Saturation mode of a bipolar transistor. Bipolar transistors complete description. The main differences between the two types of bipolar transistors

The bipolar transistor is one of the oldest but most famous type of transistor, and is still used in modern electronics. A transistor is indispensable when you need to control a fairly powerful load for which the control device cannot provide sufficient current. They come in different types and capacities, depending on the tasks performed. Basic knowledge and formulas about transistors can be found in this article.

Introduction

Before starting the lesson, let's agree that we are discussing only one type of way to turn on a transistor. A transistor can be used in an amplifier or receiver, and typically each transistor model is manufactured with specific characteristics to make it more specialized for better work in a certain inclusion.

The transistor has 3 terminals: base, collector and emitter. It is impossible to say unambiguously which of them is the input and which is the output, since they are all connected and influence each other in one way or another. When a transistor is turned on in switch mode (load control), it acts like this: the base current controls the current from the collector to the emitter or vice versa, depending on the type of transistor.

There are two main types of transistors: NPN and PNP. To understand this, we can say that the main difference between these two types is the direction of the electric current. This can be seen in Figure 1.A, where the direction of the current is indicated. In an NPN transistor, one current flows from the base into the transistor and the other current flows from the collector to the emitter, but in a PNP transistor the opposite is true. From a functional point of view, the difference between these two types of transistors is the voltage across the load. As you can see in the picture, the NPN transistor provides 0V when it is turned on, and the PNP provides 12V. You'll understand later why this affects transistor selection.

For simplicity, we will only study NPN transistors, but all this applies to PNP, taking into account that all currents are reversed.

The figure below shows the analogy between a switch (S1) and a transistor switch, where it can be seen that the base current closes or opens the path for current from the collector to the emitter:

Knowing exactly the characteristics of the transistor, you can get from it maximum return. The main parameter is the DC gain of the transistor, which is usually denoted H fe or β. It is also important to know the maximum current, power and voltage of the transistor. These parameters can be found in the documentation for the transistor, and they will help us determine the value of the base resistor, which is described below.

Using an NPN transistor as a switch

The figure shows the inclusion of an NPN transistor as a switch. You will encounter this inclusion very often when analyzing various electronic circuits. We will study how to run a transistor in the selected mode, calculate the base resistor, transistor current gain and load resistance. I suggest the simplest and most exact way for this.

1. Assume that the transistor is in saturation mode: Wherein mathematical model transistor becomes very simple, and we know the voltage at point V c. We will find the value of the base resistor at which everything will be correct.

2. Determination of collector saturation current: The voltage between collector and emitter (V ce) is taken from the transistor documentation. The emitter is connected to GND, respectively V ce = V c - 0 = V c. Once we know this value, we can calculate the collector saturation current using the formula:

Sometimes, the load resistance R L is unknown or cannot be as accurate as the relay coil resistance; In this case, it is enough to know the current required to start the relay.
Make sure that the load current does not exceed the maximum collector current of the transistor.

3. Calculation of the required base current: Knowing the collector current, you can calculate the minimum required base current to achieve this collector current using the following formula:

It follows from it that:

4. Exceeding permissible values: After you have calculated the base current, and if it turns out to be lower than that specified in the documentation, then you can overload the transistor by multiplying the calculated base current, for example, by 10 times. Thus, the transistor switch will be much more stable. In other words, the transistor's performance will decrease if the load increases. Be careful not to exceed the maximum base current stated in the documentation.

5. Calculation of the required value of R b: Considering an overload of 10 times, the resistance R b can be calculated using the following formula:

where V 1 is the transistor control voltage (see Fig. 2.a)

But if the emitter is connected to ground, and the base-emitter voltage is known (about 0.7V for most transistors), and assuming that V 1 = 5V, the formula can be simplified to the following:

It can be seen that the base current is multiplied by 10 taking into account the overload.
When the value of Rb is known, the transistor is "set" to operate as a switch, also called "saturation and cutoff mode", where "saturation" is when the transistor is fully open and conducting current, and "cutting" is when it is closed and not conducting current .

Note: When we say , we are not saying that the collector current must be equal to . This simply means that the transistor's collector current can rise to this level. The current will follow Ohm's laws, just like any electrical current.

Load calculation

When we considered that the transistor was in saturation mode, we assumed that some of its parameters did not change. This is not entirely true. In fact, these parameters were changed mainly by increasing the collector current, and therefore it is safer for overload. The documentation indicates a change in transistor parameters during overload. For example, the table in Figure 2.B shows two parameters that change significantly:

H FE (β) varies with collector current and voltage V CEsat. But V CEsat itself changes depending on the collector and base current, as shown in the table below.

The calculation can be very complex, since all the parameters are closely and complexly interrelated, so it is better to take the worst values. Those. the smallest H FE, the largest V CEsat and V CEsat.

Typical application of a transistor switch

In modern electronics, a transistor switch is used to control electromagnetic relays, which consume up to 200 mA. If you want to control the relay logic chip or a microcontroller, then the transistor is irreplaceable. In Figure 3.A, the resistance of the base resistor is calculated depending on the current required by the relay. Diode D1 protects the transistor from the pulses that the coil generates when turned off.

2. Connecting an open collector transistor:

Many devices, such as the 8051 family of microcontrollers, have open-collector ports. The base resistor resistance of the external transistor is calculated as described in this article. Note that the ports can be more complex, and often use FETs instead of bipolar ones and are called open-drain outputs, but everything remains exactly the same as in Figure 3.B

3. Creating a logical element OR-NOT (NOR):

Sometimes a circuit needs to use one logic element, and you don't want to use a 14-pin 4-element IC either due to cost or board space. It can be replaced with a pair of transistors. Note that the frequency characteristics of such elements depend on the characteristics and type of transistors, but are usually below 100 kHz. Reducing the output resistance (Ro) will increase power consumption but increase the output current.
You need to find a compromise between these parameters.

The figure above shows a NOR gate built using 2 2N2222 transistors. This can be done with PNP 2N2907 transistors, with minor modifications. You just have to consider that everything electric currents then flow into opposite direction.

Finding errors in transistor circuits

When a problem occurs in circuits containing many transistors, it can be quite difficult to know which one is bad, especially when they are all soldered in. I give you some tips that will help you find the problem in such a scheme quickly:

1. Temperature: If the transistor gets very hot, there is probably a problem somewhere. It is not necessary that the problem is a hot transistor. Usually the defective transistor does not even heat up. This temperature increase may be caused by another transistor connected to it.

2. Measuring V CE of transistors: If they are all the same type and all work, then they should have approximately the same VCE. Search for transistors having different V CE is quick way detection of defective transistors.

3. Measuring the voltage across the base resistor: The voltage across the base resistor is quite important (if the transistor is turned on). For a 5V NPN transistor driver, the voltage drop across the resistor should be greater than 3V. If there is no voltage drop across the resistor, then either the transistor or the transistor control device is defective. In both cases, the base current is 0.

PNP transistor is electronic device, in a certain sense the opposite 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 symbol instrument, the arrow, which also determines the emitter terminal, 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 three possible connection schemes: with common base, With common emitter and with a 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 opened by battery voltage B1, and the collector-base junction is biased reverse direction via 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 fundamental difference between the NPN and PNP transistor is the correct bias of the pn junctions, since the directions of the currents and the polarity 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 be wondering 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, the presence of two various types transistors - NPN and PNP - gives great benefits 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 good example complementary silicon power transistors. They both have gain direct currentβ=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 rated power 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 conclusions should act as regular diode and conduct current only 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.

Exist different kinds semiconductor devices - thyristors, triodes, they are classified according to their purpose and type of design. Semiconductor bipolar transistors are capable of carrying two types of charges simultaneously, while field-effect transistors carry only one.

Design and operating principle

Previously, instead of transistors in electrical diagrams special low-noise vacuum tubes, but they were large in size and operated by incandescence. Bipolar transistor GOST 18604.11-88 is a semiconductor electrical appliance, which is a controlled element and is characterized by a three-layer structure, is used to control microwaves. It can be in the housing or without it. They come in p-n-p and n-p-n types. Depending on the order of the layers, the base can be a p or n plate onto which a certain material is fused. Due to diffusion during manufacturing, a very thin but durable coating layer is obtained.

Photo - basic connection diagrams

To determine which transistor is in front of you, you need to find the arrow of the emitter junction. If its direction goes towards the base, then the structure is pnp, if away from it, then npn. Some polar imported analogues (IGBT and others) may have letter designation transition. In addition, there are also bipolar complementary transistors. These are devices that have the same characteristics, but different types conductivity. This pair has found application in various radio circuits. This feature must be taken into account if it is necessary to replace individual elements of the circuit.

Photo - design

The area that is located in the center is called the base, on both sides of it are the emitter and collector. The base is very thin, often its thickness does not exceed a couple of 2 microns. In theory, there is such a thing as an ideal bipolar transistor. This is a model in which the distance between the emitter and collector regions is the same. But, often, the emitter junction (the area between the base and the emitter) is twice as large as the collector junction (the area between the base and the collector).

Photos - types of bipolar triodes

According to the type of connection and the level of transmitted power, they are divided into:

High frequency;
Low frequency.

By power:

Low-power;
Medium power;
Power (a transistor driver is required for control).

The operating principle of bipolar transistors is based on the fact that the two middle junctions are located in close proximity to each other. This allows you to significantly enhance the electrical impulses passing through them. If applied to different areas (areas) electrical energy different potentials, then a certain area of the transistor will shift. In this way they are very similar to diodes.

Photo - example

For example, when positive, it opens p-n region, and when negative it closes. Main feature The action of transistors is that when any area is displaced, the base is saturated with electrons or vacancies (holes), this allows the potential to be reduced and the conductivity of the element to be increased.

There are the following key species works:

Active mode;
Cutoff;
Double or saturation;
Inversion.

Before determining the operating mode in bipolar triodes, you need to understand how they differ from each other. High-voltage ones most often operate in active mode (also known as key mode), here, when the power is turned on, the emitter junction is shifted, and in the collector section there is reverse voltage. The inversion mode is the opposite of the active one; here everything is shifted in direct proportion. Thanks to this, electronic signals are greatly amplified.

During cutoff, all types of voltage are excluded, the transistor current level is reduced to zero. In this mode, the transistor switch or field-effect triode with an insulated gate opens, and the device turns off. There is also a dual mode or operation in saturation; with this type of operation, the transistor cannot act as an amplifier. Based on this connection principle, circuits operate where it is not necessary to amplify signals, but to open and close contacts.

Due to the difference in voltage and current levels in various modes, to determine them, you can check the bipolar transistor with a multimeter, for example, in amplification mode, a working n-p-n transistor should show a change in stages from 500 to 1200 Ohms. The measuring principle is described below.

The main purpose of transistors is to change certain signals electrical network depending on current and voltage indicators. Their properties make it possible to control the gain by changing the frequency of the current. In other words, it is an impedance converter and signal amplifier. Used in various audio and video equipment to control low-power flows of electricity and as UMZCH, transformers, control of motors of machine tool equipment, etc.

Video: how bipolar transistors work

Examination

The easiest way to measure h21e of powerful bipolar transistors is to test them with a multimeter. To turn on a pnp semiconductor triode, a negative voltage is applied to the base. To do this, the multimeter is switched to ohmmeter mode at -2000 Ohm. The norm for resistance fluctuations is from 500 to 1200 Ohms.

To check other areas, you need to apply positive resistance to the base. During this test, the indicator should show more resistance, otherwise the triode is faulty.

Sometimes the output signals are interrupted by resistors, which are installed to reduce the resistance, but now this bypass technology is rarely used. To check the resistance characteristics of pulsed n-p-n transistors you need to connect the plus to the base, and the minus to the emitter and collector terminals.

Technical characteristics and markings

The main parameters by which these semiconductor elements are selected are pinout and color marking.

Photo - pinout of low-power bipolar triodes

Photo - power pinout

Color coding is also used.

Photos - examples color coding

Photo - color table

Many modern domestic transistors are also designated by a letter code, which includes information about the group (field-effect, bipolar), type (silicon, etc.), year and month of manufacture.

Photo - transcript

Basic properties (parameters) of triodes:

Voltage gain;
Input voltage;
Composite frequency characteristics.

They are also used to select static characteristics, which include comparison of input and output current-voltage characteristics.

The necessary parameters can be calculated by calculating the main characteristics (distribution of cascade currents, calculation of the key mode). Collector current: Ik=(Ucc-Ukanas)/Rн

Ucc – network voltage;
Ukenas – saturation;
Rн – network resistance.

Power loss during operation:

P=Ik*Ukanas

You can buy bipolar transistors SMD, IGBT and others at any electrical store. Their price varies from a few cents to tens of dollars, depending on the purpose and characteristics.

Bipolar transistor— electronic semiconductor device, a type of transistor designed to amplify, generate and convert electrical signals. The transistor is called bipolar, since two types of charge carriers simultaneously participate in the operation of the device - electrons And holes. This is how it differs from(field-effect) transistor, in which only one type of charge carrier is involved.

The principle of operation of both types of transistors is similar to the operation of a water tap that regulates the flow of water, only a flow of electrons passes through the transistor. In bipolar transistors, two currents pass through the device - the main “large” current, and the control “small” current. The power of the main current depends on the power of the controller.

With field-effect transistors, only one current passes through the device, the power of which depends on the electromagnetic field.

In this article we will take a closer look at the operation of a bipolar transistor.

Bipolar transistor design. A bipolar transistor consists of three semiconductor layers and two PN junctions. PNP and NPN transistors are distinguished by the type of alternation of hole and electron conductivity. It is similar to two diodes connected face to face or vice versa. And A bipolar transistor has three contacts (electrodes). The contact coming out of the central layer is called (base. And The extreme electrodes are called collector emitter collector

emitter ). The base layer is very thin relative to the collector and emitter. In addition to this, the semiconductor regions at the edges of the transistor are asymmetrical. The semiconductor layer on the collector side is slightly thicker than on the emitter side. This is necessary for

proper operation

transistor.

Now let's connect the voltage between the base and emitter V BE , but significantly lower than V CE (for silicon transistors the minimum required V BE is 0.6V). Since the layer P is very thin, plus a voltage source connected to the base, it will be able to “reach” with its electric field the N region of the emitter. Under its influence, electrons will be directed to the base.

Some of them will begin to fill the holes located there (recombine).

The other part will not find a free hole, because the concentration of holes in the base is much lower than the concentration of electrons in the emitter.

As a result, the central layer of the base is enriched with free electrons. Most of them will go towards the collector, since the voltage is much higher there. This is also facilitated by the very small thickness of the central layer. Some part of the electrons, although much smaller, will still flow towards the plus side of the base. As a result, we get two currents: a small one - from the base to the emitter I BE, and a large one - from the collector to the emitter I CE. If you increase the voltage at the base, then even more electrons will accumulate in the P layer. As a result, the base current will increase slightly and the collector current will increase significantly. Thus, with a slight change in base current I B β , , the collector current I changes greatly S. That’s what happens. signal amplification in a bipolar transistor.

The ratio of the collector current I C to the base current I B is called the current gain.

Designated hfe or

h21e

, depending on the specifics of the calculations carried out with the transistor. The simplest bipolar transistor amplifier power supply at 20V, due to the energy of which amplification will occur. From the base of the transistor we connect a weak 2V power source. We will connect to it in series an alternating voltage source in the form of a sine wave, with an oscillation amplitude of 0.1V. This will be a signal that needs to be amplified. Resistor Rb near the base is necessary in order to limit the current coming from signal source

, usually with low power.

2. Calculation of base input current I b Now let's calculate the base current I b. Since we are dealing with alternating voltage, we need to calculate two current values - at maximum voltage

(V max) and minimum (V min). Let's call these current values respectively - I bmax and I bmin. Also, in order to calculate the base current, you need to know the base-emitter voltage V BE. There is one PN junction between the base and emitter. It turns out that the base current “meets” on its way

semiconductor diode

. The voltage at which a semiconductor diode begins to conduct is about 0.6V. We will not go into details of the current-voltage characteristics of the diode, and for simplicity of calculations we will take an approximate model, according to which the voltage on the current-carrying diode is always 0.6V. This means that the voltage between the base and emitter is V BE = 0.6V. And since the emitter is connected to ground (V E = 0), the voltage from base to ground is also 0.6V (V B = 0.6V).

Let's calculate I bmax and I bmin using Ohm's law: 2. Calculation of collector output current I C Now, knowing the gain (β = 200), you can easily calculate the maximum and

minimum value

collector current (I cmax and I cmin).

3. Calculation of output voltage V out

The collector current flows through the resistor Rc, which we have already calculated. It remains to substitute the values:

So, let's summarize the principle of operation of an amplifier based on a bipolar transistor.

A current I b flows through the base, carrying constant and variable components.

A constant component is needed so that the PN junction between the base and emitter begins to conduct - “opens”.

The variable component is, in fact, the signal itself (useful information).

The collector-emitter current inside the transistor is the result of the base current multiplied by the gain β.
In turn, the voltage across the resistor Rc above the collector is the result of multiplying the amplified collector current by the resistor value.
Thus, the V out pin receives a signal with an increased oscillation amplitude, but with the same shape and frequency. It is important to emphasize that the transistor takes energy for amplification from the VCC power source. If the supply voltage is insufficient, the transistor will not be able to operate fully, and the output signal may be distorted.
Operating modes of a bipolar transistor

In accordance with the voltage levels on the electrodes of the transistor, there are four modes of its operation:

Cut off mode. Active mode..

Active mode

Saturation mode. Reverse mode. Cut-off mode

When the base-emitter voltage is lower than 0.6V - 0.7V, the PN junction between the base and emitter is closed.

In this state, the transistor has 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.

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.

Saturation mode. Inverse mode this mode

the collector and emitter change roles: the collector PN junction is biased in the forward direction, and the emitter junction is biased in the opposite direction.

As a result, current flows from the base to the collector. The collector semiconductor region is asymmetrical to the emitter, and the gain in inverse mode is lower than in normal active mode. The transistor is designed in such a way that it operates as efficiently as possible in active mode. β , , the collector current I changes greatly S. That’s what happens. signal amplification in a bipolar transistor Therefore, the transistor is practically not used in inverse mode.

Basic parameters of a bipolar transistor.

Current gain– ratio of collector current I C to base current I B. Designated (, depending on the specifics of the calculations carried out with transistors.β is a constant value for one transistor, and depends on the physical structure of the device. A high gain is calculated in hundreds of units, a low gain - in tens. For two separate transistors of the same type, even if they were “pipeline neighbors” during production, β may be slightly different. This characteristic of a bipolar transistor is perhaps the most important. If other parameters of the device can often be neglected in calculations, then the current gain is almost impossible. Input impedance

– resistance in the transistor that “meets” the base current. Designated R in R in

). The larger it is, the better for the amplification characteristics of the device, since the source is usually located on the base side weak signal, which needs to consume as little current as possible.

The ideal option is when the input impedance is infinity. R input for an average bipolar transistor is several hundred KΩ (kilo-ohm)., which the amplifier can withstand with minor losses in the overall gain. For example, if a transistor with low output conductivity amplifies the signal 100 times without a load, then when a 1 KΩ load is connected, it will already amplify only 50 times. A transistor with the same gain but higher output conductance will have a smaller gain drop.

The ideal option is when the output conductivity is infinity (or output resistance R out = 0 (R out = 0)). The name of the semiconductor device transistor is formed from two words: transfer - transfer + resist

- resistance. Because it can really be represented in the form of some resistance, which will be regulated by the voltage of one electrode. A transistor is sometimes also called a semiconductor triode.

The first bipolar transistor was created in 1947, and in 1956, three scientists were awarded the Nobel Prize in Physics for its invention. A bipolar transistor is a semiconductor device that consists of three semiconductors with alternating types of impurity conductivity. An electrode is connected and output to each layer. A bipolar transistor uses simultaneously charges whose carriers are electrons ( n - “negative”) and holes (p – “positive

"), that is, carriers of two types, hence the formation of the name prefix “bi” - two.

Transistors differ in the type of layer alternation: P n p

-transistor (direct conduction); Npn-

transistor (reverse conduction). Base

(B) is an electrode that is connected to the central layer of the bipolar transistor. The electrodes from the outer layers are called emitter (E) and collector (K).

Figure 1 – Bipolar transistor design The diagrams indicate “ VT

", in old Russian-language documentation you can find the designations "T", "PP" and "PT". Bipolar transistors are depicted on electrical circuits, depending on the alternation of semiconductor conductivity, as follows:

Figure 2 – Designation of bipolar transistors In Figure 1 above, the difference between the collector and emitter is not visible. If you look at a simplified cross-sectional representation of a transistor, you can see that the area p-n

The collector transition is greater than that of the emitter.

Figure 3 – Cross-section of transistor The base is made of a semiconductor with weak conductivity, that is, the resistance of the material is high. Required condition In Figure 1 above, the difference between the collector and emitter is not visible. If you look at a simplified cross-sectional representation of a transistor, you can see that the area Since the collector and emitter junctions are different, the connection polarity cannot be changed. This characteristic classifies the transistor as an asymmetrical device.

A bipolar transistor has two current-voltage characteristics (volt-ampere characteristics): input and output.

The input current-voltage characteristic is the dependence of the base current ( I B ) from base-emitter voltage ( U BE).

Figure 4 – Input current-voltage characteristic of a bipolar transistor

The output current-voltage characteristic is the dependence of the collector current ( I K ) from the collector-emitter voltage ( U KE).

Figure 5 – Output current-voltage characteristic of the transistor

Let's look at the principle of operation of a bipolar transistor npn type, for pnp similarly, only it is not electrons that are considered, but holes.The transistor has two p-n junctions. In the active operating mode, one of them is connected with forward bias, and the other with reverse bias. When the EB junction is open, electrons from the emitter easily move to the base (recombination occurs). But, as mentioned earlier, the base layer is thin and its conductivity is low, so some electrons have time to move to the base-collector junction. The electric field helps to overcome (strengthens) the layer transition barrier, since electrons are minority carriers here. As the base current increases, the emitter-base junction will open more and more electrons will be able to flow from the emitter to the collector. The collector current is proportional to the base current and with a small change in the latter (control), the collector current changes significantly. This is how the signal is amplified in a bipolar transistor.

Figure 6 – Active mode of transistor operation

Looking at the picture you can explain principle of operation of a transistor a little simpler. Imagine that KE is a water pipe, and B is a faucet with which you can control the flow of water. That is, the more current you apply to the base, the more you will get at the output.

The value of the collector current is almost equal to the emitter current, excluding recombination losses in the base, which forms the base current, so the formula is valid:

I E = I B + I K.

Basic parameters of the transistor:

Current gain is the ratio of the effective value of the collector current to the base current.

Input resistance - following Ohm's law, it will be equal to the emitter-base voltage ratio U EB to control current I B .

Voltage gain – the parameter is determined by the ratio of the output voltage U EC to input U BE.

Frequency response describes the ability of a transistor to operate up to a certain cutoff frequency input signal. After exceeding the maximum frequency, the physical processes in the transistor will not have time to occur and its amplifying abilities will be reduced to nothing.

Switching circuits for bipolar transistors

To connect the transistor, only its three terminals (electrodes) are available to us. Therefore for him normal operation Two power supplies are required. One electrode of the transistor will connect to two sources simultaneously. Consequently, there are 3 connection schemes for a bipolar transistor: OE - with a common emitter, OB - a common base, OK - a common collector. Each has both advantages and disadvantages; depending on the application and required characteristics, the choice of connection is made.

The connection circuit with a common emitter (CE) is characterized by the greatest amplification of current and voltage, and, accordingly, power. At this connection the output alternating voltage is shifted by 180 electrical degrees relative to the input. The main disadvantage is the low frequency response, that is, the low value of the cutoff frequency, which does not make it possible to use it with a high-frequency input signal.

(OB) provides excellent frequency response. But it does not provide such a large signal voltage gain as with OE. And current amplification does not occur at all, therefore this diagram often called a current follower because it has the property of current stabilization.

The circuit with a common collector (CC) has almost the same current gain as with the OE, but the voltage gain is almost equal to 1 (slightly less). The voltage offset is not typical for this connection diagram. I also call it an emitter follower, since the output voltage ( U EB ) correspond to the input voltage.

Application of transistors:

Amplifier circuits;

Signal generators;

Electronic keys.