So, the third and final part of the story about bipolar transistors on our website =) Today we will talk about using these wonderful devices as amplifiers, consider possible bipolar transistor switching circuits and their main advantages and disadvantages. Let's get started!

This circuit is very good when using high frequency signals. In principle, this is why the transistor is turned on in the first place. Very big disadvantages are the low input resistance and, of course, the lack of current amplification. See for yourself, at the input we have the emitter current, at the output.

That is, the emitter current is greater than the collector current by a small amount of the base current. This means that there is not just no current gain, moreover, the output current is slightly less than the input current. Although, on the other hand, this circuit has a fairly large voltage transfer coefficient) These are the advantages and disadvantages, let’s continue….

Connection diagram for a bipolar transistor with a common collector

This is what the wiring diagram for a bipolar transistor with a common collector looks like. Does it remind you of anything?) If we look at the circuit from a slightly different angle, we recognize our old friend here - the emitter follower. There was almost a whole article about it (), so we have already covered everything related to this scheme. Meanwhile, we are waiting for the most commonly used circuit - with a common emitter.

Connection circuit for a bipolar transistor with a common emitter.

This circuit has earned popularity for its amplifying properties. Of all the circuits, it gives the greatest gain in current and voltage; accordingly, the increase in signal power is also large. The disadvantage of the circuit is that the amplification properties are strongly influenced by increasing temperature and signal frequency.

We got acquainted with all the circuits, now let’s take a closer look at the last (but not the least important) amplifier circuit based on a bipolar transistor (with a common emitter). First, let's depict it a little differently:

There is one minus here - the grounded emitter. When the transistor is turned on in this way, there are nonlinear distortions at the output, which, of course, must be combated. Nonlinearity occurs due to the influence of the input voltage on the emitter-base junction voltage. Indeed, there is nothing “extra” in the emitter circuit; the entire input voltage turns out to be applied precisely to the base-emitter junction. To cope with this phenomenon, we add a resistor to the emitter circuit. So we get negative feedback.

What is this?

To put it briefly, then negative inverse principle th communications lies in the fact that some part of the output voltage is transferred to the input and subtracted from the input signal. Naturally, this leads to a decrease in the gain, since the input of the transistor, due to the influence of feedback, will receive a lower voltage value than in the absence of feedback.

Nevertheless, negative feedback is very useful for us. Let's see how it will help reduce the influence of the input voltage on the voltage between the base and emitter.

So, even if there is no feedback, an increase in the input signal by 0.5 V leads to the same increase. Everything is clear here 😉 And now let’s add feedback! And in the same way, we increase the input voltage by 0.5 V. Following this, , increases, which leads to an increase in the emitter current. And an increase leads to an increase in the voltage across the feedback resistor. It would seem, what's wrong with this? But this voltage is subtracted from the input! Look what happened:

The input voltage has increased - the emitter current has increased - the voltage across the negative feedback resistor has increased - the input voltage has decreased (due to subtraction) - the voltage has decreased.

That is, negative feedback prevents the base-emitter voltage from changing when the input signal changes.

As a result, our amplifier circuit with a common emitter was supplemented with a resistor in the emitter circuit:

There is another problem with our amplifier. If a negative voltage value appears at the input, the transistor will immediately close (the base voltage will become less than the emitter voltage and the base-emitter diode will close), and nothing will happen at the output. This is somehow not very good) Therefore, it is necessary to create bias. This can be done using a divisor as follows:

We got such a beauty 😉 If the resistors are equal, then the voltage on each of them will be equal to 6V (12V / 2). Thus, in the absence of a signal at the input, the base potential will be +6V. If a negative value, for example -4V, comes to the input, then the base potential will be equal to +2V, that is, the value is positive and does not interfere with the normal operation of the transistor. This is how useful it is to create an offset in the base circuit)

How else could we improve our scheme...

Let us know what signal we will amplify, that is, we know its parameters, in particular the frequency. It would be great if there was nothing at the input except the useful amplified signal. How to ensure this? Of course, using a high-pass filter) Let's add a capacitor, which, in combination with a bias resistor, forms a high-pass filter:

This is how the circuit, in which there was almost nothing except the transistor itself, was overgrown with additional elements 😉 Perhaps we’ll stop there; soon there will be an article devoted to the practical calculation of an amplifier based on a bipolar transistor. In it we will not only compose amplifier circuit diagram, but we will also calculate the ratings of all elements, and at the same time select a transistor suitable for our purposes. See you soon! =)

Bipolar transistors are made of alloyed materials and can be of two types - NPN and PNP. A transistor has three terminals known as emitter (E), base (B) and collector (K). The figure below shows an NPN transistor where, in the main operating modes (active, saturation, cutoff), the collector has a positive potential, the emitter is negative, and the base is used to control the state of the transistor.

The physics of semiconductors will not be discussed in this article, however, it is worth mentioning that a bipolar transistor consists of three separate parts, separated by two p-n junctions. A PNP transistor has one N region separated by two P regions:

An NPN transistor has one P region sandwiched between two N regions:

The junctions between the N and P regions are similar to the junctions in , and they can also be forward biased or reverse biased p-n junctions. These devices can operate in different modes depending on the type of displacement:

• Cut-off: work in this mode also occurs when switching. No current flows between the emitter and collector, practically an “open circuit”, that is, “the contact is open”.
• Active mode: The transistor operates in amplifier circuits. In this mode, its characteristic is almost linear. A current flows between the emitter and collector, the magnitude of which depends on the value of the bias (control) voltage between the emitter and the base.
• Saturation: works when switching. There is practically a “short circuit” between the emitter and the collector, that is, “the contact is closed.”
• Inverse active mode: As in active mode, the transistor current is proportional to the base current, but flows in the opposite direction. Very rarely used.

In an NPN transistor, a positive voltage is applied to the collector to create a current from the collector to the emitter. In a PNP transistor, a positive voltage is applied to the emitter to create a current from the emitter to the collector. In NPN, current flows from the collector (K) to the emitter (E):

And in PNP, the current flows from the emitter to the collector:

It is clear that the directions of current and voltage polarity in PNP and NPN are always opposite to each other. NPN transistors require a supply with positive polarity relative to the common terminals, and PNP transistors require a negative supply.

PNP and NPN work almost identically, but their modes are different due to the polarities. For example, to put NPN into saturation mode, U B must be higher than U K and U E. Below is a brief description of the operating modes depending on their voltage:

The basic operating principle of any bipolar transistor is to control the base current to regulate the flow of current between the emitter and collector. The operating principle of NPN and PNP transistors is the same. The only difference is the polarity of the voltages applied to their N-P-N and P-N-P junctions, that is, the emitter-base-collector.

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

A PNP transistor is an electronic device, in a certain sense the inverse of an NPN transistor. In this type of transistor design, its PN junctions are opened by voltages of reverse polarity with respect to the NPN type. In the symbol of the device, the arrow, which also determines the emitter output, this time points inside the transistor symbol.

Device design

The design circuit of a PNP-type transistor consists of two regions of p-type semiconductor material on either side of a region of n-type material, as shown in the figure below.

The arrow identifies the emitter and the generally accepted direction of its current ("inward" for a PNP transistor).

The PNP transistor has very similar characteristics to its NPN bipolar counterpart, except that the directions of currents and voltage polarities in it are reversed for any of the possible three connection schemes: common base, common emitter and common collector.

The main differences between the two types of bipolar transistors

The main difference between them is that holes are the main current carriers for PNP transistors, NPN transistors have electrons in this capacity. Therefore, the polarities of the voltages supplying the transistor are reversed, and its input current flows from the base. In contrast, with an NPN transistor, the base current flows into it, as shown below in the circuit diagram for connecting both types of devices with a common base and a common emitter.

The operating principle of a PNP-type transistor is based on the use of a small (like the NPN-type) base current and a negative (unlike the NPN-type) base bias voltage to control a much larger emitter-collector current. In other words, for a PNP transistor, the emitter is more positive with respect to the base and also with respect to the collector.

Let's look at the differences between the PNP type in the connection diagram with a common base

Indeed, it can be seen that the collector current IC (in the case of an NPN transistor) flows from the positive terminal of battery B2, passes through the collector terminal, penetrates into it and must then exit through the base terminal to return to the negative terminal of the battery. In the same way, looking at the emitter circuit, you can see how its current from the positive terminal of battery B1 enters the transistor through the base terminal and then penetrates into the emitter.

Thus, both the collector current I C and the emitter current I E pass through the base terminal. Since they circulate along their circuits in opposite directions, the resulting base current is equal to their difference and is very small, since IC is slightly less than I E. But since the latter is still larger, the direction of flow of the difference current (base current) coincides with I E, and therefore a PNP-type bipolar transistor has a current flowing out of the base, and an NPN-type one has an inflowing current.

Differences between PNP type using the example of a connection circuit with a common emitter

In this new circuit, the base-emitter PN junction is biased by battery voltage B1 and the collector-base junction is reverse biased by battery voltage B2. The emitter terminal is thus common to the base and collector circuits.

The total emitter current is given by the sum of two currents I C and I B; passing through the emitter terminal in one direction. Thus, we have I E = I C + I B.

In this circuit, the base current I B simply “branches off” from the emitter current I E, also coinciding with it in direction. In this case, a PNP-type transistor still has a current flowing from the base I B, and an NPN-type transistor has an inflowing current.

In the third of the known transistor switching circuits, with a common collector, the situation is exactly the same. Therefore, we do not present it in order to save space and time for readers.

PNP transistor: connecting voltage sources

The base-to-emitter voltage source (V BE) is connected negative to the base and positive to the emitter because the PNP transistor operates when the base is biased negatively relative to the emitter.

The emitter supply voltage is also positive with respect to the collector (V CE). Thus, with a PNP-type transistor, the emitter terminal is always more positive in relation to both the base and collector.

The voltage sources are connected to the PNP transistor as shown in the figure below.

This time the collector is connected to the supply voltage VCC through a load resistor, R L, which limits the maximum current flowing through the device. A base voltage VB, which biases it negatively relative to the emitter, is applied to it through a resistor RB, which again is used to limit the maximum base current.

Operation of a PNP transistor stage

So, to cause base current to flow in a PNP transistor, the base must be more negative than the emitter (current must leave the base) by about 0.7 volts for a silicon device or 0.3 volts for a germanium device. The formulas used to calculate base resistor, base current or collector current are the same as those used for an equivalent NPN transistor and are presented below.

We see that the fundamental difference between an NPN and a PNP transistor is the correct biasing of the pn junctions, since the directions of the currents and the polarities of the voltages in them are always opposite. Thus, for the above circuit: I C = I E - I B, since the current must flow from the base.

Generally, a PNP transistor can be replaced by an NPN transistor in most electronic circuits, the only difference being the voltage polarity and current direction. Such transistors can also be used as switching devices, and an example of a PNP transistor switch is shown below.

Transistor characteristics

The output characteristics of a PNP transistor are very similar to those of an equivalent NPN transistor, except that they are rotated 180° to allow for reverse polarity of voltages and currents (the base and collector currents of a PNP transistor are negative). Similarly, to find the operating points of a PNP transistor, its dynamic load line can be depicted in the third quarter of the Cartesian coordinate system.

Typical characteristics of the 2N3906 PNP transistor are shown in the figure below.

Transistor pairs in amplifier stages

You may wonder what is the reason to use PNP transistors when there are many NPN transistors available that can be used as amplifiers or solid state switches? However, having two different types of transistors - NPN and PNP - provides great advantages when designing power amplifier circuits. These amplifiers use “complementary” or “matched” pairs of transistors (representing one PNP transistor and one NPN transistor connected together, as shown in the figure below) in the output stage.

Two corresponding NPN and PNP transistors with similar characteristics, identical to each other, are called complementary. For example, TIP3055 (NPN type) and TIP2955 (PNP type) are a good example of complementary silicon power transistors. They both have DC current gain β=I C /I B matched within 10% and high collector current of around 15A, making them ideal for motor control or robotic applications.

In addition, class B amplifiers use matched pairs of transistors in their output power stages. In them, the NPN transistor conducts only the positive half-wave of the signal, and the PNP transistor only conducts its negative half.

This allows the amplifier to pass the required power through the speaker in both directions at a given power rating and impedance. As a result, the output current, which is usually on the order of several amperes, is evenly distributed between the two complementary transistors.

Transistor pairs in electric motor control circuits

They are also used in H-bridge control circuits for reversible DC motors, which make it possible to regulate the current through the motor evenly in both directions of its rotation.

The H-bridge circuit above is so called because the basic configuration of its four transistor switches resembles the letter "H" with the motor located on the cross line. The transistor H-bridge is probably one of the most commonly used types of reversible DC motor control circuit. It uses “complementary” pairs of NPN and PNP transistors in each branch to act as switches to control the motor.

Control input A allows the motor to run in one direction, while input B is used for reverse rotation.

For example, when transistor TR1 is on and TR2 is off, input A is connected to the supply voltage (+Vcc), and if transistor TR3 is off and TR4 is on, then input B is connected to 0 volts (GND). Therefore, the motor will rotate in one direction, corresponding to the positive potential of input A and the negative potential of input B.

If the switch states are changed so that TR1 is off, TR2 is on, TR3 is on, and TR4 is off, the motor current will flow in the opposite direction, causing it to reverse.

By using opposite logic levels "1" or "0" on inputs A and B, you can control the direction of rotation of the motor.

Determining the type of transistors

Any bipolar transistors can be thought of as consisting essentially of two diodes connected together back to back.

We can use this analogy to determine whether a transistor is a PNP or NPN type by testing its resistance between its three terminals. Testing each pair of them in both directions using a multimeter, after six measurements we get the following result:

1. Emitter - Base. These leads should act like a normal diode and only conduct current in one direction.

2.Collector - Base. These leads should also act like a normal diode and only conduct current in one direction.

3. Emitter - Collector. These conclusions should not be drawn in any direction.

Transition resistance values ​​of transistors of both types

Then we can determine the PNP transistor to be healthy and closed. A small output current and negative voltage at its base (B) relative to its emitter (E) will open it and allow much more emitter-collector current to flow. PNP transistors conduct at a positive emitter potential. In other words, a PNP bipolar transistor will conduct only if the base and collector terminals are negative with respect to the emitter.

Page 1 of 2

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) .