What is the difference between NPN and PNP transistors? Bipolar transistor connection circuits

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The design and principle of operation of a bipolar transistor

A bipolar transistor is a semiconductor device that has two electron-hole junctions formed in one semiconductor single crystal. These transitions form three regions in the semiconductor with different types of electrical conductivity. One extreme region is called the emitter (E), the other - the collector (K), the middle - the base (B). Metal leads are soldered to each area to connect the transistor to the electrical circuit.
The electrical conductivity of the emitter and collector is opposite to the electrical conductivity of the base. Depending on the order of alternation of p- and n-regions, transistors with p-n-p and n-p-n structures are distinguished. Conventional graphic symbols for p-n-p and n-p-n transistors differ only in the direction of the arrow at the electrode indicating the emitter.

The operating principles of p-n-p and n-p-n transistors are the same, so in the future we will only consider the operation of a transistor with a p-n-p structure.
The electron-hole junction formed by the emitter and the base is called the emitter, and the collector and base is called the collector. The distance between the junctions is very small: for high-frequency transistors it is less than 10 micrometers (1 μm = 0.001 mm), and for low-frequency transistors it does not exceed 50 μm.
When the transistor is operating, its junctions receive external voltages from the power source. Depending on the polarity of these voltages, each junction can be turned on in either the forward or reverse direction. There are three operating modes of the transistor: 1) cutoff mode - both transitions and, accordingly, the transistor are completely closed; 2) saturation mode - the transistor is completely open; 3) active mode - this is a mode intermediate between the first two. The cutoff and saturation modes are used together in key stages, when the transistor is alternately completely open or completely closed with the frequency of the pulses arriving at its base. Cascades operating in switching mode are used in pulsed circuits (switching power supplies, horizontal scan output stages of televisions, etc.). The output stages of power amplifiers can operate partially in cutoff mode.
Transistors are most often used in active mode. This mode is determined by applying a small voltage to the base of the transistor, which is called bias voltage (U cm). The transistor opens slightly and current begins to flow through its transitions. The principle of operation of the transistor is based on the fact that a relatively small current flowing through the emitter junction (base current) controls a larger current in the collector circuit. The emitter current is the sum of the base and collector currents.

Operating modes of a bipolar transistor

Cut-off mode transistor is obtained when the emitter and collector p-n junctions are connected to external sources in the opposite direction. In this case, very small reverse emitter currents flow through both pn junctions ( I EBO) And collector ( I KBO). The base current is equal to the sum of these currents and, depending on the type of transistor, ranges from units of microamps - µA (for silicon transistors) to units of milliamps - mA (for germanium transistors).

If the emitter and collector p-n junctions are connected to external sources in the forward direction, the transistor will be in saturation mode . The diffusion electric field of the emitter and collector junctions will be partially weakened by the electric field created by external sources U EB And U KB. As a result, the potential barrier that limited the diffusion of the main charge carriers will decrease, and the penetration (injection) of holes from the emitter and collector into the base will begin, that is, currents called emitter saturation currents will flow through the emitter and collector of the transistor ( I E.us) and collector ( I K.us).

Used to amplify signals active mode of operation of the transistor .
When the transistor is operating in the active mode, its emitter junction is switched on in the forward direction, and the collector junction is switched on in the reverse direction.

Under direct voltage U EB injects holes from the emitter into the base. Once in the n-type base, holes become minority charge carriers in it and, under the influence of diffusion forces, move (diffuse) to the collector p-n junction. Some of the holes in the base are filled (recombined) with the free electrons present in it. However, the width of the base is small - from several units to 10 microns. Therefore, the main part of the holes reaches the collector p-n junction and is transferred by its electric field to the collector. Obviously, the collector current I Kp cannot be greater than the emitter current, since some of the holes recombine in the base. That's why I Kp = h 21B I uh
Magnitude h 21B is called the static transfer coefficient of the emitter current. For modern transistors h 21B= 0.90…0.998. Since the collector junction is switched in the opposite direction (often said - biased in the opposite direction), reverse current also flows through it I KBO, formed by minority carriers of the base (holes) and collector (electrons). Therefore, the total collector current of a transistor connected according to a circuit with a common base
I To = h 21B I uh +I BWC
Holes that did not reach the collector junction and recombined (filled) in the base give it a positive charge. To restore the electrical neutrality of the base, the same number of electrons is supplied to it from the external circuit. The movement of electrons from the external circuit to the base creates a recombination current in it I B.rec. In addition to the recombination current, the reverse collector current flows through the base in the opposite direction and the full base current
I B = I B.rek - I KBO
In active mode, the base current is tens and hundreds of times less than the collector current and emitter current.

Bipolar transistor connection circuits

In the previous diagram, the electrical circuit formed by the source U EB, emitter and base of the transistor, is called input, and the circuit formed by the source U KB, collector and base of the same transistor, - output. The base is the common electrode of the transistor for the input and output circuits, so this connection is called a circuit with a common base, or for short "OB scheme".
The following figure shows a circuit in which the emitter is the common electrode for the input and output circuits. This is a common emitter circuit, or "OE diagram".

KI– current gain

K U– voltage gain

KP– power gain

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Good afternoon, friends!

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

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 power supplies.

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 a 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 large.

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 are simply sequentially checking the health of the p-n junctions.

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 corresponding 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 transfer 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!

There is an obvious difference between a simple switching circuit and a linear transistor amplifier. In a normally operating linear amplifier, the collector current is always directly proportional to the base current. In a switching circuit such as in Fig. 1., the collector current is mainly determined by the supply voltage V CC and the load resistance R L. The saturation mode of the transistor is quite important and deserves detailed discussion.

Rice. 1. Illustration of saturation mode. The transistor acts as a switch to turn on the lamp.

Let's consider what happens to the collector current in the circuit in Fig. 1 if the base current is gradually increased starting from zero. When switch S 1 is open, no base current flows and the collector current is negligible. The short circuit of S 1 leads to the appearance of a base current I B = V CC /R B, where we have neglected the potential difference at the base-emitter junction. The collector current flowing through the load R L is equal to I C = h FE V CC / R B . For the specific circuit shown in the figure, with h FE = 100 and at the maximum value of R B (50 kOhm), we obtain:

I C =100x10/5000 A=20 mA

The voltage drop across R L is determined by the product R L I C and in our case is equal to 50 x 0.02 = 1 V. The transistor is in linear mode; a decrease in R B results in an increase in the base current, an increase in the collector current and, therefore, an increase in the voltage drop across R L . Under these conditions, the circuit could be used as a voltage amplifier.

Now consider the case when

and the base current is

I B =V CC /R B =V CC /(h FE R L)

Therefore, the collector current is equal to

I C =(h FE V CC)/(h FE R L)=V CC /R L

From a load point of view, the transistor behaves like a pair of switch contacts. It follows from Ohm's law that the load current in this situation cannot exceed the value V CC /R L. Therefore, a further increase in the base current cannot increase the collector current, which is now determined only by the load resistance and supply voltage. The transistor is in saturation. In practice, when a transistor is saturated, there is always a small voltage remaining between the collector and emitter, usually denoted V CE(sat). As a rule, it is less than 1 V and can reach up to 0.1 V for transistors specifically designed to work as switches. Typically, V CE(sat) decreases as more current flows through the base-emitter junction, that is, when the ratio of collector current IC to base current I B becomes significantly less than the transistor current gain h FE.

Roughly speaking, deep saturation (small value of V CE(sat)) occurs when

I C /I B< h FE /5

For a circuit like the one shown in Fig. 1, when the base current is given by simply connecting a resistor to the power supply, we select

R B /R L< h FE /5

Therefore, for the circuit in Fig. 1, taking the value of the current gain h FE = 150 typical for the 2N3053 transistor (analogue of KT630B - see analogues of domestic and foreign transistors), we have

R B /R L< 150/5 = 30.

Therefore, with R L = 50 Ohm we choose

R B< 30 х 50 Ом = 1,5 кОм.

So, if a lamp with a resistance of 50 Ohms is used as a load, then to turn it on effectively we should choose a base resistor value of less than 1.5 kOhm. If this is not possible, when, for example, a photoresistor with a minimum resistance of 10 kOhm is used as RB, then a Darlington circuit should be used to increase the current gain.

If the bipolar transistor operates with a collector current close to the maximum, and it is necessary to maintain the voltage V CE (sat) at the level of fractions of a volt, then due to the decrease in h FE, a base current may be required greater than I s /10.

It may be surprising that V CE(sat) can be much less than the voltage V BE, which for a silicon transistor is approximately 0.6 V. This happens because in saturation mode the collector-base junction is forward biased. Consequently, we have two p-n junctions, biased in the forward direction, connected towards each other so that the voltage drops across them are mutually compensated. This ability of a bipolar transistor to have a very small voltage drop between collector and emitter in saturation mode makes it a very useful switching device. Many of the most important applications of electronics, including the vast field of digital electronics, use switching circuits.

In switching mode, the transistor operates with either virtually zero collector current (transistor off) or virtually zero collector voltage (transistor on). In both cases, the power dissipated by the transistor is very small. Significant power is dissipated only when switching occurs: at this time, both the collector-emitter voltage and the collector current have finite values.

A low-power transistor such as the 2N3053, with a maximum power dissipation of less than one watt, can switch power into a load of several watts. Please note that the maximum values ​​of collector voltage and current should not exceed permissible limits; In addition, it is desirable to switch as quickly as possible to avoid dissipating excessive power.

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)

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- strengthening of a weak signal due to an additional power source.

Bipolar transistor. Operating principle. Main Features

Before considering the physics of how a transistor operates, let us 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 the active mode of 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 strongly, 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. A small force (control action) is enough to increase the flow of water from the tap.

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 first give 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 excessive current through the collector will be enough for this.

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. It is equal to the ratio of the amplitude or effective values ​​of the 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 as the frequency of the input signal increases, the gain decreases. This is due to the fact that the time of occurrence of the main physical processes (the time of movement of carriers from the emitter to the collector, the charge and discharge of capacitive barrier 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

The symbols for n-p-n and p-n-p transistors 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

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, in general, 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 a common emitter (CE), a common collector (OC) and a common base (CB). 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

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 must 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

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

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, automatic adjustment occurs when the input signal parameters accidentally change

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, the output voltage matches the input voltage in phase and amplitude, 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 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

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In this article we will talk about the transistor. We will show the diagrams for its connection and the calculation of a transistor cascade with a common emitter.

TRANSISTOR is a semiconductor device for amplifying, generating and converting electrical vibrations, made on the basis of a single-crystal semiconductor ( Si– silicon, or Ge- germanium), containing at least three areas with different electronic ( n) and hole ( p) - conductivity. Invented in 1948 by Americans W. Shockley, W. Brattain and J. Bardeen. Based on their physical structure and current control mechanism, transistors are divided into bipolar (more often called simply transistors) and unipolar (more often called field-effect transistors). In the first, containing two or more electron-hole transitions, both electrons and holes serve as charge carriers; in the second, either electrons or holes. The term “transistor” is often used to refer to portable broadcast receivers based on semiconductor devices.

The current in the output circuit is controlled by changing the input voltage or current. A small change in input quantities can lead to a significantly larger change in output voltage and current. This amplifying property of transistors is used in analog technology (analog TV, radio, communications, etc.).

Bipolar transistor

A bipolar transistor can be n-p-n And p-n-p conductivity. Without looking into the insides of the transistor, one can note the difference in conductivity only in the polarity of the connection in practical circuits of power supplies, capacitors, and diodes that are part of these circuits. The figure on the right graphically shows n-p-n And p-n-p transistors.

The transistor has three terminals. If we consider a transistor as a four-terminal network, then it should have two input and two output terminals. Therefore, one of the pins must be common for both the input and output circuits.

Transistor connection circuits

Connection circuit for a transistor with a common emitter– designed to amplify the amplitude of the input signal in voltage and current. In this case, the input signal, amplified by the transistor, is inverted. In other words, the phase of the output signal is rotated 180 degrees. This circuit is the main one for amplifying signals of different amplitudes and shapes. The input resistance of a transistor cascade with an OE ranges from hundreds of ohms to a few kilo-ohms, and the output resistance ranges from a few to tens of kilo-ohms.

Connection diagram for a transistor with a common collector– designed to amplify the amplitude of the input current signal. There is no voltage gain in such a circuit. It would be more correct to say that the voltage gain is even less than unity. The input signal is not inverted by the transistor.
The input resistance of a transistor cascade with OK ranges from tens to hundreds of kilo-ohms, and the output resistance is within hundreds of ohms - units of kilo-ohms. Due to the fact that there is usually a load resistor in the emitter circuit, the circuit has a high input resistance. In addition, due to the amplification of the input current, it has a high load capacity. These properties of a common-collector circuit are used to match transistor stages—as a “buffer stage.” Since the input signal, without increasing in amplitude, is “repeated” at the output, the circuit for switching on a transistor with a common collector is also called Emitter follower.

There are also Connection circuit for a transistor with a common base. This inclusion scheme exists in theory, but in practice it is very difficult to implement. This switching circuit is used in high-frequency technology. Its peculiarity is that it has a low input impedance, and it is difficult to match such a cascade to the input. I have quite a bit of experience in electronics, but speaking about this transistor circuit, I’m sorry, I don’t know anything! I used it a couple of times as “someone else’s” circuit, but never figured it out. Let me explain: according to all physical laws, a transistor is controlled by its base, or rather by the current flowing along the base-emitter path. Using the input terminal of the transistor - the base at the output - is not possible. In fact, the base of the transistor is “connected” to the body at high frequency through a capacitor, but it is not used at the output. And galvanically, through a high-resistance resistor, the base is connected to the output of the cascade (bias is applied). But you can essentially apply the offset from anywhere, even from an additional source. All the same, a signal of any shape entering the base is extinguished through the same capacitor. For such a cascade to work, the input terminal - the emitter through a low-resistance resistor is “planted” on the housing, hence the low input resistance. In general, the connection circuit for a transistor with a common base is a topic for theorists and experimenters. In practice it is extremely rare. In my practice in circuit design, I have never encountered the need to use a transistor circuit with a common base. This is explained by the properties of this connection circuit: the input resistance is from units to tens of ohms, and the output resistance is from hundreds of kilo-ohms to several mega-ohms. Such specific parameters are a rare need.

A bipolar transistor can operate in switching and linear (amplifying) modes. The key mode is used in various control circuits, logic circuits, etc. In the key mode, the transistor can be in two operating states - open (saturated) and closed (locked) state. Linear (amplification) mode is used in circuits for amplifying harmonic signals and requires maintaining the transistor in a “half” open, but not saturated state.

To study the operation of a transistor, we will consider the connection circuit of a common emitter transistor as the most important connection circuit.

The diagram is shown in the figure. On the diagram VT- the transistor itself. Resistors R b1 And R b2– a transistor bias circuit, which is an ordinary voltage divider. It is this circuit that ensures that the transistor is biased to the “operating point” in the harmonic signal amplification mode without distortion. Resistor R to– load resistor of the transistor cascade, designed to supply electric current from the power source to the transistor collector and limit it in the “open” transistor mode. Resistor R e– a feedback resistor inherently increases the input resistance of the cascade, while reducing the gain of the input signal. Capacitors C perform the function of galvanic isolation from the influence of external circuits.

To make it clearer to you how a bipolar transistor works, we will draw an analogy with a conventional voltage divider (see figure below). To begin with, a resistor R 2 Let's make the voltage divider controllable (variable). By changing the resistance of this resistor, from zero to an “infinitely” large value, we can obtain a voltage at the output of such a divider from zero to the value supplied to its input. Now let's imagine that the resistor R 1 The voltage divider is the collector resistor of the transistor stage, and the resistor R 2 The voltage divider is the collector-emitter junction of the transistor. At the same time, by applying a control action in the form of an electric current to the base of the transistor, we change the resistance of the collector-emitter junction, thereby changing the parameters of the voltage divider. The difference from a variable resistor is that the transistor is controlled by a weak current. This is exactly how a bipolar transistor works. The above is depicted in the figure below:

In order for the transistor to operate in signal amplification mode, without distorting the latter, it is necessary to ensure this very operating mode. They talk about shifting the base of the transistor. Competent specialists amuse themselves with the rule: The transistor is controlled by current - this is an axiom. But the bias mode of the transistor is set by the base-emitter voltage, and not by the current - this is reality. And for someone who does not take into account the bias voltage, no amplifier will work. Therefore, its value must be taken into account in calculations.

So, the operation of a bipolar transistor cascade in amplification mode occurs at a certain bias voltage at the base-emitter junction. For a silicon transistor, the bias voltage is in the range of 0.6...0.7 volts, for a germanium transistor - 0.2...0.3 volts. Knowing about this concept, you can not only calculate transistor stages, but also check the serviceability of any transistor amplifier stage. It is enough to use a multimeter with high internal resistance to measure the base-emitter bias voltage of the transistor. If it does not correspond to 0.6...0.7 volts for silicon, or 0.2...0.3 volts for germanium, then look for the fault here - either the transistor is faulty, or the bias or decoupling circuits of this transistor cascade are faulty.

The above is depicted on the graph - current-voltage characteristic (volt-ampere characteristic).

Most of the “specialists”, looking at the presented current-voltage characteristic, will say: What kind of nonsense is drawn on the central graph? This is not what the output characteristic of a transistor looks like! It is shown on the right graph! I’ll answer, everything is correct there, and it started with electron vacuum tubes. Previously, the current-voltage characteristic of a lamp was considered to be the voltage drop across the anode resistor. Now, they continue to measure on the collector resistor, and on the graph they add letters indicating the voltage drop across the transistor, which is deeply mistaken. On the left graph I b – U b the input characteristic of the transistor is presented. On the central chart I k – U k The output current-voltage characteristic of the transistor is presented. And on the right graph I R – U R shows the current-voltage graph of the load resistor R to, which is usually passed off as the current-voltage characteristic of the transistor itself.

The graph has a linear section used to linearly amplify the input signal, limited by points A And WITH. Midpoint – IN, is exactly the point at which it is necessary to contain a transistor operating in amplification mode. This point corresponds to a certain bias voltage, which is usually taken in calculations: 0.66 volts for a silicon transistor, or 0.26 volts for a germanium transistor.

According to the current-voltage characteristic of the transistor, we see the following: in the absence or low bias voltage at the base-emitter junction of the transistor, there is no base current and collector current. At this moment, the entire voltage of the power source drops at the collector-emitter junction. With a further increase in the base-emitter bias voltage of the transistor, the transistor begins to open, the base current appears and, along with it, the collector current increases. Upon reaching the “working area” at the point WITH, the transistor enters linear mode, which continues until the point A. At the same time, the voltage drop at the collector-emitter junction decreases, and at the load resistor R to, on the contrary, it increases. Dot IN– the operating bias point of the transistor is the point at which, as a rule, a voltage drop equal to exactly half the voltage of the power source is established at the collector-emitter junction of the transistor. Frequency response segment from point WITH, to the point A called the displacement working area. After the point A, the base current and therefore the collector current increase sharply, the transistor opens completely and enters saturation. At this moment, at the collector-emitter junction the voltage caused by the structure drops n-p-n transitions, which is approximately equal to 0.2...1 volt, depending on the type of transistor. The rest of the power supply voltage drops across the load resistance of the transistor - the resistor R to., which also limits further growth of the collector current.

From the lower “additional” figures, we see how the voltage at the output of the transistor changes depending on the signal supplied to the input. The output voltage (collector voltage drop) of the transistor is out of phase (180 degrees) with the input signal.

Calculation of a transistor cascade with a common emitter (CE)

Before proceeding directly to the calculation of the transistor stage, let us pay attention to the following requirements and conditions:

The calculation of a transistor cascade is carried out, as a rule, from the end (i.e. from the output);

To calculate a transistor cascade, you need to determine the voltage drop across the collector-emitter junction of the transistor in rest mode (when there is no input signal). It is selected in such a way as to obtain the most undistorted signal. In a single-ended circuit of a transistor stage operating in mode “A”, this is, as a rule, half the value of the power source voltage;

Two currents flow in the emitter circuit of the transistor - the collector current (along the collector-emitter path) and the base current (along the base-emitter path), but since the base current is quite small, it can be neglected and it can be assumed that the collector current is equal to the emitter current;

A transistor is an amplifying element, so it is fair to note that its ability to amplify signals should be expressed by some quantity. The magnitude of the amplification is expressed by an indicator taken from the theory of four-terminal networks - the base current amplification factor in a switching circuit with a common emitter (CE) and is designated - h 21. Its value is given in reference books for specific types of transistors, and usually a plug is given in reference books (for example: 50 - 200). For calculations, the minimum value is usually selected (from the example we select the value - 50);

Collector ( R to) and emitter ( R e) resistances affect the input and output resistances of the transistor stage. We can assume that the input impedance of the cascade R in =R e *h 21, and the output is R out = R to. If the input resistance of the transistor stage is not important to you, then you can do without a resistor at all R e;

Resistor values R to And R e limit the currents flowing through the transistor and the power dissipated by the transistor.

The procedure and example of calculating a transistor cascade with OE

Initial data:

Supply voltage U i.p.=12 V.

Select a transistor, for example: Transistor KT315G, for it:

Pmax=150 mW; Imax=150 mA; h 21>50.

We accept R k =10*R e

The voltage b-e of the transistor operating point is taken U bae= 0.66 V

Solution:

1. Let's determine the maximum static power that will be dissipated by the transistor at the moments of passage of the alternating signal through the operating point B of the static mode of the transistor. It should be a value 20 percent less (coefficient 0.8) of the maximum transistor power specified in the directory.

We accept P dis.max =0.8*P max=0.8*150 mW=120 mW

2. Let's determine the collector current in static mode (without a signal):

I k0 =P ras.max /U ke0 =P ras.max /(U i.p. /2)= 120mW/(12V/2) = 20mA.

3. Considering that half of the supply voltage drops across the transistor in static mode (without a signal), the second half of the supply voltage will drop across resistors:

(R to +R e)=(U i.p. /2)/I to0= (12V/2)/20mA=6V/20mA = 300 Ohm.

Taking into account the existing range of resistor values, as well as the fact that we have chosen the ratio R k =10*R e, we find the resistor values:

R to= 270 Ohm; R e= 27 Ohm.

4. Let's find the voltage at the collector of the transistor without a signal.

U k0 =(U kе0 + I k0 *R e)=(U i.p. - I k0 *R k)= (12 V - 0.02A * 270 Ohm) = 6.6 V.

5. Let's determine the base current of the transistor control:

I b =I k /h 21 =/h 21= / 50 = 0.8 mA.

6. The total base current is determined by the base bias voltage, which is set by the voltage divider R b1,R b2. The resistive base divider current should be much greater (5-10 times) the base control current I b, so that the latter does not affect the bias voltage. We choose a divider current that is 10 times greater than the base control current:

R b1,R b2: I case. =10*I b= 10 * 0.8 mA = 8.0 mA.

Then the total resistance of the resistors

R b1 + R b2 = U i.p. /I del.= 12 V / 0.008 A = 1500 Ohm.

7. Let's find the voltage at the emitter in rest mode (no signal). When calculating a transistor stage, it is necessary to take into account: the base-emitter voltage of the working transistor cannot exceed 0.7 volts! The voltage at the emitter in the mode without an input signal is approximately equal to:

U e =I k0 *R e= 0.02 A * 27 Ohm = 0.54 V,

Where I k0— quiescent current of the transistor.

8. Determining the voltage at the base

U b =U e +U be=0.54 V+0.66 V=1.2 V

From here, through the voltage divider formula we find:

R b2 = (R b1 +R b2 )*U b /U i.p.= 1500 Ohm * 1.2 V / 12V = 150 Ohm R b1 = (R b1 +R b2 )-R b2= 1500 Ohm - 150 Ohm = 1350 Ohm = 1.35 kOhm.

According to the resistor series, due to the fact that through the resistor R b1 The base current also flows, we select the resistor in the direction of decreasing: R b1=1.3 kOhm.

9. Separating capacitors are selected based on the required amplitude-frequency characteristics (bandwidth) of the cascade. For normal operation of transistor stages at frequencies up to 1000 Hz, it is necessary to select capacitors with a nominal value of at least 5 μF.

At lower frequencies, the amplitude-frequency response (AFC) of the cascade depends on the recharging time of the separating capacitors through other elements of the cascade, including elements of neighboring cascades. The capacity should be such that the capacitors do not have time to recharge. The input resistance of the transistor stage is much greater than the output resistance. The frequency response of the cascade in the low-frequency region is determined by the time constant t n =R in *C in, Where R in =R e *h 21, C in— separating input capacitance of the cascade. C out transistor stage, this C in the next cascade and it is calculated in the same way. Lower cutoff frequency of the cascade (cutoff frequency cutoff frequency) f n =1/t n. For high-quality amplification, when designing a transistor stage, it is necessary to choose the ratio 1/t n =1/(R input *C input)< 30-100 times for all cascades. Moreover, the more cascades, the greater the difference should be. Each stage with its own capacitor adds its own frequency response decline. Typically, a 5.0 µF isolation capacitance is sufficient. But the last stage, through Cout, is usually loaded with the low-resistance resistance of the dynamic heads, so the capacitance is increased to 500.0-2000.0 µF, sometimes more.

The calculation of the key mode of the transistor stage is carried out in exactly the same way as the previously carried out calculation of the amplifier stage. The only difference is that the key mode assumes two states of the transistor in rest mode (without a signal). It is either closed (but not shorted) or open (but not oversaturated). At the same time, the operating points of “rest” are located outside of points A and C shown on the current-voltage characteristic. When the transistor should be closed in the circuit in a state without a signal, it is necessary to remove the resistor from the previously depicted cascade circuit R b1. If you want the transistor to be open at rest, you need to increase the resistor in the cascade circuit R b2 10 times the calculated value, and in some cases, it can be removed from the diagram.