The principle of semiconductor control of electric current was known at the beginning of the twentieth century. Even though electronics engineers knew how a transistor worked, they continued to design devices based on vacuum tubes. The reason for such distrust of semiconductor triodes was the imperfection of the first point-point transistors. The family of germanium transistors did not have stable characteristics and were highly dependent on temperature conditions.

Monolithic silicon transistors began to seriously compete with vacuum tubes only in the late 50s. Since that time, the electronics industry began to develop rapidly, and compact semiconductor triodes actively replaced energy-intensive lamps from electronic device circuits. With the advent of integrated circuits, where the number of transistors can reach billions, semiconductor electronics has won a landslide victory in the fight to miniaturize devices.

What is a transistor?

In its modern meaning, a transistor is a semiconductor radio element designed to change the parameters of an electric current and control it. A conventional semiconductor triode has three terminals: a base, which receives control signals, an emitter, and a collector. There are also high power composite transistors.

The scale of sizes of semiconductor devices is striking - from several nanometers (unpackaged elements used in microcircuits) to centimeters in diameter for powerful transistors intended for power plants and industrial equipment. Reverse voltages of industrial triodes can reach up to 1000 V.

Device

Structurally, the triode consists of semiconductor layers enclosed in a housing. Semiconductors are materials based on silicon, germanium, gallium arsenide and other chemical elements. Today, research is being conducted to prepare certain types of polymers, and even carbon nanotubes, for the role of semiconductor materials. Apparently in the near future we will learn about new properties of graphene field-effect transistors.

Previously, semiconductor crystals were located in metal cases in the form of caps with three legs. This design was typical for point-point transistors.

Today, the designs of most flat, including silicon semiconductor devices are made on the basis of a single crystal doped in certain parts. They are pressed into plastic, metal-glass or metal-ceramic cases. Some of them have protruding metal plates for heat dissipation, which are attached to the radiators.

The electrodes of modern transistors are arranged in one row. This arrangement of the legs is convenient for automatic board assembly. The terminals are not marked on the housings. The type of electrode is determined from reference books or by measurements.

For transistors, semiconductor crystals with different structures, such as p-n-p or n-p-n, are used. They differ in the polarity of the voltage on the electrodes.

Schematically, the structure of a transistor can be represented as two semiconductor diodes separated by an additional layer. (See Figure 1). It is the presence of this layer that allows you to control the conductivity of the semiconductor triode.

Rice. 1. Structure of transistors

Figure 1 schematically shows the structure of bipolar triodes. There is also a class of field-effect transistors, which will be discussed below.

Basic operating principle

At rest, no current flows between the collector and emitter of a bipolar triode. Electric current is prevented by the resistance of the emitter junction, which arises as a result of the interaction of the layers. To turn on the transistor, you need to apply a small voltage to its base.

Figure 2 shows a diagram explaining the working principle of a triode.

Rice. 2. Operating principle

By controlling the base currents, you can turn the device on and off. If an analog signal is applied to the base, it will change the amplitude of the output currents. In this case, the output signal will exactly repeat the oscillation frequency at the base electrode. In other words, the electrical signal received at the input will be amplified.

Thus, semiconductor triodes can operate in electronic switch mode or in input signal amplification mode.

The operation of the device in electronic key mode can be understood from Figure 3.

Rice. 3. Triode in switch mode

Designation on diagrams

Common designation: "VT" or "Q", followed by a positional index. For example, VT 3. On earlier diagrams you can find outdated designations: “T”, “PP” or “PT”. The transistor is depicted as symbolic lines indicating the corresponding electrodes, circled or not. The direction of current in the emitter is indicated by an arrow.

Figure 4 shows a ULF circuit in which transistors are designated in a new way, and Figure 5 shows schematic images of different types of field-effect transistors.

Rice. 4. Example of a ULF circuit using triodes

Types of transistors

Based on their operating principle and structure, semiconductor triodes are distinguished:

• field;
• bipolar;
• combined.

These transistors perform the same functions, but there are differences in the principle of their operation.

Field

This type of triode is also called unipolar, due to its electrical properties - they carry current of only one polarity. Based on their structure and type of control, these devices are divided into 3 types:

1. Transistors with a control p-n junction (Fig. 6).
2. With an insulated gate (available with a built-in or induced channel).
3. MIS, with structure: metal-dielectric-conductor.

A distinctive feature of an insulated gate is the presence of a dielectric between it and the channel.

Parts are very sensitive to static electricity.

Circuits of field triodes are shown in Figure 5.

Rice. 5. Field effect transistors
Rice. 6. Photo of a real field-effect triode

Pay attention to the names of the electrodes: drain, source and gate.

Field effect transistors consume very little power. They can work for more than a year on a small battery or rechargeable battery. Therefore, they are widely used in modern electronic devices such as remote controls, mobile gadgets, etc.

Bipolar

Much has been said about this type of transistor in the subsection “Basic operating principle.” Let us only note that the device received the name “Bipolar” because of its ability to pass charges of opposite signs through one channel. Their feature is low output impedance.

Transistors amplify signals and act as switching devices. A fairly powerful load can be connected to the collector circuit. Due to the high collector current, the load resistance can be reduced.

Let's look at the structure and principle of operation in more detail below.

Combined

In order to achieve certain electrical parameters from the use of one discrete element, transistor developers invent combined designs. Among them are:

• with embedded resistors and their circuit;
• combinations of two triodes (same or different structures) in one package;
• lambda diodes - a combination of two field-effect triodes forming a section with negative resistance;
• designs in which a field-effect triode with an insulated gate controls a bipolar triode (used to control electric motors).

Combined transistors are, in fact, an elementary microcircuit in one package.

How does a bipolar transistor work? Instructions for dummies

The operation of bipolar transistors is based on the properties of semiconductors and their combinations. To understand the principle of operation of triodes, let's understand the behavior of semiconductors in electrical circuits.

Semiconductors.

Some crystals, such as silicon, germanium, etc., are dielectrics. But they have one feature - if you add certain impurities, they become conductors with special properties.

Some additives (donors) lead to the appearance of free electrons, while others (acceptors) create “holes”.

If, for example, silicon is doped with phosphorus (donor), we obtain a semiconductor with an excess of electrons (n-Si structure). By adding boron (an acceptor), the doped silicon will become a hole-conducting semiconductor (p-Si), that is, its structure will be dominated by positively charged ions.

One-way conduction.

Let's conduct a thought experiment: connect two different types of semiconductors to a power source and supply current to our design. Something unexpected will happen. If you connect the negative wire to an n-type crystal, the circuit will be completed. However, when we reverse the polarity, there will be no electricity in the circuit. Why is this happening?

As a result of connecting crystals with different types of conductivity, a region with a p-n junction is formed between them. Some electrons (charge carriers) from an n-type crystal will flow into a crystal with hole conductivity and recombine holes in the contact zone.

As a result, uncompensated charges arise: in the n-type region - from negative ions, and in the p-type region from positive ions. The potential difference reaches values ​​from 0.3 to 0.6 V.

The relationship between voltage and impurity concentration can be expressed by the formula:

φ= V T*ln( Nn* Np)/n 2 i , where

V T – value of thermodynamic stress, Nn And Np – the concentration of electrons and holes, respectively, and n i denotes the intrinsic concentration.

When connecting a plus to a p-conductor and a minus to an n-type semiconductor, the electric charges will overcome the barrier, since their movement will be directed against the electric field inside the p-n junction. In this case, the transition is open. But if the poles are reversed, the transition will be closed. Hence the conclusion: the p-n junction forms one-way conductivity. This property is used in the design of diodes.

From diode to transistor.

Let's complicate the experiment. Let's add another layer between two semiconductors with the same structures. For example, between p-type silicon wafers we insert a conductivity layer (n-Si). It is not difficult to guess what will happen in the contact zones. By analogy with the process described above, regions with p-n junctions are formed that will block the movement of electrical charges between the emitter and collector, regardless of the polarity of the current.

The most interesting thing will happen when we apply a slight voltage to the layer (base). In our case, we will apply a current with a negative sign. As in the case of a diode, an emitter-base circuit is formed through which current will flow. At the same time, the layer will begin to become saturated with holes, which will lead to hole conduction between the emitter and collector.

Look at Figure 7. It shows that positive ions have filled the entire space of our conditional structure and now nothing interferes with the conduction of current. We have obtained a visual model of a bipolar transistor with a p-n-p structure.

Rice. 7. Principle of operation of the triode

When the base is de-energized, the transistor very quickly returns to its original state and the collector junction closes.

The device can also operate in amplification mode.

The collector current is directly proportional to the base current : ITo= ß* IB , Where ß – current gain, IB – base current.

If you change the value of the control current, the intensity of hole formation on the base will change, which will entail a proportional change in the amplitude of the output voltage, while maintaining the signal frequency. This principle is used to amplify signals.

By applying weak pulses to the base, at the output we get the same amplification frequency, but with a much larger amplitude (set by the voltage applied to the collector-emitter circuit).

NPN transistors work in a similar way. Only the polarity of the voltages changes. Devices with an n-p-n structure have direct conductivity. Transistors of the pnp type have reverse conductivity.

It remains to add that the semiconductor crystal reacts in a similar way to the ultraviolet spectrum of light. By turning the photon flow on and off, or adjusting its intensity, you can control the operation of a triode or change the resistance of a semiconductor resistor.

Bipolar transistor connection circuits

Circuit designers use the following connection schemes: with a common base, common emitter electrodes, and connection with a common collector (Fig. 8).

Rice. 8. Connection diagrams for bipolar transistors

Amplifiers with a common base are characterized by:

• low input impedance, which does not exceed 100 Ohms;
• good temperature properties and frequency characteristics of the triode;
• high permissible voltage;
• two different power sources are required.

Common emitter circuits have:

• high current and voltage gain;
• low power gain;
• inversion of the output voltage relative to the input.

With this connection, one power source is sufficient.

The connection diagram based on the “common collector” principle provides:

• high input and low output resistance;
• low voltage gain factor (< 1).

How does a field effect transistor work? Explanation for dummies

The structure of a field-effect transistor differs from a bipolar one in that the current in it does not cross the p-n junction zone. The charges move through a controlled area called the gate. The gate throughput is controlled by voltage.

The space of the p-n zone decreases or increases under the influence of the electric field (see Fig. 9). The number of free charge carriers changes accordingly - from complete destruction to extreme saturation. As a result of this effect on the gate, the current at the drain electrodes (contacts that output the processed current) is regulated. The incoming current flows through the source contacts.

Figure 9. Field-effect transistor with p-n junction

Field triodes with a built-in and induced channel operate on a similar principle. You saw their diagrams in Figure 5.

Field-effect transistor connection circuits

In practice, connection diagrams are used by analogy with a bipolar triode:

• with a common source - produces a large gain in current and power;
• common gate circuits provide low input impedance and low gain (has limited use);
• common-drain circuits that operate in the same way as common-emitter circuits.

Figure 10 shows various connection schemes.

Rice. 10. Image of field triode connection diagrams

Almost every circuit is capable of operating at very low input voltages.

Videos explaining the principle of operation of the transistor in simple language

Bipolar transistor is a semiconductor device with two interacting R-n-transitions and with three terminals (Fig. 1.15). Depending on the alternation of doped areas, transistors are distinguished n-p-n-type (Fig. 1.15, A) And R-n-r-type (Fig. 1.15, b).

In Fig. 1.15, V, G symbols of transistors are given p-p-p- And R-n-r- types, respectively. The transistor terminals are designated: E– emitter, B– base, TO– collector.

The emitter and collector regions differ in that the concentration of impurities in the emitter region is much higher than in the collector region. The transition that occurs between the emitter and the base is called emitter junction , and the transition that occurs between the collector and the base is collector .

In Fig. Figure 1.16 shows a circuit diagram for connecting a transistor with connected constant voltage sources and a collector resistor. In this circuit, the base terminal of the transistor is connected to the housing. Therefore this scheme is called circuit for connecting a transistor with a common base (CB).

Distinguish four operating modes of the bipolar transistor :

1) active mode – the emitter junction is open and the collector junction is closed (Fig. 1.16);

2) cut-off mode - both R-n- the junctions are closed, and there is no significant current through the transistor.

To obtain this mode, it is necessary to change the polarity of the source in the circuit (see Fig. 1.16) E E to the opposite;

1) saturation mode - two R-n-transistor junctions are open and direct currents flow through them. To obtain this mode, it is necessary to change the polarity of the source in the circuit (see Fig. 1.16) E K to the opposite;

2) inverse mode – the collector junction is open and the emitter junction is closed. To obtain this mode, it is necessary to change the source polarities in the circuit (see Fig. 1.16) to opposite polarities E K And E E.

The active mode of operation is mainly used to amplify and convert signals. The operation of a bipolar transistor in active mode is based on the phenomenon of diffusion, as well as the effect of charge carrier drift in an electric field.

Transistor operation in active mode

Let's consider the operation of a transistor in active mode using the example of a pnp-type transistor (Fig. 1.16). In this mode, the emitter junction of the transistor is open. The opening voltage is E E= 0.4…0.7 V.

Current flows through the open emitter junction i E (i E= 0.1…10 mA for a low-power transistor). As a rule, in the emitter region of the transistor the concentration of acceptor impurities is many times greater than the concentration of donor impurities in the base region. n- transistor area. Therefore, the concentration of holes in the emitter region is much greater than the concentration of electrons in the base region, and almost the entire emitter current is a hole current.

In singles p-n-transition during hole diffusion into P-region, complete recombination of injected holes with electrons occurs P-regions The same process occurs in the emitter junction of the transistor. Thanks to this process, a base current arises i B(see Fig. 1.16). However, more complex processes occur in the transistor.

The main feature of the transistor design is the relative thin base area b. Base width ( W) in a transistor is much less than the free path of holes ( L). In modern silicon transistors W» 1 µm, and the diffusion length L= 5…10 µm. Consequently, the vast majority of holes reach the collector junction without having time to recombine with base electrons. Once in the reverse-biased collector junction, the holes drift (and accelerate) in the existing junction field.

Having passed through the collector junction, the holes recombine with electrons flowing to the collector from the power source ( E K). Note that this hole current is many times greater than the intrinsic reverse current of the closed collector junction and almost completely determines the collector current ( i K) transistor.

From the analysis of the active mode (Fig. 1.16), the equation for transistor currents follows:

In this equation, the base current is much less than the emitter current and collector current, and
The collector current is almost equal to the emitter current of the transistor.

The relationship between the currents in the transistor is characterized by two parameters:

emitter current transfer coefficient

And base current transfer coefficient

Using formula (1.2), we obtain the formula relationship between transmission coefficients :

Coefficient values α And β depend on the design of the transistor. For most low-power transistors used in communications devices and computers, the coefficient b= 20...200, and the coefficient a = 0,95…0,995.

Transistor amplification properties

Let's consider the amplifying properties of the transistor. Let there be a voltage at the input of the transistor E E= 0.5 V. And let this voltage create a current i E= 5 mA. The power consumed to control the transistor is equal to:

R VX= E Ei E= 0.5 × 5 ×10 -3 = 2.5 mW.

Let the payload resistance in the collector circuit of the transistor (Fig. 1.17) be equal to R K= 1 kOhm. A collector current flows through the load resistor, approximately equal to the emitter current of the transistor: iK» i E. The output power released at the load is equal to:

R N =i K 2R K = 25 mW .

Consequently, the circuit (see Fig. 1.17) provides tenfold power amplification. Note that to provide such amplification, it is required that a large blocking voltage be applied to the collector junction:

E K >U K,

Where U K = i K RK– voltage drop across the load resistance in the collector circuit.

The increased output signal energy is provided by the power supply in the collector circuit.

Let's consider other modes of operation of the transistor:

· in mode saturation a forward current of the collector junction arises. Its direction is opposite to the direction of the diffusion current of holes. The resulting collector current decreases sharply, and the amplifying properties of the transistor sharply deteriorate;

Rarely used transistor inverse mode, since the injection properties of the collector are much worse than the injection properties of the emitter;

· V mode cutoffs all currents through the transistor are practically equal to zero - both junctions of the transistor are closed, and the amplifying properties of the transistor do not appear.

In addition to the considered circuit for connecting a transistor with a common base, two other circuits are used:

1) when connected to the transistor emitter body, we get common emitter (CE) circuit (Fig. 1.17). The OE scheme is most often found in practice;

2) when connected to the transistor collector body we get circuit with a common collector (OK) . In these circuits, the control voltage is applied to the base terminal of the transistor.

The dependence of the currents through the terminals of the transistor on the voltages applied to the transistor is called current-voltage characteristics (volt-ampere characteristics) transistor.

For a circuit with a common emitter (Fig. 1.17), the current-voltage characteristics of the transistor look like (Fig. 1.18, 1.19). Similar graphs can be obtained for a scheme with a common base. Curves (see Fig. 1.18) are called input characteristics of the transistor , since they show the dependence of the input current on the control input voltage supplied between the base and emitter of the transistor. The input characteristics of the transistor are close to the characteristics R-n-transition.

The dependence of the input characteristics on the collector voltage is explained by an increase in the width of the collector junction and, consequently, a decrease in the base thickness with an increase in the reverse voltage at the transistor collector (Early effect).

Curves (see Fig. 1.19) are called output characteristics of the transistor . They are used to determine the collector current of the transistor. An increase in the collector current corresponds to an increase in the control voltage at the base of the transistor:

u BE4 > u BE3 > u BE2 > u BE1..

At u FE£ U US(see Fig. 1.19) the voltage at the collector of the transistor becomes less than the voltage at the base. In this case, the collector junction of the transistor opens, and the saturated mode occurs
iation, in which the collector current sharply decreases.

At a high voltage on the collector, the collector current begins to increase, as a process of avalanche (or thermal) breakdown of the collector junction of the transistor occurs.

From the analysis of the current-voltage characteristics of the transistor it follows that the transistor, like the diode, belongs to nonlinear elements. However, in active mode with u FE> U US The collector current of the transistor changes approximately in direct proportion to the increments of the input control voltage at the base of the transistor, i.e. The output circuit of the transistor is close in properties to an ideal controlled current source. The collector current in active mode is practically independent of the load connected to the transistor collector.

In Fig. 1.20 shows the simplest linear equivalent transistor circuit , obtained for the active operating mode when applying small amplitude alternating signals to the transistor ( U m < 0,1 В). Основным элементом этой схемы является источник тока, управляемый входным напряжением:

I K =SU BE,

Where S– transistor transconductance, equal to 10...100 mA/V for low-power transistors.

Resistance r CE characterizes energy losses in the collector circuit. Its value for low-power transistors is tens and hundreds of kilo-ohms. Emitter junction resistance ( r BE) is equal to hundreds of ohms or units of kilo-ohms. This resistance characterizes the energy lost to control the transistor. The values ​​of the parameters of the equivalent circuit can be found by indicating the operating points at the input and output I-V characteristics of the transistor and determining the corresponding derivatives at these operating points (or specifying the increments of the corresponding currents and voltages at the operating points).

Greetings, dear friends! Today we will talk about bipolar transistors and the information will be useful primarily to beginners. So, if you are interested in what a transistor is, its operating principle and in general what it is used for, then take a more comfortable chair and come closer.

Let's continue, and we have content here, it will be more convenient to navigate the article :)

Types of transistors

Transistors are mainly of two types: bipolar transistors and field-effect transistors. Of course, it was possible to consider all types of transistors in one article, but I don’t want to cook porridge in your head. Therefore, in this article we will look exclusively at bipolar transistors, and I will talk about field-effect transistors in one of the following articles. Let's not lump everything together, but pay attention to each one individually.

Bipolar transistor

The bipolar transistor is a descendant of tube triodes, those that were in televisions of the 20th century. Triodes went into oblivion and gave way to more functional brothers - transistors, or rather bipolar transistors.

With rare exceptions, triodes are used in equipment for music lovers.

Bipolar transistors may look like this.

As you can see, bipolar transistors have three terminals and structurally they can look completely different. But on electrical diagrams they look simple and always the same. And all this graphic splendor looks something like this.

This image of transistors is also called UGO (Conventional graphic symbol).

Moreover, bipolar transistors can have different types of conductivity. There are NPN type and PNP type transistors.

The difference between an n-p-n transistor and a p-n-p transistor is only that it is a “carrier” of electric charge (electrons or “holes”). Those. For a pnp transistor, electrons move from the emitter to the collector and are driven by the base. For an n-p-n transistor, electrons go from the collector to the emitter and are controlled by the base. As a result, we come to the conclusion that in order to replace a transistor of one conductivity type with another in a circuit, it is enough to change the polarity of the applied voltage. Or stupidly change the polarity of the power source.

Bipolar transistors have three terminals: collector, emitter and base. I think that it will be difficult to get confused with the UGO, but in a real transistor it’s easier than ever to get confused.

Usually where which output is determined is from the reference book, but you can simply. The terminals of the transistor sound like two diodes connected at a common point (in the area of ​​the base of the transistor).

On the left is a picture for a p-n-p type transistor; when testing, you get the feeling (through multimeter readings) that in front of you are two diodes that are connected at one point by their cathodes. For an n-p-n transistor, the diodes at the base point are connected by their anodes. I think after experimenting with a multimeter it will be more clear.

The principle of operation of a bipolar transistor

Now we will try to figure out how a transistor works. I will not go into details of the internal structure of transistors as this information will only confuse. Better take a look at this drawing.

This image best explains the working principle of a transistor. In this image, a person controls the collector current using a rheostat. He looks at the base current; if the base current increases, then the person also increases the collector current, taking into account the gain of the transistor h21E. If the base current drops, then the collector current will also decrease - the person will correct it using a rheostat.

This analogy has nothing to do with the actual operation of a transistor, but it makes it easier to understand the principles of its operation.

For transistors, rules can be noted to help make things easier to understand. (These rules are taken from the book).

1. The collector has a more positive potential than the emitter
2. As I already said, the base-collector and base-emitter circuits work like diodes
3. Each transistor is characterized by limiting values ​​such as collector current, base current and collector-emitter voltage.
4. If rules 1-3 are followed, then the collector current Ik is directly proportional to the base current Ib. This relationship can be written as a formula.

From this formula we can express the main property of a transistor - a small base current controls a large collector current.

Current gain.

It is also denoted as

Based on the above, the transistor can operate in four modes:

1. Transistor cut-off mode— in this mode the base-emitter junction is closed, this can happen when the base-emitter voltage is insufficient. As a result, there is no base current and therefore there will be no collector current either.
2. Transistor active mode- this is the normal mode of operation of the transistor. In this mode, the base-emitter voltage is sufficient to cause the base-emitter junction to open. The base current is sufficient and the collector current is also available. The collector current is equal to the base current multiplied by the gain.
3. Transistor saturation mode - The transistor switches to this mode when the base current becomes so large that the power of the power source is simply not enough to further increase the collector current. In this mode, the collector current cannot increase following an increase in the base current.
4. Inverse transistor mode— this mode is used extremely rarely. In this mode, the collector and emitter of the transistor are swapped. As a result of such manipulations, the gain of the transistor suffers greatly. The transistor was not originally designed to operate in such a special mode.

To understand how a transistor works, you need to look at specific circuit examples, so let's look at some of them.

Transistor in switch mode

A transistor in switch mode is one of the cases of transistor circuits with a common emitter. The transistor circuit in switching mode is used very often. This transistor circuit is used, for example, when it is necessary to control a powerful load using a microcontroller. The controller leg is not capable of pulling a powerful load, but the transistor can. It turns out that the controller controls the transistor, and the transistor controls a powerful load. Well, first things first.

The main idea of ​​this mode is that the base current controls the collector current. Moreover, the collector current is much greater than the base current. Here you can see with the naked eye that the current signal is amplified. This amplification is carried out using the energy of the power source.

The figure shows a diagram of the operation of a transistor in switching mode.

For transistor circuits, voltages do not play a big role, only currents matter. Therefore, if the ratio of the collector current to the base current is less than the gain of the transistor, then everything is okay.

In this case, even if we have a voltage of 5 volts applied to the base and 500 volts in the collector circuit, then nothing bad will happen, the transistor will obediently switch the high-voltage load.

The main thing is that these voltages do not exceed the limit values ​​for a specific transistor (set in the transistor characteristics).

As far as we know, the current value is a characteristic of the load.

We don't know the resistance of the light bulb, but we know the operating current of the light bulb is 100 mA. In order for the transistor to open and allow such current to flow, you need to select the appropriate base current. We can adjust the base current by changing the value of the base resistor.

Since the minimum value of the transistor gain is 10, then for the transistor to open, the base current must become 10 mA.

The current we need is known. The voltage across the base resistor will be This voltage value across the resistor is due to the fact that 0.6V-0.7V is dropped at the base-emitter junction and we must not forget to take this into account.

As a result, we can easily find the resistance of the resistor

All that remains is to choose a specific value from a number of resistors and it’s done.

Now you probably think that the transistor switch will work as it should? That when the base resistor is connected to +5 V the light bulb lights up, when it is turned off the light bulb goes out? The answer may or may not be yes.

The thing is that there is a small nuance here.

The light bulb will go out when the resistor potential is equal to the ground potential. If the resistor is simply disconnected from the voltage source, then everything is not so simple. The voltage on the base resistor can miraculously arise as a result of interference or some other otherworldly evil spirits :)

To prevent this effect from happening, do the following. Another resistor Rbe is connected between the base and emitter. This resistor is chosen with a value at least 10 times larger than the base resistor Rb (In our case, we took a 4.3 kOhm resistor).

When the base is connected to any voltage, the transistor works as it should, the resistor Rbe does not interfere with it. This resistor consumes only a small portion of the base current.

In the case when voltage is not applied to the base, the base is pulled up to the ground potential, which saves us from all kinds of interference.

So, in principle, we have figured out the operation of the transistor in the key mode, and as you can see, the key mode of operation is a kind of voltage amplification of the signal. After all, we controlled a voltage of 12 V using a low voltage of 5V.

Emitter follower

An emitter follower is a special case of common-collector transistor circuits.

A distinctive feature of a circuit with a common collector from a circuit with a common emitter (option with a transistor switch) is that this circuit does not amplify the voltage signal. What went in through the base came out through the emitter, with the same voltage.

Indeed, let’s say we applied 10 volts to the base, while we know that at the base-emitter junction somewhere around 0.6-0.7V is dropped. It turns out that at the output (at the emitter, at the load Rн) there will be a base voltage of minus 0.6V.

It turned out 9.4V, in a word, almost as much as went in and out. We made sure that this circuit will not increase the voltage for us.

“What is the point then of turning on the transistor like this?” you ask. But it turns out that this scheme has another very important property. The circuit for connecting a transistor with a common collector amplifies the signal in terms of power. Power is the product of current and voltage, but since voltage does not change, power increases only due to current! The load current is the sum of the base current plus the collector current. But if you compare the base current and the collector current, the base current is very small compared to the collector current. It turns out that the load current is equal to the collector current. And the result is this formula.

Now I think it’s clear what the essence of the emitter follower circuit is, but that’s not all.

The emitter follower has another very valuable quality - high input impedance. This means that this transistor circuit consumes almost no input current and creates no load on the signal source circuit.

To understand the principle of operation of a transistor, these two transistor circuits will be quite sufficient. And if you experiment with a soldering iron in your hands, the epiphany simply won’t keep you waiting, because theory is theory, and practice and personal experience are hundreds of times more valuable!

Where can I buy transistors?

Like all other radio components, transistors can be purchased at any nearby radio parts store. If you live somewhere on the outskirts and have not heard of such stores (like I did before), then the last option remains - order transistors from an online store. I myself often order radio components through online stores, because something may simply not be available in a regular offline store.

However, if you are assembling a device purely for yourself, then you can not worry about it, but extract it from the old one, and, so to speak, breathe new life into the old radio component.

Well friends, that’s all for me. I told you everything that I planned today. If you have any questions, then ask them in the comments, if you don’t have any questions, then write comments anyway, your opinion is always important to me. By the way, don’t forget that everyone who leaves a comment for the first time will receive a gift.

Also, be sure to subscribe to new articles, because a lot of interesting and useful things await you further.

I wish you good luck, success and a sunny mood!

From n/a Vladimir Vasiliev

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Depending on the voltage at the terminals, the transistor can be in the following main modes:

• Cut-off mode;
• Active mode;
• Saturation mode.

In addition to these modes, there is also an inverse mode, which is used very rarely.

Cut-off mode

When the voltage between base and emitter is lower than 0.6V - 0.7V, then the p-n junction between base and emitter is closed. In this state, the transistor has virtually no base current. As a result, there will be no collector current either, since there are no free electrons in the base ready to move towards the collector voltage. It turns out that the transistor is locked, and it is said to be in cutoff mode.

Active mode

In the active mode, a voltage sufficient to cause the p-n junction between the base and the emitter to open is applied to the base. Base and collector currents arise. The collector current equals the base current multiplied by the gain. That is, the active mode is the normal operating mode of the transistor, which is used for amplification.

Saturation mode

If you increase the base current, then a moment may come when the collector current stops increasing, because the transistor will open completely, and the current will be determined only by the voltage of the power source and the load resistance in the collector circuit. The transistor reaches saturation. In saturation mode, the collector current will be the maximum that can be provided by the power source at a given load resistance, and will not depend on the base current. In this state, the transistor is not able to amplify the signal, since the collector current does not respond to changes in the base current. In saturation mode, the conductivity of the transistor is maximum, and it is more suitable for the function of a switch (switch) in the “on” state. Similarly, in the cut-off mode, the conductivity of the transistor is minimal, and this corresponds to the switch in the off state. All these modes can be explained using the output characteristics of the transistor.

Let's consider an amplification stage on a transistor connected in a circuit with a common emitter (Fig. 4.14). When the input signal changes, the base current Ib will change. The collector current Ik varies in proportion to the base current:

Iк = β I b. (4.5.1)

Rice. 4.14. Diagram of the amplifier stage (drawing made by the authors)

The change in collector current can be traced by the output characteristics of the transistor (Fig. 4.15). On the abscissa axis we will plot a segment equal to E K - the voltage of the power source of the collector circuit, and on the ordinate axis we will plot a segment corresponding to the maximum possible current in the circuit of this source:

I to max = E to /R to (4.5.2)

Between these points we draw a straight line, which is called the load line and is described by the equation:

I k = (E k - U k e)/R k (4.5.3)

Where U CE is the voltage between the collector and emitter of the transistor; R K - load resistance in the collector circuit.

Rice. 4.15. Operating modes of a bipolar transistor (drawing made by the authors)

From (4.5.3) it follows that

Rk = Ek/Ik max = tanα. (4.5.4)

And, therefore, the slope of the load line is determined by the resistance R K. From Fig. 4.15 it follows that, depending on the base current Ib flowing in the input circuit of the transistor, the operating point of the transistor, which determines its collector current and voltage U CE, will move along the load line from the lowest position (point 1, determined by the intersection of the load line with the output characteristic at I b =0), to point 2, determined by the intersection of the load line with the initial steeply increasing section of the output characteristics.

The zone located between the abscissa axis and the initial output characteristic corresponding to I b = 0 is called the cut-off zone and is characterized by the fact that both transitions of the transistor - emitter and collector - are biased in the opposite direction. The collector current in this case represents the reverse current of the collector junction - I K0, which is very small and therefore almost the entire voltage of the power source E K drops between the emitter and collector of the closed transistor:

U ke ≈ E ke.

And the voltage drop across the load is very small and equal to:

U Rк = I к0 Rк (4.5.5)

They say that in this case the transistor operates in cutoff mode. Since in this mode the current flowing through the load is vanishingly small, and almost the entire voltage of the power source is applied to the closed transistor, then in this mode the transistor can be represented as an open switch.

If we now increase the base current I b, then the operating point will move along the load line until it reaches point 2. The base current corresponding to the characteristic passing through point 2 is called the saturation base current I b us. Here the transistor enters the saturation mode and a further increase in the base current will not lead to an increase in the collector current I K. The zone between the ordinate axis and the sharply changing section of the output characteristics is called the saturation zone. In this case, both junctions of the transistor are forward biased; The collector current reaches its maximum value and is almost equal to the maximum current of the collector power source:

I k max ≈ I to us (4.5.6)

and the voltage between the collector and emitter of an open transistor turns out to be very small. Therefore, in saturation mode, the transistor can be represented as a closed switch.

The intermediate position of the operating point between the cutoff zone and the saturation zone determines the operation of the transistor in amplification mode, and the region where it is located is called the active region. When working in this area, the emitter junction is biased in the forward direction, and the collector junction is biased in the opposite direction (Petrovich V.P., 2008).

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The design and principle of operation of a bipolar transistor

A bipolar transistor is a semiconductor device that has two electron-hole junctions formed in one semiconductor single crystal. These transitions form three regions in the semiconductor with different types of electrical conductivity. One extreme region is called the emitter (E), the other - the collector (K), the middle - the base (B). Metal leads are soldered to each area to connect the transistor to the electrical circuit.
The electrical conductivity of the emitter and collector is opposite to the electrical conductivity of the base. Depending on the order of alternation of p- and n-regions, transistors with p-n-p and n-p-n structures are distinguished. Conventional graphic symbols for p-n-p and n-p-n transistors differ only in the direction of the arrow at the electrode indicating the emitter.

The operating principles of p-n-p and n-p-n transistors are the same, so in the future we will only consider the operation of a transistor with a p-n-p structure.
An electron-hole junction formed by an emitter and a base is called an emitter junction, and a collector and base junction is called a collector junction. The distance between the junctions is very small: for high-frequency transistors it is less than 10 micrometers (1 μm = 0.001 mm), and for low-frequency transistors it does not exceed 50 μm.
When the transistor is operating, its junctions receive external voltages from the power source. Depending on the polarity of these voltages, each junction can be turned on in either the forward or reverse direction. There are three operating modes of the transistor: 1) cutoff mode - both transitions and, accordingly, the transistor are completely closed; 2) saturation mode - the transistor is completely open; 3) active mode - this is a mode intermediate between the first two. The cutoff and saturation modes are used together in key stages, when the transistor is alternately completely open or completely closed with the frequency of the pulses arriving at its base. Cascades operating in switching mode are used in switching circuits (switching power supplies, horizontal scanning output stages of televisions, etc.). The output stages of power amplifiers can operate partially in cutoff mode.
Transistors are most often used in active mode. This mode is determined by applying a small voltage to the base of the transistor, which is called bias voltage (U cm). The transistor opens slightly and current begins to flow through its transitions. The principle of operation of the transistor is based on the fact that a relatively small current flowing through the emitter junction (base current) controls a larger current in the collector circuit. The emitter current is the sum of the base and collector currents.

Operating modes of a bipolar transistor

Cut-off mode transistor is obtained when the emitter and collector p-n junctions are connected to external sources in the opposite direction. In this case, very small reverse emitter currents flow through both pn junctions ( I EBO) And collector ( I KBO). The base current is equal to the sum of these currents and, depending on the type of transistor, ranges from units of microamps - µA (for silicon transistors) to units of milliamps - mA (for germanium transistors).

If the emitter and collector p-n junctions are connected to external sources in the forward direction, the transistor will be in saturation mode . The diffusion electric field of the emitter and collector junctions will be partially weakened by the electric field created by external sources U EB And U KB. As a result, the potential barrier that limited the diffusion of the main charge carriers will decrease, and the penetration (injection) of holes from the emitter and collector into the base will begin, that is, currents called emitter saturation currents will flow through the emitter and collector of the transistor ( I E.us) and collector ( I K.us).

Used to amplify signals active mode of operation of the transistor .
When the transistor is operating in the active mode, its emitter junction is switched on in the forward direction, and the collector junction is switched on in the reverse direction.

Under direct voltage UEB holes are injected from the emitter into the base. Once in the n-type base, holes become minority charge carriers in it and, under the influence of diffusion forces, move (diffuse) to the collector p-n junction. Some of the holes in the base are filled (recombined) with the free electrons present in it. However, the width of the base is small - from several units to 10 microns. Therefore, the main part of the holes reaches the collector p-n junction and is transferred by its electric field to the collector. Obviously, the collector current I K p there cannot be more emitter current, since some of the holes recombine in the base. That's why I K p = h 21B I uh
Magnitude h 21B is called the static transfer coefficient of the emitter current. For modern transistors h 21B= 0.90...0.998. Since the collector junction is switched in the opposite direction (often said - biased in the opposite direction), reverse current also flows through it I BWC , formed by minority carriers of the base (holes) and collector (electrons). Therefore, the total collector current of a transistor connected according to a circuit with a common base

ITo = h 21B I uh +IBWC
Holes that did not reach the collector junction and recombined (filled) in the base give it a positive charge. To restore the electrical neutrality of the base, the same number of electrons is supplied to it from the external circuit. The movement of electrons from the external circuit to the base creates a recombination current in it I B.rec. In addition to the recombination current, the reverse collector current flows through the base in the opposite direction and the full base current
I B = I B.rek - I KBO
In active mode, the base current is tens and hundreds of times less than the collector current and emitter current.

Bipolar transistor connection circuits

In the previous diagram, the electrical circuit formed by the source U EB, emitter and base of the transistor, is called input, and the circuit formed by the source U KB, collector and base of the same transistor, is the output. The base is the common electrode of the transistor for the input and output circuits, therefore such its inclusion is called a circuit with a common base, or for short "OB scheme".

The following figure shows a circuit in which the emitter is the common electrode for the input and output circuits. This is a common emitter circuit, or "OE diagram".

In it, the output current, as in the OB circuit, is the collector current I K, slightly different from the emitter current I e, and the input is the base current I B, significantly less than the collector current. Communication between currents I B And I K in the OE scheme is determined by the equation: I K= h 21 E I B + I KEO
Proportionality factor h 21 E is called the static base current transfer coefficient. It can be expressed in terms of the static transfer coefficient of the emitter current h 21B
h 21 E = h 21B / (1 —h 21B )
If h 21B is within the range of 0.9...0.998, the corresponding values h 21 E will be within 9...499.
Component I keo is called the reverse collector current in the OE circuit. Its value is 1+ h 21 E times more than I BWC, i.e. I KEO =(1+ h 21 E ) I KBO. Reverse currents I BWC and I CEOs do not depend on input voltages U EB And U BE and as a result are called uncontrolled components of the collector current. These currents strongly depend on the ambient temperature and determine the thermal properties of the transistor. It has been established that the reverse current value I The BER doubles with a temperature increase of 10 °C for germanium and 8 °C for silicon transistors. In the OE circuit, temperature changes in the uncontrolled reverse current I KEO can be tens and hundreds of times higher than the temperature changes of the uncontrolled reverse current I BWC and completely disrupt the operation of the transistor. Therefore, in transistor circuits, special measures are used for thermal stabilization of transistor cascades, helping to reduce the influence of temperature changes in currents on the operation of the transistor.
In practice, there are often circuits in which the common electrode for the input and output circuits of the transistor is the collector. This is a connection circuit with a common collector, or “OK circuit” (emitter follower) .