Reverse collector current of a bipolar transistor. MSTU "Mami" - Department of Automation and Control Processes. PNP transistor: connecting voltage sources

The necessary explanations have been given, let's get to the point.

Transistors. Definition and history

Transistor- an electronic semiconductor device in which the current in a circuit of two electrodes is controlled by a third electrode. (transistors.ru)

Field-effect transistors were the first to be invented (1928), and bipolar transistors appeared in 1947 at Bell Labs. And it was, without exaggeration, a revolution in electronics.

Very quickly, transistors replaced vacuum tubes in various electronic devices. In this regard, the reliability of such devices has increased and their size has decreased significantly. And to this day, no matter how “sophisticated” the microcircuit is, it still contains many transistors (as well as diodes, capacitors, resistors, etc.). Only very small ones.

By the way, initially “transistors” were resistors whose resistance could be changed using the amount of applied voltage. If we ignore the physics of processes, then a modern transistor can also be represented as a resistance that depends on the signal supplied to it.

What is the difference between field-effect and bipolar transistors? The answer lies in their very names. In a bipolar transistor, charge transfer involves And electrons, And holes (“encore” - twice). And in the field (aka unipolar) - or electrons, or holes.

Also, these types of transistors differ in application areas. Bipolar ones are used mainly in analog technology, and field ones - in digital technology.

And finally: the main area of ​​application of any transistors- gain weak signal due to additional source nutrition.

Bipolar transistor. Principle of operation. Main characteristics

A bipolar transistor consists of three regions: emitter, base and collector, each of which is supplied with voltage. Depending on the type of conductivity of these areas, n-p-n and p-n-p transistors are distinguished. Typically the collector area is wider than the emitter area. The base is made of a lightly doped semiconductor (which is why it has high resistance) and is made very thin. Since the emitter-base contact area is significantly smaller than the base-collector contact area, it is impossible to swap the emitter and collector by changing the connection polarity. Thus, the transistor is an asymmetrical device.

Before considering the physics of how a transistor operates, let's outline the general problem.


It is as follows: a strong current flows between the emitter and collector ( collector current), and between the emitter and base there is a weak control current ( base current). The collector current will change depending on the change in base current. Why?
Let's consider the p-n junctions of the transistor. There are two of them: emitter-base (EB) and base-collector (BC). IN active mode operation of the transistor, the first of them is connected with forward bias, and the second with reverse bias. What happens at the p-n junctions? For greater certainty, we will consider an n-p-n transistor. For p-n-p everything is similar, only the word “electrons” needs to be replaced with “holes”.

Since the EB junction is open, electrons easily “run across” to the base. There they partially recombine with holes, but O Most of them, due to the small thickness of the base and its low doping, manage to reach the base-collector transition. Which, as we remember, is reverse biased. And since electrons in the base are minority charge carriers, the electric field of the transition helps them overcome it. Thus, the collector current is only slightly less than the emitter current. Now watch your hands. If you increase the base current, the EB junction will open more, and more electrons will be able to slip between the emitter and collector. And since the collector current is initially greater than the base current, this change will be very, very noticeable. Thus, the weak signal received at the base will be amplified. Once again, a large change in collector current is a proportional reflection of a small change in base current.

I remember that the principle of operation of a bipolar transistor was explained to my classmate using the example of a water tap. The water in it is the collector current, and the base control current is how much we turn the knob. Enough little effort(control action) so that the flow of water from the tap increases.

In addition to the processes considered, a number of other phenomena can occur at the p-n junctions of the transistor. For example, with a strong increase in voltage at the base-collector junction, avalanche charge multiplication may begin due to impact ionization. And coupled with the tunnel effect, this will give first an electrical breakdown, and then (with increasing current) a thermal breakdown. However, thermal breakdown in a transistor can occur without electrical breakdown (i.e., without increasing the collector voltage to breakdown voltage). One will be enough for this excessive current through the collector.

Another phenomenon is due to the fact that when the voltages on the collector and emitter junctions change, their thickness changes. And if the base is too thin, then a closing effect may occur (the so-called “puncture” of the base) - a connection between the collector junction and the emitter junction. In this case, the base region disappears and the transistor stops working normally.

The collector current of the transistor in the normal active mode of operation of the transistor is greater than the base current by a certain number of times. This number is called current gain and is one of the main parameters of the transistor. It is designated h21. If the transistor is turned on without load on the collector, then at a constant collector-emitter voltage the ratio of the collector current to the base current will give static current gain. It can be equal to tens or hundreds of units, but it is worth considering the fact that in real circuits this coefficient is smaller due to the fact that when the load is turned on, the collector current naturally decreases.

The second important parameter is transistor input resistance. According to Ohm's law, it is the ratio of the voltage between the base and emitter to the control current of the base. The larger it is, the lower the base current and the higher the gain.

The third parameter of a bipolar transistor is voltage gain. He equal to the ratio amplitude or effective values output (emitter-collector) and input (base-emitter) alternating voltages. Since the first value is usually very large (units and tens of volts), and the second is very small (tenths of volts), this coefficient can reach tens of thousands of units. It is worth noting that each base control signal has its own voltage gain.

Transistors also have frequency response, which characterizes the transistor’s ability to amplify a signal whose frequency approaches the cut-off amplification frequency. The fact is that with increasing frequency input signal the gain decreases. This is due to the fact that the passage time of the main physical processes(the time of movement of carriers from the emitter to the collector, the charge and discharge of barrier capacitive junctions) becomes commensurate with the period of change of the input signal. Those. the transistor simply does not have time to react to changes in the input signal and at some point simply stops amplifying it. The frequency at which this happens is called boundary.

Also, the parameters of the bipolar transistor are:

• reverse current collector-emitter
• on time
• reverse collector current
• maximum permissible current

Conditional n-p-n notation And pnp transistors They differ only in the direction of the arrow indicating the emitter. It shows how current flows in a given transistor.

Operating modes of a bipolar transistor

The option discussed above represents the normal active mode of operation of the transistor. However, there are several other open/closed combinations p-n junctions, each of which represents separate mode transistor operation.

1. Inverse active mode. Here the BC transition is open, but on the contrary, the EB is closed. The amplification properties in this mode, of course, are worse than ever, so transistors are used very rarely in this mode.
2. Saturation mode. Both crossings are open. Accordingly, the main charge carriers of the collector and emitter “run” to the base, where they actively recombine with its main carriers. Due to the resulting excess of charge carriers, the resistance of the base and p-n junctions decreases. Therefore, a circuit containing a transistor in saturation mode can be considered short-circuited, and this radio element itself can be represented as an equipotential point.
3. Cut-off mode. Both transitions of the transistor are closed, i.e. the current of the main charge carriers between the emitter and collector stops. Flows of minority charge carriers create only small and uncontrollable thermal transition currents. Due to the poverty of the base and transitions with charge carriers, their resistance increases greatly. Therefore, it is often believed that a transistor operating in cutoff mode represents an open circuit.
4. Barrier mode In this mode, the base is directly or through a low resistance connected to the collector. A resistor is also included in the collector or emitter circuit, which sets the current through the transistor. This creates the equivalent of a diode circuit with a resistor in series. This mode is very useful, as it allows the circuit to operate at almost any frequency, over a wide temperature range and is undemanding to the parameters of the transistors.

Switching circuits for bipolar transistors

Since the transistor has three contacts, then general case Power must be supplied to it from two sources, which together produce four outputs. Therefore, one of the transistor contacts has to be supplied with a voltage of the same sign from both sources. And depending on what kind of contact it is, there are three circuits for connecting bipolar transistors: with common emitter(OE), common collector (OK) and common base(ABOUT). Each of them has both advantages and disadvantages. The choice between them is made depending on which parameters are important to us and which can be sacrificed.

Connection circuit with common emitter

This circuit provides the greatest gain in voltage and current (and hence in power - up to tens of thousands of units), and therefore is the most common. Here the emitter-base junction is turned on directly, and the base-collector junction is turned on reversely. And since both the base and the collector are supplied with voltage of the same sign, the circuit can be powered from one source. In this circuit, the output phase AC voltage changes relative to the phase of the input AC voltage by 180 degrees.

But in addition to all the goodies, the OE scheme also has a significant drawback. It lies in the fact that an increase in frequency and temperature leads to a significant deterioration in the amplification properties of the transistor. Thus, if the transistor is to operate at high frequencies, then it is better to use a different switching circuit. For example, with a common base.

Connection diagram with a common base

This circuit does not provide significant signal amplification, but is good at high frequencies, since it allows more full use of the frequency response of the transistor. If the same transistor is connected first according to a circuit with a common emitter, and then with a common base, then in the second case one will observe significant increase its limiting amplification frequency. Since with such a connection the input resistance is low and the output resistance is not very high, transistor cascades assembled according to the circuit with OB are used in antenna amplifiers, Where characteristic impedance cables usually does not exceed 100 ohms.

In a common-base circuit, the signal phase does not invert, and the noise level at high frequencies is reduced. But, as already mentioned, its current gain is always slightly less than unity. True, the voltage gain here is the same as in a circuit with a common emitter. The disadvantages of a common base circuit also include the need to use two power supplies.

Connection diagram with a common collector

The peculiarity of this circuit is that the input voltage is completely transmitted back to the input, i.e. the negative feedback is very strong.

Let me remind you that negative feedback is such feedback in which the output signal is fed back to the input, thereby reducing the level of the input signal. Thus it happens automatic adjustment in case of accidental change of input signal parameters

The current gain is almost the same as in the common emitter circuit. But the voltage gain is small (the main drawback of this circuit). It approaches unity, but is always less than it. Thus, the power gain is equal to only a few tens of units.

In a common collector circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, output voltage the phase and amplitude coincides with the input one, i.e., repeats it. That is why such a circuit is called an emitter follower. Emitter - because the output voltage is removed from the emitter relative to the common wire.

This connection is used to match transistor stages or when the input signal source has a high input impedance (for example, a piezoelectric pickup or a condenser microphone).

Two words about cascades

There are times when you need to increase output power(i.e. increase collector current). In this case, parallel connection of the required number of transistors is used.

Naturally, they should be approximately the same in characteristics. But it must be remembered that the maximum total collector current should not exceed 1.6-1.7 of the maximum collector current of any of the cascade transistors.
However (thanks for the note), it is not recommended to do this in the case of bipolar transistors. Because two transistors, even of the same type, are at least slightly different from each other. Accordingly, when connected in parallel, currents will flow through them different sizes. To equalize these currents, balanced resistors are installed in the emitter circuits of the transistors. The value of their resistance is calculated so that the voltage drop across them in the operating current range is at least 0.7 V. It is clear that this leads to a significant deterioration in the efficiency of the circuit.

There may also be a need for a transistor with good sensitivity and at the same time good gain. In such cases, a cascade of a sensitive but low-power transistor (VT1 in the figure) is used, which controls the power supply of a more powerful fellow (VT2 in the figure).

Other applications of bipolar transistors

Transistors can be used not only in signal amplification circuits. For example, due to the fact that they can operate in saturation and cutoff modes, they are used as electronic keys. It is also possible to use transistors in signal generator circuits. If they operate in key mode, then a square wave signal will be generated, and if in amplification mode, then a signal free form, depending on the control action.

Marking

Since the article has already grown to an indecently large volume, at this point I will simply give two good links, which describe in detail the main marking systems semiconductor devices(including transistors): http://kazus.ru/guide/transistors/mark_all.html and file.xls (35 kb).

Helpful comments:
http://habrahabr.ru/blogs/easyelectronics/133136/#comment_4419173

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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 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 bipolar transistor The PNP type has current flowing out of the base, and the NPN type has flowing 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 with an NPN one in most electronic circuits, the only difference is the polarity of the voltage and the direction of the current. 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 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 loudspeaker 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 opposite direction, which will entail its reversal.

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.

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 gain of the transistor according to DC, which is usually denoted Hfe 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 that 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 R b is known, the transistor is "set" to act 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 you need to use a single gate in a circuit and you don't want to use a 14-pin 4-gate chip either due to cost or board space. It can be replaced with a pair of transistors. Note that frequency characteristics such elements depend on the characteristics and type of transistors, but 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 in the opposite direction.

Finding errors in transistor circuits Oh

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.

Let's briefly consider the work n-р-n-transistor. At the interface between semiconductors and n(electronic) - and R(hole)-type conductivities, due to diffusion, a region of opposite space charges arises. It is formed by ionized atoms of acceptor and donor impurities and is depleted of mobile charge carriers: electrons and holes. The field of contact potential difference formed between charges is a potential barrier that prevents the diffusion transition of carriers.

If the emitter junction is forward biased (as shown in Fig. 4), then the potential barrier decreases and electrons will be injected from the emitter into the base. The hole concentration in the base is usually significantly lower than the electron concentration in the emitter, and the injection of holes into the emitter can be neglected. Therefore the emitter current i 3 is formed by the electronic component of the carrier flow. Electrons injected from the emitter are minority charge carriers in the base and will, mainly due to diffusion, move through the base towards the collector junction. A positive voltage is applied to the collector relative to the base, which corresponds to the reverse bias of the collector junction. Electrons that reach the collector junction are drawn by its field into the collector region and form a collector current i j. Since the thickness of the base is small and the concentration of holes in it is small, then only small part electrons recombine (unite) with holes in the base; the remaining electrons reach the collector junction. The recombination of electrons in the base causes a corresponding current in external circuit- base current i b.

There are obvious relationships between the emitter, base and collector currents:

where α is the emitter current transfer coefficient; it takes, depending on the type of transistor, values ​​in the range from 0.95 to 0.99. From the above relations we obtain the dependence of the collector current on the base current:

Parameter (3)

is called the base current transfer coefficient and is 20÷100. They say that the base current is amplified in the transistor.

3.3. Current-voltage characteristics of bipolar
transistor in a common emitter circuit

The properties of a bipolar transistor are determined by families of static current-voltage characteristics that express the relationship between its currents and voltages. The type of these characteristics depends on the transistor connection circuit. The most popular is the common emitter circuit (Fig. 5). The input characteristics are the family i b = F(u b) at u ke = const (Fig. 6, a). They are similar to characteristics semiconductor diode. The output characteristics represent the family i k = F(u ke) at

i b = const (Fig. 6, b).

At low u ke when i b >0 (i.e. u be ≥ 0.6 V), the collector junction (like the emitter junction) turns out to be forward biased, so not all electrons injected into the base end up in the collector region.

The transistor operates here in mode saturation , since an increase in the base current does not lead to an increase in the collector current. The characteristics corresponding to this mode are merged into a line B. Further with growth u ke collector current i at first it grows quickly and then remains almost unchanged.

As the base current increases, which is part of the emitter current, the collector current also increases, and static characteristics move upward. The transistor works here in active mode and acts as a current regulator. It should be noted that the connection between the collector and base currents is quite linear, which is manifested in the equidistant arrangement of flat sections of the collector characteristics. Finally, when the emitter junction is reverse biased (i.e. u bae< 0,6 В) последний заперт, и через транзистор протекает неуправляемый (его называют сквозным) ток i kes. This mode is called current cutoff mode. Characteristic i b = 0 (line A) separates the active mode and cutoff regions.

3.4. Description of the transistor by h-parameters and its
equivalent circuit

When analyzing transistor circuits in small-signal mode, it is convenient to represent the transistor as a linear two-terminal network (Fig. 7) and describe the relationship of currents and voltages at the input and output with four parameters. To describe transistors, so-called hybrid transistors are usually used, which are easy to measure. h-options; let's introduce them.

Let us take the input current as independent variables i 1 and output voltage u 2. Then the input voltage u 1 and output current i 2 will be some nonlinear functions of the selected independent variables:

For small changes in currents and voltages, the increments of input voltage and output current for the active region can be written as

Here the derivatives are calculated for some constant values ​​of current and voltage I 1,0 , U 2.0, which characterize the DC mode of the transistor. Let's denote these constants

The role of small increments can be played by small alternating currents and voltages with amplitudes I 1 , I 2 and U 1 , U 2. Then the dependence between alternating currents and the voltages in the transistor will be described by the system linear equations With h-parameters:

(4a)

. (4b)

According to (4), the parameter h 11 is the input resistance of the transistor, and h 21 - current transfer coefficient at short circuit exit ( U 2 = 0); h 22 - output conductivity, and h 12 - coefficient feedback by voltage with the input open ( I 1 = 0). Parameter h 21 is equal to α for a common base circuit and β for a common emitter circuit.

Specific values h-parameters vary for different types transistors, their connection circuits and DC mode I 1,0 , U 2,0 ; h-parameters can also be calculated from static current-voltage characteristics transistor, if the latter are known.

In accordance with equations (4), the transistor can be formally represented by the equivalent circuit shown in Fig. 8. Current generator h 21 I 1, in the output circuit takes into account the effect of current amplification, and the generator h 12 U 2 reflects the presence of feedback voltage in the input circuit.

An equivalent circuit of this type can be used to study transistor circuits with a small harmonic signal in wide range frequency In this case, equations (4) are written for complex amplitudes of currents and voltages, and they themselves h-parameters will be frequency-dependent complex quantities. For relatively low frequencies h-parameters can be considered constants for the selected DC mode of the transistor. For example, for silicon n-р-n-transistor KT315B at I k0 = 1 mA, U ke0 = 10 V h-parameters in a circuit with a common emitter usually lie in the ranges of values:

Device and principle of operation

The first transistors were made from germanium. Currently, they are made primarily from silicon and gallium arsenide. The latter transistors are used in high-frequency amplifier circuits. A bipolar transistor consists of three in various ways doped semiconductor zones: emitter E, bases B and collector C. Depending on the type of conductivity of these zones, NPN (emitter - n-semiconductor, base - p-semiconductor, collector - n-semiconductor) and PNP transistors are distinguished. Conductive contacts are connected to each of the zones. The base is located between the emitter and collector and is made of a lightly doped semiconductor with high resistance. The total base-emitter contact area is significantly smaller than the collector-base contact area (this is done for two reasons - the large area of ​​the collector-base junction increases the likelihood of minority charge carriers being extracted into the collector, and since in operating mode the collector-base junction is usually switched on in reverse bias, which increases heat generation and promotes heat removal from the collector), therefore the bipolar transistor general view is asymmetrical device(it is impossible to swap the emitter and collector by changing the polarity of the connection and resulting in a bipolar transistor absolutely similar to the original one).

In the active operating mode, the transistor is turned on so that its emitter junction is biased in the forward direction (open), and the collector junction is biased in the opposite direction (closed). For definiteness, let's consider npn transistor, all reasoning is repeated absolutely similarly for the case pnp transistor, replacing the word “electrons” with “holes”, and vice versa, as well as replacing all voltages with opposite signs. IN npn transistor, electrons, the main current carriers in the emitter, pass through open passage emitter-base (injected) into the base area. Some of these electrons recombine with the majority charge carriers in the base (holes). However, because the base is made very thin and relatively lightly doped, most of the electrons injected from the emitter diffuse into the collector region. The strong electric field of the reverse-biased collector junction captures electrons and carries them into the collector. The collector current is thus practically equal to the emitter current, with the exception of a small recombination loss in the base, which forms the base current (I e = I b + I k). The coefficient α connecting the emitter current and the collector current (I k = α I e) is called the emitter current transfer coefficient. The numerical value of the coefficient α is 0.9 - 0.999. The higher the coefficient, the more efficiently the transistor transmits current. This coefficient depends little on the collector-base and base-emitter voltages. Therefore, over a wide range of operating voltages, the collector current is proportional to the base current, the proportionality coefficient is equal to β = α / (1 − α) = (10..1000). Thus, by changing the small base current, you can control significantly high current collector.

Operating modes of a bipolar transistor

Normal active mode

The emitter-base junction is connected in the forward direction (open), and the collector-base junction is in the reverse direction (closed)
U EB >0;U KB<0 (для транзистора p-n-p типа, для транзистора n-p-n типа условие будет иметь вид U ЭБ <0;U КБ >0);

Inverse active mode

The emitter junction has a reverse connection, and the collector junction has a direct connection.

Saturation mode

Both pn junctions are forward biased (both open). 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 Ueb and Ukb. 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 saturation currents of the emitter (IE.sat) and collector (IK) will flow through the emitter and collector of the transistor. us).

Cut-off mode

In this mode, both p-n junctions of the device are biased in the opposite direction (both are closed). The cutoff mode of the 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 currents of the emitter (IEBO) and collector (ICBO) flow through both p-n junctions. 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).

Barrier mode

In this mode base transistor for direct current is connected short-circuited or through a small resistor with its collector, and in collector or in emitter The transistor circuit is turned on by a resistor that sets the current through the transistor. In this connection, the transistor is a kind of diode connected in series with a current-setting resistor. Such cascade circuits are distinguished by a small number of components, good high-frequency isolation, a large operating temperature range, and insensitivity to transistor parameters.

Connection schemes

Any transistor connection circuit is characterized by two main indicators:

• Current gain I out / I in.
• Input resistance Rin =Uin /Iin

Connection diagram with a common base

Amplifier with a common base.

• Among all three configurations, it has the lowest input and highest output impedance. It has a current gain close to unity and a high voltage gain. The signal phase is not inverted.
• Current gain: I out /I in =I to /I e =α [α<1]
• Input resistance R in =U in /I in =U be /I e.

The input resistance for a circuit with a common base is small and does not exceed 100 Ohms for low-power transistors, since the input circuit of the transistor is an open emitter junction of the transistor.

Advantages:

• Good temperature and frequency properties.
• High permissible voltage

Disadvantages of a common base scheme:

• Low current gain because α< 1
• Low input impedance
• Two different voltage sources for power.

Connection circuit with common emitter

• Current gain: I out /I in =I to /I b =I to /(I e -I to) = α/(1-α) = β [β>>1]
• Input resistance: R in =U in /I in =U be /I b

Advantages:

• High current gain
• High voltage gain
• Highest power gain
• You can get by with one power source
• The output AC voltage is inverted relative to the input.

Flaws:

• Worse temperature and frequency properties compared to a common base circuit

Common collector circuit

• Current gain: I out /I in =I e /I b =I e /(I e -I k) = 1/(1-α) = β [β>>1]
• Input resistance: R in = U in / I in = (U b e + U k e) / I b

Advantages:

• High input impedance
• Low output impedance

Flaws:

• The voltage gain is less than 1.

A circuit with this connection is called an “emitter follower”

Main settings

• Current transfer coefficient
• Input impedance
• Output conductivity
• Reverse current collector-emitter
• On time
• Limit frequency of base current transfer coefficient
• Reverse collector current
• Maximum permissible current
• Cutoff frequency of current transfer coefficient in a circuit with a common emitter

Transistor parameters are divided into intrinsic (primary) and secondary. Intrinsic parameters characterize the properties of the transistor, regardless of its connection circuit. The following are taken as the main own parameters:

• current gain α;
• resistance of the emitter, collector and base to alternating current r e, r k, r b, which are:
  • r e - the sum of the resistances of the emitter region and the emitter junction;
  • r k - the sum of the resistances of the collector area and the collector junction;
  • r b - transverse resistance of the base.

Equivalent circuit of a bipolar transistor using h-parameters

Secondary parameters are different for various schemes switching on of the transistor and, due to its nonlinearity, are valid only for low frequencies and small amplitudes of signals. For secondary parameters, several systems of parameters and their corresponding equivalent circuits. The main ones are mixed (hybrid) parameters, denoted by the letter “h”.

Input impedance- transistor resistance to input alternating current in the event of a short circuit at the output. The change in input current is the result of a change in the input voltage, without the influence of feedback from the output voltage.

H 11 = U m1 /I m1 at U m2 = 0.

Voltage feedback factor shows what proportion of the output alternating voltage is transferred to the input of the transistor due to feedback in it. There is no alternating current in the input circuit of the transistor, and a change in the input voltage occurs only as a result of a change in the output voltage.

H 12 = U m1 /U m2 at I m1 = 0.

Current transfer coefficient(current gain) shows the gain of AC current at zero load resistance. The output current depends only on the input current without being influenced by the output voltage.

H 21 = I m2 /I m1 at U m2 = 0.

Output conductivity- internal conductivity for alternating current between output terminals. The output current changes under the influence of the output voltage.

H 22 = I m2 /U m2 at I m1 = 0.

The relationship between alternating currents and transistor voltages is expressed by the equations:

U m1 = h 11 I m1 + h 12 U m2 ;
I m2 = h 21 I m1 + h 22 U m2.

Depending on the transistor connection circuit, letters are added to the digital indices of the h-parameters: “e” - for the OE circuit, “b” - for the OB circuit, “k” - for the OK circuit.

For the OE circuit: I m1 = I mb, I m2 = I mk, U m1 = U mb-e, U m2 = U mk-e. For example, for this scheme:

H 21e = I mк /I mb = β.

For the OB circuit: I m1 = I mе, I m2 = I mк, U m1 = U mе-b, U m2 = U mк-b.

The transistor's own parameters are related to the h-parameters, for example for an OE circuit:

;

;

;

.

With increasing frequency, the collector junction capacitance C k begins to have a harmful effect on the operation of the transistor. The resistance of the capacitance decreases, the current through the load resistance and, consequently, the gain factors α and β decreases. The resistance of the emitter junction capacitance C e also decreases, however, it is shunted by a small junction resistance r e and in most cases may not be taken into account. In addition, with increasing frequency, an additional decrease in the coefficient β occurs as a result of a lag in the phase of the collector current from the phase of the emitter current, which is caused by the inertia of the process of moving carriers through the base from the emitter junction to the collector and the inertia of the processes of accumulation and resorption of charge in the base. Frequencies at which the coefficients α and β decrease by 3 dB are called cutoff frequencies current transfer coefficient for the OB and OE schemes, respectively.

In the pulse mode, the collector current pulse begins with a delay by a delay time τ з relative to the input current pulse, which is caused by the finite travel time of the carriers through the base. As carriers accumulate in the base, the collector current increases during the rise time τ f. On time transistor is called τ on = τ h + τ f.

Transistor manufacturing technology

• Epitaxial-planar
• Splavnaya
  • Diffusion
  • Diffusion-alloy

Application of transistors

• Demodulator (Detector)
• Inverter (logic element)
• Microcircuits based on transistor logic (see transistor-transistor logic, diode-transistor logic, resistor-transistor logic)

see also

Literature

Notes

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