Which transistor is called bipolar? Bipolar transistors: switching circuits. Connection circuit for a bipolar transistor with a common emitter

Good afternoon, friends!

Today we will continue to get acquainted with the electronic “building blocks” of computer hardware. We have already looked at how they work field effect transistors, which are necessarily present on every motherboard computer.

Sit back comfortably - now we will make an intellectual effort and try to figure out how it works

Bipolar transistor

A bipolar transistor is a semiconductor device that is widely used in electronic products, including computer units nutrition.

The word “transistor” is derived from two English words – “translate” and “resistor”, which means “resistance converter”.

The word “bipolar” means that the current in the device is caused by charged particles of two polarities – negative (electrons) and positive (so-called “holes”).

“Hole” is not jargon, but a completely scientific term. A “hole” is an uncompensated positive charge or, in other words, the absence of an electron in the crystal lattice of a semiconductor.

A bipolar transistor is a three-layer structure with alternating types of semiconductors.

Since there are two types of semiconductors, positive (positive, p-type) and negative (negative, n-type), there can be two types of such a structure - p-n-p and n-p-n.

The middle region of such a structure is called the base, and the outer regions are called the emitter and collector.

In the diagrams, bipolar transistors are designated in a certain way (see figure). We see that the transistor is, essentially, yes p-n junction, connected in series.

A quick question: why can’t the transistor be replaced with two diodes? After all, each of them has a p-n junction, right? I connected two diodes in series - and that’s it!

No! The fact is that the base in the transistor is made very thin during manufacturing, which cannot be achieved by connecting two separate diodes.

The principle of operation of a bipolar transistor

The ratio of the collector current to the base current is called the current gain and can range from several units to several hundred.

It is interesting to note that for low-power transistors it is most often greater than for high-power ones (and not vice versa, as one might think).

The difference is that, unlike the PT gate, during control the base current is always present, i.e. Some power is always spent on control.

The greater the voltage between the emitter and the base, the greater the base current and, accordingly, the greater the collector current. However, any transistor has maximum permissible voltage values ​​between the emitter and base and between the emitter and collector. If you exceed these parameters, you will have to pay with a new transistor.

In operating mode, usually the base-emitter junction is open and the base-collector junction is closed.

A bipolar transistor, like a relay, can also operate in switching mode. If you apply some sufficient current to the base (close button S1), the transistor will be well open. The lamp will light up.

In this case, the resistance between the emitter and collector will be small.

The voltage drop across the emitter-collector section will be several tenths of a volt.

If you then stop supplying current to the base (open S1), the transistor will close, i.e. the resistance between the emitter and collector will become very high.

The lamp will go out.

How to test a bipolar transistor?

Since a bipolar transistor consists of two pn junctions, checking it with a digital tester is quite simple.

It is necessary to set the tester operation switch to position, connecting one probe to the base, and the second – alternately to the emitter and collector.

Essentially, we just check sequentially p-n serviceability transitions.

Such a transition can be either open or closed.

Then you need to change the polarity of the probes and repeat the measurements.

In one case, the tester will show a voltage drop at the emitter-base and collector-base junctions of 0.6 - 0.7 V (both junctions are open).

In the second case, both transitions will be closed, and the tester will record this.

It should be noted that in operating mode, most often one of the transistor transitions is open and the second is closed.

Measuring the current gain of a bipolar transistor

If the tester has the ability to measure the current transfer coefficient, then you can check the operation of the transistor by installing the transistor leads into the appropriate sockets.

Current transfer coefficient is the ratio of the collector current to the base current.

The higher the gain, the more collector current can be controlled by the base current, all other things being equal.

The pinout (pin name) and other data can be taken from the data sheets (reference data) for the corresponding transistor. Data sheets can be found on the Internet through search engines.

The tester will show on the display the current transfer (gain) coefficient, which must be compared with the reference data.

The current transfer coefficient of low-power transistors can reach several hundred.

For powerful transistors it is significantly smaller - several units or tens.

However, there are powerful transistors with a transmission coefficient of several hundred or thousand. These are the so-called Darlington pairs.

A Darlington pair consists of two transistors. The output current of the first transistor is the input current for the second.

The overall current transfer coefficient is the product of the coefficients of the first and second transistors.

The Darlington pair is made in a common housing, but it can also be made from two separate transistors.

Built-in diode protection

Some transistors (power and high voltage) can be protected from reverse voltage by a built-in diode.

Thus, if you connect the tester probes to the emitter and collector in diode testing mode, it will show the same 0.6 - 0.7 V (if the diode is forward biased) or a “blocked diode” (if the diode is reverse biased) .

If the tester shows some small voltage, and in both directions, then The transistor is definitely broken and needs to be replaced. A short can also be determined in resistance measurement mode - the tester will show low resistance.

There is (fortunately, quite rarely) a “mean” malfunction of transistors. This is when it works at first, but after some time (or after warming up) it changes its parameters or fails altogether.

If you unsolder such a transistor and check it with a tester, it will have time to cool before the probes are connected, and the tester will show that it is normal. The best way to verify this is by replacing the “suspicious” transistor in the device.

In conclusion, let’s say that the bipolar transistor is one of the main “pieces of hardware” in electronics. It would be nice to learn to find out whether these “pieces of iron” are “alive” or not. Of course, I have given you, dear readers, a very simplified picture.

In fact, the operation of a bipolar transistor is described by many formulas, there are many varieties of them, but it is a complex science. For those wishing to dig deeper, I can recommend the wonderful book by Horowitz and Hill, “The Art of Circuit Design.”

Transistors for your experiments can be purchased

See you on the blog!

A PNP transistor is an electronic device, in a certain sense the inverse of an NPN transistor. In this type of transistor design, its PN junctions are opened by voltages of reverse polarity with respect to the NPN type. IN 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 possible three connection schemes: with a 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 from it that the collector current I C (in the case 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 scheme The base-emitter PN junction is biased by battery voltage B1, and the collector-base junction is reverse biased by battery voltage B2. The emitter terminal is thus common to the base and collector circuits.

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

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

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

PNP transistor: connecting voltage sources

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

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

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

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

Operation of a PNP transistor stage

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

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

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

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

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

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

Transistor pairs in electric motor control circuits

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

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

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

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

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

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

Determining the type of transistors

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

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

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

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

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

Transition resistance values ​​of transistors of both types

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

In this article we will try to describe operating principle The most common type of transistor is bipolar. Bipolar transistor is one of the main active elements of radio-electronic devices. Its purpose is to work to enhance power electrical signal arriving at its input. Power amplification is carried out through external source energy. A transistor is a radio-electronic component with three terminals

Design feature of a bipolar transistor

To produce a bipolar transistor, you need a semiconductor of hole or electronic conductivity type, which is obtained by diffusion or alloying with acceptor impurities. As a result, regions with polar types of conductivities are formed on both sides of the base.

Bipolar transistors are of two types based on conductivity: n-p-n and p-n-p. The operating rules that govern a bipolar transistor having n-p-n conductivity (for p-n-p it is necessary to change the polarity of the applied voltage):

1. The positive potential at the collector is more important compared to the emitter.
2. Any transistor has its maximum valid parameters Ib, Ik and Uke, exceeding which is in principle unacceptable, since this can lead to destruction of the semiconductor.
3. The base-emitter and base-collector terminals function like diodes. As a rule, the diode in the base-emitter direction is open, and in the base-collector direction it is biased in the opposite direction, that is, the incoming voltage interferes with the flow electric current through him.
4. If steps 1 to 3 are completed, then the current Ik is directly proportional to the current Ib and has the form: Ik = he21*Ib, where he21 is the current gain. This rule characterizes the main quality of the transistor, namely that the low base current controls the powerful collector current.

For different bipolar transistors of the same series, the he21 indicator can fundamentally vary from 50 to 250. Its value also depends on the flowing collector current, the voltage between the emitter and the collector, and on the ambient temperature.

Let's study rule No. 3. It follows from this that the voltage applied between the emitter and the base should not be significantly increased, since if the base voltage is 0.6...0.8 V greater than the emitter (forward voltage of the diode), then an extremely large current will appear. Thus, in a working transistor, the voltages at the emitter and base are interconnected according to the formula: Ub = Ue + 0.6V (Ub = Ue + Ube)

Let us remind you once again that all these points apply to transistors with n-p-n conductivity. For p-n-p type everything should be reversed.

You should also pay attention to the fact that the collector current has no connection with the conductivity of the diode, since, as a rule, the collector-base diode receives reverse voltage. In addition, the current flowing through the collector depends very little on the potential on the collector (this diode is similar to a small current source)

When the transistor is turned on in amplification mode, the emitter junction is open and the collector junction is closed. This is achieved by connecting power supplies.

Since the emitter junction is open, the emitter current will pass through it, arising due to the transition of holes from the base to the emitter, as well as electrons from the emitter to the base. Thus, the emitter current contains two components - hole and electron. The injection ratio determines the efficiency of the emitter. Charge injection is the transfer of charge carriers from the zone where they were the majority to the zone where they become minority.

In the base, electrons recombine, and their concentration in the base is replenished from the plus of the EE source. As a result of this, in electrical circuit a rather weak current will flow from the base. The remaining electrons that did not have time to recombine in the base, under the accelerating influence of the field of the locked collector junction, as minority carriers, will move into the collector, creating a collector current. The transfer of charge carriers from the zone where they were minority to the zone where they become majority is called extraction of electrical charges.

In the considered scheme Ik = Ik0 + aIe » Ie. Therefore, the common base bipolar transistor circuit has a low current transfer coefficient. Because of this, it is rarely used, mainly in high-frequency devices, where it is preferable to others in terms of voltage amplification.

The main circuit for switching on a bipolar transistor is a circuit with a common emitter (Fig. 3).

Rice. 3. Switching on a bipolar transistor according to a common emitter circuit

For it we can write Ib = Ie – Ik = (1 – a)Ie – Ik0.

Considering that 1 – a = 0.001 - 0.1, we have Ib<< Iэ » Iк.

Let's find the ratio of the collector current to the base current:

This relationship is called base current transfer coefficient. At a = 0.99 we obtain b = 100. If a signal source is included in the base circuit, then the same signal, but amplified in current b times, will flow in the collector circuit, forming a voltage on the resistor Rk much greater than the voltage of the signal source .

To evaluate the operation of a bipolar transistor in a wide range of pulsed and direct currents, powers and voltages, as well as to calculate the bias circuit and mode stabilization, families of input and output current-voltage characteristics (volt-ampere characteristics).

Family of input current-voltage characteristics establish the dependence of the input current (base or emitter) on the input voltage Ube at Uk = const, Fig. 4, a. The input current-voltage characteristics of the transistor are similar to the current-voltage characteristics of a diode in direct connection.

The family of output I-V characteristics establishes the dependence of the collector current on the voltage across it at a certain base or emitter current (depending on the circuit with a common emitter or common base), Fig. 4, b.

Rice. 4. Current-voltage characteristics of a bipolar transistor: a – input, b – output

In addition to the n–p electrical junction, a junction based on a metal–semiconductor contact—the Schottky barrier—is widely used in high-speed circuits. In such transitions, no time is spent on the accumulation and dissolution of charges in the base, and the performance of the transistor depends only on the rate of recharging of the barrier capacitance.

Rice. 5. Bipolar transistors

Bipolar transistor parameters

To assess the maximum permissible operating modes of transistors, the following basic parameters are used:

1) maximum permissible collector-emitter voltage(for various transistors Uke max = 10 - 2000 V),

2) maximum permissible collector power dissipation Pk max– according to it, transistors are divided into low power transistors (up to 0.3 W), medium power (0.3 - 1.5 W) and high power (more than 1.5 W); medium and high power transistors are often equipped with a special heat sink device – radiator,

3) maximum permissible collector current Ik max – up to 100 A and more,

4) limiting frequency of current transmission fgr(frequency at which h21 becomes equal to unity), bipolar transistors divide by it:

• for low frequencies – up to 3 MHz,

• mid-frequency – from 3 to 30 MHz,

• high frequency – from 30 to 300 MHz,

• ultra-high frequency – more than 300 MHz.
  

Doctor of Technical Sciences, Professor L. A. Potapov

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

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

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

Connection diagram for a bipolar transistor with a common collector

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

Connection circuit for a bipolar transistor with a common emitter.

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

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

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

What is this?

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

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

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

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

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

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

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

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

How else could we improve our scheme...

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

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