the base of the transistor or the voltage of the base is very small. What happens when the voltage of the base is very small or has no voltage is that electrons or electric current do not flow between the collector and the emitter, then the transistor behaves as if it were an open switch.

The cutoff region tells us that the transistor is not working, the transistor does not let the electric current pass between its emitter and collector terminals.

Example of saturation pnp transistor

Let’s see an example of a transistor to turn on a led.

In this picture you can see a power supply from where the positive pole voltage to an LED which in turn is connected by the emitter, the collector area is connected to the power supply at the negative pole, the connection of the part of the power would be finished now the control part would be missing.

The control part consists of a switch connected to the negative pole of the power supply and from this to the base area of the transistor, in this way each time we act the switch will reach voltage to the base and there will be passage of electrons between the emitter and the collector.

Types of transistors

• Bipolar o bjt transistor, type npn or pnp, with emitter, base, collector.
• Unijunction Transistor o ujt, type channel n and channel p, with emitter, base 1 and base 2.
• junction Field Effect Transistor or jfet, type channel n and channel p, with source, door and drain.
• Field Effect Transistor mosfet, type channel n and channel p, with source, door and drain.
• Darlington Transistor, two transistors one feeding another, with emitter, base and collector.
• Phototransistor, type npn and pnp, with emitter, base (which is light) and collector.