Transistors are semiconductor triodes that have three outputs. Their main property is the ability to control high current at the outputs of the circuit using relatively low input signals.
For radio components that are used in modern complex electrical devices, field-effect transistors are used. Thanks to the properties of these elements, the current in the electrical circuits of printed circuit boards is turned on or off, or it is amplified.
What is a field effect transistor?
Field-effect transistors are three or four contact devices in which the current flowing to two contacts can be controlled by the electric field voltage of the third contact. on two contacts is regulated by the voltage of the electric field on the third. As a result, such transistors are called field-effect transistors.
Names of contacts located on the device and their functions:
- Sources – contacts with incoming electric current, which are located in section n;
- Drains are contacts with outgoing, processed current, which are located in section n;
- Gates are contacts located in section p, by changing the voltage at which the throughput of the device is adjusted.
Field effect transistors with n-p junctions are special types that allow you to control current. As a rule, they differ from simple ones in that current flows through them without crossing the section of p-n junctions, a section that is formed at the boundaries of these two zones. The dimensions of the p-n area are adjustable.
Video “In detail about field-effect transistors”
Types of field effect transistors
A field-effect transistor with n-p junctions is divided into several classes depending on:
- From the type of conductor channels: n or r. Channels influence signs, polarities, control signals. They must be opposite in sign to the n-section.
- From the structure of the devices: diffuse, alloyed along p-n junctions, with Schottky gates, thin-film.
- Of the total number of contacts: can be three or four contacts. For four contact devices, the substrates are also gates.
- From the materials used: germanium, silicon, gallium arsenide.
In turn, the division of classes occurs depending on the principle of operation of the transistor:
- devices controlled by p-n junctions;
- devices with insulated gates or Schottky barriers.
The principle of operation of a field-effect transistor
Speaking in simple words about how a field-effect transistor for dummies with control p-n junctions works, it is worth noting: radio components consist of two sections: p-junctions and n-junctions. An electric current passes through section n. Section p is an overlapping zone, a kind of valve. If you apply a certain pressure to it, it will block the area and prevent the passage of current. Or, on the contrary, as the pressure decreases, the amount of current passing will increase. As a result of this pressure, the voltage increases at the contacts of the gates located on the river section.
Devices with control p-n channel junctions are semiconductor wafers that have electrical conductivity of one of these types. The drain and source contacts are connected to the end sides of the plates, and the gate contacts are connected to the middle. The operating principle of the device is based on changing the spatial thickness of p-n junctions. Since there are practically no mobile charge carriers in the blocking regions, their conductivity is zero. In semiconductor wafers, in areas of which the blocking layer is not affected, current-conducting channels are created. If a negative voltage is applied in relation to the source, a current is formed at the gate through which charge carriers flow.
Insulated gates are characterized by the placement of a thin layer of dielectric on them. This device operates on the principle of electric fields. It only takes a little electricity to destroy it. In this regard, in order to prevent static voltage, which can exceed 1000 V, it is necessary to create special housings for devices that minimize the effect of viral types of electricity.
What is a field effect transistor used for?
When considering the operation of complex types of electrical engineering, it is worth considering the operation of such an important component of an integrated circuit as a field-effect transistor. The main task of using this element lies in five key areas, and therefore the transistor is used for:
- High frequency amplification.
- Low frequency boost.
- Modulations.
- DC amplification.
- Key devices (switches).
As a simple example, the operation of a transistor switch can be represented as a microphone and a light bulb in one arrangement. Thanks to the microphone, sound vibrations are captured, which affects the appearance of electric current flowing to the area of the locked device. The presence of current affects the switching on of the device and the switching on of the electrical circuit to which the light bulbs are connected. The latter light up after the microphone has picked up the sound, but they burn due to power sources not connected to the microphone and more powerful.
Modulation is used to control information signals. The signals control the oscillation frequencies. Modulation is used for high-quality audio radio signals, for transmitting audio frequencies to television broadcasts, for broadcasting color images and television signals with high quality. Modulation is used everywhere where it is necessary to work with high-quality materials.
As amplifiers, field-effect transistors work in a simplified form according to the following principle: graphically, any signals, in particular audio, can be represented as a broken line, where its length is the time interval, and the height of the breaks is the audio frequency. To amplify the sound, a powerful voltage flow is supplied to the radio component, acquiring the desired frequency, but with a higher value, due to the supply of weak signals to the control contacts. In other words, thanks to the device, a proportional redrawing of the original line occurs, but with a higher peak value.
How to use a field-effect transistor for dummies
The first devices that entered the market for sale, and in which field-effect transistors with control p-n junctions were used, were hearing aids. Their invention took place back in the fifties of the 20th century. On a larger scale they were used as elements for telephone exchanges.
Nowadays, the use of such devices can be seen in many types of electrical engineering. Having small sizes and a large list of characteristics, field-effect transistors are found in kitchen appliances (toasters, kettles, microwaves), in computer, audio and video equipment and other electrical appliances. They are used for fire safety alarm systems.
In industrial enterprises, transistor equipment is used to regulate power on machine tools. In the transport sector, they are installed in trains and locomotives, and in fuel injection systems for personal cars. In the housing and communal services sector, transistors make it possible to monitor dispatching and street lighting control systems.
Also, the most popular area in which transistors are used is the manufacture of components used in processors. The design of each processor includes multiple miniature radio components, which, when the frequency increases by more than 1.5 GHz, require increased energy consumption. In connection with these, processor technology developers decided to create multi-core equipment rather than increase the clock frequency.
Advantages and disadvantages of field-effect transistors
The use of field-effect transistors, due to their universal characteristics, made it possible to bypass other types of transistors. They are widely applied to integrated circuit as switch.
Advantages:
- cascades of parts consume a small amount of energy;
- amplification indicators exceed the values of other similar devices;
- high noise immunity is achieved due to the fact that there is no current in the gate;
- have a higher turn-on and turn-off speed and operate at frequencies inaccessible to other transistors.
Flaws:
- less resistant to high temperatures, which lead to destruction;
- at frequencies above 1.5 GHz, the amount of energy consumed increases rapidly;
- sensitive to static types of electricity.
Thanks to the characteristics possessed by semiconductor materials, taken as the basis for a field-effect transistor, they allow the device to be used in household and industrial applications. Various household appliances that are used by modern people are equipped with field-effect transistors.
Video “Design and principle of operation of a field-effect transistor”
Transistor - the main component in any electrical circuit. This article is about them and is written for beginner radio amateurs. A transistor is a kind of amplifier switch; the operating principle is similar to a thyristor. There is no way to do without transistors in electronics; literally everything is assembled on them - the simplest flashing lights, transistor low-frequency power amplifiers, radio receivers and transmitters, television and video equipment and many other devices. Transistors can increase or decrease the initial voltage of the power supply if they are used in converter circuits.
The transistor itself is a semiconductor device; the transistor crystal is mainly made of silicon or germanium. Transistors come in two types - unipolar and bipolar, respectively field-effect and bipolar. In terms of conductivity, there are also two types - forward conduction transistors (p - n - p) and reverse conduction transistors (n - p - n). N-P - from Latin negative and positive. In the diagrams you can easily distinguish what conductivity the transistor is used - if the emitter arrow enters the transistor, it means it has forward conductivity, but if it comes out of the transistor, it means the transistor has reverse conductivity.
To operate the transistor, a small current is supplied to the base, after which the transistor opens and can pass a larger current through the emitter - collector, that is, by supplying a relatively small current to the base, we can control larger currents. In other words, by turning the water tap with a slight force, we control a powerful flow of water. A transistor can be in two states: it is open when voltage is applied to the base (operating state of the transistor) and closed when no current flows to the base (rest state of the transistor).
Based on operating frequency, low-frequency and high-frequency transistors are often used. Low-frequency transistors are used for power circuits of voltage converters, power amplifiers in power supplies, and so on. Low-frequency transistors are usually of higher power. High-frequency transistors operating at frequencies of several gigahertz are also used very often. Basically, they have found wide application in radio receiving and transmitting equipment, in high-frequency amplifiers and in many other devices. Such transistors have relatively low power; they are indispensable in the field of radio reception and transmission.
Transistors come in a variety of shapes and sizes - from surface-mount chip elements invisible to human eyes, to mega-power transistors the size of a house.
The latter can have a power of up to hundreds of megawatts and are mainly used in power plants and factories. For better current conductivity, a thin layer of gold or silver is often applied across the contacts of a high-frequency transistor, but recently such transistors are very rare; such transistors were mainly used in radio equipment during the Soviet Union. For beginners, I’m sure this material helped them figure out what’s what and clarify questions about transistors - Arthur Kasyan (AKA).
Discuss the article WHAT IS A TRANSISTOR
Transistor called a semiconductor device designed to amplify and generate electrical oscillations. So what is a transistor? - It is a crystal placed in a housing equipped with leads. The crystal is made from a semiconductor material. In terms of their electrical properties, semiconductors occupy an intermediate position between conductors and non-conductors (insulators).
A small crystal of a semiconductor material (semiconductor), after appropriate technological processing, becomes capable of changing its electrical conductivity within a very wide range when weak electrical oscillations and a constant bias voltage are applied to it.
The crystal is placed in a metal or plastic case and equipped with three leads, hard or soft, connected to the corresponding areas of the crystal. The metal case sometimes has its own terminal, but one of the three electrodes of the transistor is connected to the case.
Currently, two types of transistors are used - bipolar and field. Bipolar transistors appeared first and became most widespread. Therefore, they are usually simply called transistors. Field-effect transistors appeared later and are still used less frequently than bipolar ones.
Bipolar transistors
Bipolar transistors called because the electric current in them is formed by electric charges of positive and negative polarity. Positive charge carriers are usually called holes, negative charges are carried by electrons. A bipolar transistor uses a crystal made of germanium or silicon, the main semiconductor materials used to make transistors and diodes.
That's why transistors are called the same silicon, other - germanium. Both types of bipolar transistors have their own characteristics, which are usually taken into account when designing devices.
To make the crystal, ultra-pure material is used, to which special strictly dosed quantities are added; impurities. They determine the appearance in the crystal of conductivity caused by holes (p-conductivity) or electrons (n-conductivity). In this way, one of the electrodes of the transistor, called the base, is formed.
If now special impurities are introduced into the surface of the base crystal by one technological method or another, changing the conductivity type of the base to the reverse so that nearby n-p-n or p-n-p zones are formed, and leads are connected to each zone, a transistor is formed.
One of the extreme zones is called an emitter, i.e., a source of charge carriers, and the second is a collector, a collector of these carriers. The area between the emitter and collector is called the base. The terminals of a transistor are usually given names similar to their electrodes.
The amplifying properties of the transistor are manifested in the fact that if now a small electrical voltage is applied to the emitter and base - the input signal, then a current will flow in the collector - emitter circuit, in shape repeating the input current of the input signal between the base and emitter, but many times greater in value .
For normal operation of the transistor, it is first necessary to apply supply voltage to its electrodes. In this case, the voltage at the base relative to the emitter (this voltage is often called the bias voltage) should be equal to several tenths of a volt, and at the collector relative to the emitter - several volts.
The inclusion of n-p-n and p-n-p transistors in the circuit differs only in the polarity of the collector voltage and bias. Silicon and germanium transistors of the same structure differ from each other only in the value of the bias voltage. For silicon it is approximately 0.45 V more than for germanium.
Rice. 1
In Fig. Figure 1 shows the graphical symbols of transistors of one and the other structure, made on the basis of germanium and silicon, and the typical bias voltage. The electrodes of the transistors are designated by the first letters of the words: emitter - E, base - B, collector - K.
The bias voltage (or, as they say, mode) is shown relative to the emitter, but in practice, the voltage at the electrodes of the transistor is indicated relative to the common wire of the device. A common wire in a device and in a diagram is a wire galvanically connected to the input, output, and often to the power source, i.e., common to the input, output, and power source.
The amplification and other properties of transistors are characterized by a number of electrical parameters, the most important of which are discussed below.
Static base current transfer coefficient h 21E shows how many times the collector current of the bipolar transistor is greater than the current of its base, which caused this current. For most types of transistors, the numerical value of this coefficient from instance to instance can vary from 20 to 200. There are transistors with a lower value - 10...15, and with a larger value - up to 50...800 (these are called super-amplification transistors).
It is often believed that good results can only be obtained with transistors that have a large value of h21e. However, practice shows that with skillful design of equipment it is quite possible to get by with transistors having h 2 l E equal to only 12...20. This is exemplified by most of the designs described in this book.
Frequency properties of the transistor takes into account the fact that the transistor is capable of amplifying electrical signals with a frequency not exceeding a certain limit for each transistor. The frequency at which the transistor loses its amplification properties is called the limiting amplification frequency of the transistor.
In order for a transistor to provide significant signal amplification, it is necessary that the maximum operating frequency of the signal be at least 10...20 times less than the limiting frequency f t of the transistor. For example, to effectively amplify low-frequency signals (up to 20 kHz), low-frequency transistors are used, the limiting frequency of which is not less than 0.2...0.4 MHz.
To amplify signals from radio stations in the long-wave and medium-wave ranges (signal frequency not higher than 1.6 MHz), only high-frequency transistors with a maximum frequency of not lower than 16...30 MHz are suitable.
Maximum permissible power dissipation- this is the greatest power that a transistor can dissipate for a long time without the risk of failure. In reference books on transistors, the maximum permissible power of the Yaktakh collector is usually indicated, since it is in the collector-emitter circuit that the greatest power is released and the highest current and voltage act.
The base and collector currents, flowing through the transistor crystal, heat it up. A germanium crystal can operate normally at a temperature of no more than 80, and a silicon crystal - no more than 120°C. The heat that is generated in the crystal is transferred to the environment through the transistor body, as well as through an additional heat sink (radiator), which is additionally supplied to high-power transistors.
Depending on the purpose, low, medium and high power transistors are produced. Low-power ones are used mainly for amplification and conversion of weak signals of low and high frequencies, high-power ones - in the final stages of amplification and generation of electrical oscillations of low and high frequencies.
The amplification capabilities of a stage on a bipolar transistor depend not only on what power it has, but also on what specific transistor is selected, in what mode of operation in alternating and direct current it operates (in particular, what is the collector current and voltage between the collector and emitter ), what is the relationship between the operating frequency of the signal and the limiting frequency of the transistor.
What is a field effect transistor
Field-effect transistor is a semiconductor device in which the current between two electrodes, formed by the directed movement of charge carriers of holes or electrons, is controlled by an electric field created by the voltage on the third electrode.
The electrodes between which a controlled current flows are called source and drain, and the source is considered to be the electrode from which the charge carriers emerge (flow).
The third, control, electrode is called the gate. The current-conducting section of semiconductor material between the source and drain is usually called a channel, hence another name for these transistors - channel transistors. Under the influence of voltage on the gate relative to the source, the resistance of the channel changes, and therefore the current through it.
Depending on the type of charge carriers, transistors are distinguished with n-channel or p-channel. In n-channel channels, the channel current is determined by the directional movement of electrons, and in p-channel channels, by holes. In connection with this feature of field-effect transistors, they are sometimes also called unipolar. This name emphasizes that the current in them is formed by carriers of only one sign, which distinguishes field-effect transistors from bipolar ones.
For the manufacture of field-effect transistors, silicon is mainly used, which is due to the peculiarities of their production technology.
Basic parameters of field-effect transistors
The slope of the input characteristic S or the conductivity of the forward current transfer Y 21 indicates how many milliamps the channel current changes when the input voltage between gate and source changes by 1 V. Therefore, the value of the slope of the input characteristic is determined in mA / V, just like the slope of the radio tube characteristic.
Modern field-effect transistors have a transconductance from tenths to tens and even hundreds of milliamps per volt. Obviously, the greater the transconductance, the greater the gain the field-effect transistor can provide. But large values of slope correspond to large channel current.
Therefore, in practice, a channel current is usually chosen at which, on the one hand, the required gain is achieved, and on the other hand, the necessary efficiency in current consumption is ensured.
The frequency properties of a field-effect transistor, as well as a bipolar transistor, are characterized by the value of the limiting frequency. Field-effect transistors are also divided into low-frequency, mid-frequency and high-frequency, and also to obtain high gain, the maximum signal frequency must be at least 10...20 times less than the limiting frequency of the transistor.
The maximum permissible constant power dissipation of a field-effect transistor is determined in exactly the same way as for a bipolar one. The industry produces field-effect transistors of low, medium and high power.
For normal operation of a field-effect transistor, a constant initial bias voltage must be applied to its electrodes. The polarity of the bias voltage is determined by the type of channel (n or p), and the value of this voltage is determined by the specific type of transistor.
It should be pointed out here that among field-effect transistors there is a much greater variety of crystal designs than among bipolar ones. The most widespread in amateur designs and in industrial products are field-effect transistors with the so-called built-in channel and p-n junction.
They are unpretentious in operation, operate over a wide frequency range, and have a high input impedance, reaching several megaohms at low frequencies, and several tens or hundreds of kiloohms at medium and high frequencies, depending on the series.
For comparison, we point out that bipolar transistors have a significantly lower input resistance, usually close to 1...2 kOhm, and only the stages on a composite transistor can have a higher input resistance. This is the great advantage of field-effect transistors over bipolar ones.
In Fig. Figure 2 shows the symbols of field-effect transistors with a built-in channel and p-n junction, and also indicates typical values of the bias voltage. The terminals are designated according to the first letters of the electrode names.
It is typical that for transistors with a p-channel the voltage at the drain relative to the source should be negative, and at the gate relative to the source - positive, and for a transistor with an n-channel - vice versa.
In industrial equipment and less often in amateur radio equipment, field-effect transistors with an insulated gate are also used. Such transistors have an even higher input resistance and can operate at very high frequencies. But they have a significant drawback - the low electrical strength of the insulated gate.
For its breakdown and failure of the transistor, even a weak charge of static electricity, which is always present on the human body, on clothes, on tools, is quite enough.
For this reason, the terminals of field-effect transistors with an insulated gate during storage should be tied together with soft bare wire, when installing transistors, hands and tools should be “grounded,” and other protective measures should be used.
Literature: Vasilyev V.A. Receivers for a beginner radio amateur (MRB 1072).
Any electronic device consists of radioelements. They can be passive, which does not require a power source, or active, which can only operate when voltage is applied. Semiconductors are called active elements. One of the most important semiconductor devices is the transistor. This radio element replaced tube devices and completely changed the circuitry of devices. All microelectronics and the operation of any microcircuit are based on it.
The name “transistor” comes from the merger of two English words: transfer - portable, and resistor - resistance. In the generally accepted concept, this is a semiconductor element with three terminals. In it, the current value at two terminals depends on the third, when changing the current or voltage at which the current value of the output circuit is controlled. Bipolar devices are controlled by current variation, and field devices are controlled by voltage.
The first developments of the transistor began in the 20th century. In Germany, scientist Julius Edgar Lilienfeld described the operating principle of a transistor, and already in 1934, physicist Oskar Heil registered a device, later called a transistor. Such a device worked on the electrostatic field effect.
Physicists William Shockley, Walter Brattain, together with scientist John Bardeen, made the first prototype of a point-point transistor in the late 40s. With the discovery of the n-p junction, the production of the point-point transistor ceased, and instead, the development of planar devices from germanium began. A working prototype of the transistor was officially presented in December 1947. On this day the first bipolar transistor appeared. In the summer of 1948, transistor-based devices began to be sold. From that moment on, the electronic tubes (triodes) that were common at that time began to become a thing of the past.
In the mid-50s, the first planar transistor was produced in series by Texas Instruments, using silicon as the material for its manufacture. At that time, the production of the radio element resulted in a lot of defects, but this did not hinder the technological development of the device. In 1953, a circuit used in hearing aids was made using transistors, and a year later, American physicists received the Nobel Prize for their discovery.
March 1959 was marked by the creation of the first silicon planar device; its developer was Swiss physicist Jean Erni. A pair of transistors was successfully placed on a single silicon chip. From this moment the development of integrated circuit technology began. Today, a single chip houses more than a billion transistors. For example, on the popular 8-core computer processor Core i7−5960X their number is 2.6 billion.
In parallel with improvements in the bipolar transistor, development of a device based on a metal-semiconductor connection began in the 60s. Such a radio element is called a MOS (metal-oxide-semiconductor) transistor, today better known as a “mosfet”.
Initially, the concept of “transistor” referred to resistance, the value of which was controlled by voltage, since a transistor can be thought of as a kind of resistor, regulated by an applied potential at one terminal. For field-effect transistors, a comparison with which is more correct, it is the potential at the gate, and for bipolar transistors, it is the potential at the base or the base current.
The basis of the device’s operation is the ability of the n-p junction to pass current in one direction. When voltage is applied, a forward drop occurs at one junction, and a reverse drop at the other. The transition zone with direct voltage has low resistance, and with reverse voltage it has high resistance. A small control current flows between the base and emitter. The value of this current changes the resistance between the collector and emitter. There are two types of bipolar device:
- p-n-p;
- n-p-n.
The only difference is in the main charge carriers, i.e., the direction of the current.
If you connect two semiconductors of different types to each other, then a region appears at the junction boundary, or, as is commonly called, a p-n junction. The type of conductivity depends on the atomic structure of the material, namely how strong the bonds in the material are. The atoms in a semiconductor are arranged in a lattice, and the material itself is not a conductor. But if atoms of another material are added to the lattice, the physical properties of the semiconductor change. Mixed atoms form, depending on their nature, free electrons or holes.
The free electrons formed form a negative charge, and the holes form a positive charge. There is a potential barrier in the transition area. It is formed by a contact potential difference, and its height does not exceed tenths of a volt, preventing the flow of charge carriers deep into the material. If the junction is under direct voltage, then the magnitude of the potential barrier decreases, and the magnitude of the current passing through it increases. When a reverse voltage is applied, the magnitude of the barrier increases and the barrier's resistance to the passage of current increases. By understanding the operation of a pn junction, you can understand how a transistor works.
First of all, such devices are divided into single and composite. There are also so-called complex radioelements. They have three terminals and are made as a single unit. Such assemblies contain both transistors of the same type and different types. The main division of devices occurs according to the following criteria:
The general definition for a radio element can be formulated as follows: a transistor is a semiconductor element designed to convert electrical quantities. Its main use is to amplify the signal or operate in key mode.
The principle of operation of a transistor for a “teapot” is easier to describe by analogy with a water supply. The element itself can be represented as a valve. By turning the faucet slightly, you can regulate the flow of water (current strength). If you turn the handle a little, water will flow through the pipe (conductor), if you open the tap even more, the water flow will also increase. Thus, the output of the water flow is proportional to its input, multiplied by a certain value. This value is called the gain.
A bipolar transistor has three terminals: emitter, base, collector. The emitter and collector have the same type of conductivity, which is different from the base. Hole-type transistors consist of two regions of p-type conductivity, and one n-type. Electronic type is the opposite. Each area has its own output.
When a signal of the required conductivity is applied to the emitter, the current in the base area increases. The main charge carriers move to the base area, which leads to an increase in current in the reverse connection area. A space charge arises. The electric field begins to draw carriers of a different sign into the reverse connection zone. Partial recombination (destruction) of charges of the opposite sign occurs in the base, due to which the base current arises.
The emitter is the area of the device that serves to transfer charge carriers to the base. A collector is a zone designed to extract charge carriers from the base. And the base is the area for the emitter to transfer the opposite amount of charge. The main characteristic of the device is the current-voltage characteristic, the function of which describes the relationship between current and voltage.
In the diagram, the device is signed with the Latin letters VT or Q. It looks like a circle with an arrow inside, where the arrow indicates the direction of current flow. For PNP (forward conduction) the arrow is inward, and for NPN (reverse conduction) the arrow is outward. To make a transistor, germanium or silicon is used. These materials differ in the operating voltage range of the base junction. For germanium it lies in the range of 0.1-0.4 V, and for silicon from 0.4 to 1.2 V. Silicon is usually used.
The difference between a field-effect transistor and a bipolar transistor is that the voltage applied to the controlled contact is responsible for the passage of current.
The main purpose of mosfets is associated with their good switching speed with very little power applied to the control pin. The field element has three terminals: gate, drain, source. When a mosfet operates with a control n-p junction, the potential on the gate is either zero (the device is open) or has a certain value greater than zero (the device is closed). When the reverse voltage reaches a certain level, the blocking layer opens and the device goes into cut-off mode.
In a mosfet with a p-n junction, the control electrode (gate) is a semiconductor layer with p-type conductivity, and the opposite conductivity is an n-type channel.
Its image in the diagram is similar to a bipolar device, only all the lines are straight, and the arrow inside emphasizes the type of device. The operating principle of MOS devices is based on the effect of changes in the conductivity of the semiconductor at the boundary of the region with the dielectric when exposed to an electric field. Field devices, depending on the controlled p-n junction, can be:
Each species can have both p-type and n-type conductivity. In a general understanding, the operating principle does not depend on conductivity; only the polarity of the voltage source changes.
A transistor is a complex device, the physical processes taking place in which are difficult for beginner radio amateurs (dummies) to understand. How a transistor works can be explained as follows: A transistor is an electronic switch whose degree of opening depends on the level of current or voltage applied to its controlled terminal (base or gate).
Why a transistor is needed can be described in a generalized form. For example, the base (shutter) of the device is a door. It opens by external influence, i.e., voltage of the same polarity as the collector (source). The greater the tension, the more the door will open. In front of the door there is a line of people (charge carriers) who want to run through it (collector-emitter or source-drain). The greater the impact on the door, the more it is open, which means more people will pass through.
Therefore, imagining the door as a transition resistance, we can conclude: the greater the impact on the base (gate), the less resistance to the main charge carriers (people) in the case of direct polarity. If the polarity is reversed (the door is locked), then there will be no movement of charges (people).
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 electrical 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).
- The collector has a more positive potential than the emitter
- As I already said, the base-collector and base-emitter circuits work like diodes
- Each transistor is characterized by limiting values such as collector current, base current and collector-emitter voltage.
- 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:
- 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.
- 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.
- 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.
- 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.
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I wish you good luck, success and a sunny mood!
From n/a Vladimir Vasiliev
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