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

This circuit is very good at using high frequency signals. In principle, for this, such an inclusion of the transistor is used in the first place. Very big disadvantages are the low input impedance 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 the current amplification is not simply absent, 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, we continue….

Scheme for switching on a bipolar transistor with a common collector

This is how the circuit for switching on a bipolar transistor with a common collector looks like. Does it look like anything?) If you look at the circuit from a slightly different angle, then we recognize here our old friend - an emitter follower. There was almost a whole article about him (), so we have already considered everything related to this scheme there. In the meantime, the most commonly used circuit is waiting for us - with a common emitter.

Scheme for switching on a bipolar transistor with a common emitter.

This circuit has gained popularity for its amplifying properties. Of all the circuits, it gives the greatest current and voltage gain, respectively, the increase in the signal in terms of power is also large. The disadvantage of this circuit is that the gain properties are highly influenced by the rise in 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 on a bipolar transistor (with a common emitter). First, let's depict it a little differently:

There is one drawback here - a grounded emitter. When the transistor is turned on in this way, nonlinear distortions are present at the output, which, of course, must be combated. Non-linearity arises from the effect of the input voltage on the emitter-base junction voltage. Indeed, there is nothing "superfluous" in the emitter circuit, all the input voltage is applied precisely to the base-emitter junction. To deal with this phenomenon, let's add a resistor to the emitter circuit. Thus, we get negative feedback.

What is it?

In short, negative back principle th connections consists 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 a lower voltage value will be supplied to the input of the transistor due to the influence of feedback than in the absence of feedback.

Nevertheless, negative feedback is very beneficial for us. Let's see how it will help reduce the effect of the input voltage on the base-to-emitter voltage.

So, suppose there is no feedback, increasing the input signal by 0.5V results in the same increase. Everything is clear here 😉 And now we add feedback! And in the same way, we increase the voltage at the input by 0.5 V. Following this, it increases, which leads to an increase in the emitter current. And an increase leads to an increase in the voltage across the feedback resistor. It would seem, what's the big deal? But this voltage is subtracted from the input voltage! See what happened:

The voltage at the input 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 common emitter amplifier circuit was replenished 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 there will be nothing 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 6V (12V / 2). Thus, in the absence of a signal at the input, the base potential will be + 6V. If a negative value comes to the input, for example, -4V, then the base potential will be + 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 chain)

How else to improve our scheme ...

Let us know which 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 can this be ensured? Of course, using a high-pass filter) 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 for the transistor itself, was overgrown with additional elements 😉 Perhaps we will stop at this, there will soon be an article devoted to the practical calculation of an amplifier 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 we will choose a transistor that is suitable for our purposes. See you soon! =)

Are bipolar transistors. Switching circuits depend on what kind of conductivity they have (hole or electronic) and the functions they perform.

Classification

Transistors are divided into groups:

1. Based on materials: gallium arsenide and silicon are most commonly used.
2. By signal frequency: low (up to 3 MHz), medium (up to 30 MHz), high (up to 300 MHz), ultra-high (above 300 MHz).
3. According to the maximum power dissipation: up to 0.3 W, up to 3 W, more than 3 W.
4. By the type of device: three connected semiconductor layers with alternating changes in the forward and reverse methods of impurity conductivity.

How do transistors work?

The outer and inner layers of the transistor are connected to the lead electrodes, called respectively the emitter, collector and base.

The emitter and collector do not differ from each other in the types of conductivity, but the degree of doping with impurities in the latter is much lower. This ensures an increase in the permissible output voltage.

The base, which is the middle layer, has high resistance because it is made of lightly doped semiconductor. It has a significant contact area with the collector, which improves the removal of heat generated due to the reverse bias of the transition, and also facilitates the passage of minority carriers - electrons. Despite the fact that the transition layers are based on the same principle, the transistor is an unbalanced device. When changing the places of the extreme layers with the same conductivity, it is impossible to obtain similar parameters of a semiconductor device.

Inclusion circuits are capable of maintaining it in two states: it can be open or closed. In active mode, when the transistor is on, the emitter biasing of the junction is done in the forward direction. To clearly see this, for example, on a semiconductor triode of the n-p-n type, voltage should be applied to it from sources, as shown in the figure below.

In this case, the border at the second collector junction is closed, and no current should flow through it. But in practice, the opposite happens due to the close location of the transitions to each other and their mutual influence. Since the "minus" of the battery is connected to the emitter, the open junction allows electrons to enter the base zone, where they partially recombine with holes - the main carriers. A base current I b is formed. The stronger it is, the proportionally greater is the output current. Bipolar transistor amplifiers work on this principle.

Through the base there is an exclusively diffusive movement of electrons, since there is no action of an electric field. Due to the insignificant layer thickness (microns) and the large amount of negatively charged particles, almost all of them fall into the collector area, although the base resistance is quite high. There they are drawn in by the transition electric field, which promotes their active transfer. The collector and emitter currents are practically equal to each other, if we neglect the insignificant loss of charges caused by recombination in the base: I e = I b + I to.

Transistor parameters

1. Gain factors for voltage U eq / U be and current: β = I to / I b (actual values). Typically, the β coefficient does not exceed 300, but can reach 800 and higher.
2. Input impedance.
3. Frequency response - the operability of the transistor up to a given frequency, when exceeding which the transient processes in it do not keep pace with the changes in the supplied signal.

Bipolar transistor: switching circuits, operating modes

The modes of operation differ depending on how the circuit is assembled. The signal must be applied and removed at two points for each case, and there are only three terminals available. It follows that one electrode must simultaneously belong to the input and output. This turns on any bipolar transistors. Inclusion schemes: OB, OE and OK.

1. Scheme with OK

Connection diagram with a common collector: the signal is fed to the resistor R L, which is also included in the collector circuit. This connection is called a common collector circuit.

This option only creates current gain. The advantage of the emitter follower is the creation of a large input resistance (10-500 kOhm), which makes it possible to conveniently match the stages.

2. Scheme with OB

Scheme for switching on a bipolar transistor with a common base: the input signal enters through C 1, and after amplification is removed in the output collector circuit, where the base electrode is common. In this case, a voltage gain is created similar to working with an OE.

The disadvantage is the low resistance of the input (30-100 Ohm), and the circuit with OB is used as an oscillator.

3. Scheme with OE

In many cases, when bipolar transistors are used, switching circuits are predominantly made with a common emitter. The supply voltage is supplied through the load resistor R L, and the negative pole of the external power supply is connected to the emitter.

The alternating signal from the input goes to the emitter and base electrodes (V in), and in the collector circuit it becomes larger in value (V CE). The main elements of the circuit: a transistor, a resistor R L and an amplifier output circuit with external power supply. Auxiliary: capacitor C 1, which prevents the passage of direct current into the circuit of the supplied input signal, and resistor R 1, through which the transistor opens.

In the collector circuit, the voltages at the output of the transistor and across the resistor R L are together equal to the EMF value: V CC = I C R L + V CE.

Thus, a small signal V in at the input sets the law of changing the constant supply voltage into an alternating one at the output of the controlled transistor converter. The circuit provides an increase in the input current 20-100 times, and voltage - 10-200 times. Accordingly, the power is also increased.

Disadvantage of the circuit: low input resistance (500-1000 ohms). For this reason, there are problems in shaping. The output impedance is 2-20 kOhm.

These diagrams show how a bipolar transistor works. If you do not take additional measures, external influences such as overheating and signal frequency will greatly affect their performance. Also, emitter grounding creates harmonic distortion at the output. To increase the reliability of operation, feedbacks, filters, etc. are connected in the circuit. In this case, the gain decreases, but the device becomes more efficient.

Modes of operation

The function of the transistor is influenced by the value of the connected voltage. All modes of operation can be shown if the previously presented circuit for switching on a bipolar transistor with a common emitter is applied.

1. Cutoff mode

This mode is created when the value of the voltage V BE decreases to 0.7 V. In this case, the emitter junction closes, and there is no collector current, since there are no free electrons in the base. Thus, the transistor is locked.

2. Active mode

If a voltage sufficient to open the transistor is applied to the base, a small input current appears and an increased output current, depending on the magnitude of the gain. Then the transistor will act as an amplifier.

3. Saturation mode

The mode differs from the active mode in that the transistor opens completely and the collector current reaches the maximum possible value. Its increase can be achieved only by changing the applied EMF or load in the output circuit. When the base current changes, the collector current does not change. The saturation mode is characterized by the fact that the transistor is extremely open, and here it serves as a switch in the on state. Circuits for switching on bipolar transistors when combining cut-off and saturation modes make it possible to create electronic keys with their help.

All modes of operation depend on the nature of the output characteristics shown in the graph.

They can be clearly demonstrated if a circuit for switching on a bipolar transistor with an OE is assembled.

If we put aside the segments corresponding to the maximum possible collector current and the value of the supply voltage V CC on the ordinate and abscissa axes, and then connect their ends together, we get a load line (red). It is described by the expression: I C = (V CC - V CE) / R C. It follows from the figure that the operating point, which determines the collector current I C and voltage V CE, will shift along the load line from bottom to top with increasing base current I V.

The area between the V CE axis and the first output characteristic (shaded), where I B = 0, characterizes the cutoff mode. In this case, the reverse current I C is negligible, and the transistor is closed.

The uppermost characteristic at point A intersects with the direct load, after which, with a further increase in I B, the collector current no longer changes. The saturation zone on the graph is the shaded area between the IC axis and the steepest curve.

How does the transistor behave in different modes?

The transistor operates with variable or constant signals entering the input circuit.

Bipolar transistor: switching circuits, amplifier

For the most part, the transistor serves as an amplifier. An alternating signal at the input leads to a change in its output current. Here you can apply schemes with OK or with OE. The signal requires a load in the output circuit. Usually a resistor is used installed in the output collector circuit. If selected correctly, the output voltage will be significantly higher than the input voltage.

The operation of the amplifier is clearly visible on the timing diagrams.

When pulsed signals are converted, the mode remains the same as for sinusoidal ones. The quality of the transformation of their harmonic components is determined by the frequency characteristics of the transistors.

Switch mode operation

Designed for contactless switching of connections in electrical circuits. The principle is a stepwise change in the resistance of the transistor. The bipolar type is quite suitable for the key device requirements.

Conclusion

Semiconductor elements are used in electrical signal conversion circuits. Universal capabilities and a large classification allow widespread use of bipolar transistors. Connection diagrams determine their functions and modes of operation. Much also depends on the characteristics.

The basic circuits for switching on bipolar transistors amplify, generate and convert input signals, and also switch electrical circuits.

Depending on the principle of operation and design features, transistors are divided into two large classes: bipolar and field.

Bipolar transistoris a semiconductor device with two interacting pn junctions and three or more leads.

A semiconductor crystal of a transistor consists of three regions with alternating types of electrical conductivity, between which there are two pn-transition. The middle region is usually very thin (fractions of a micron), therefore pn-transitions are located close to each other.

Depending on the order of alternation of semiconductor regions with different types of electrical conductivity, transistors are distinguished p-p-p and p-p-p- types . Simplified structures and UGOs of different types of transistors are shown in Figure 1.23, a, b.

Figure 1.23 - Structure and UGO of bipolar transistors

The bipolar transistor is the most common active semiconductor device. Silicon is currently used as the main material for the manufacture of bipolar transistors. In this case, transistors are mainly manufactured p-p-p-type, in which the main charge carriers are electrons, which have a mobility two to three times higher than the mobility of holes.

The control of the amount of current flowing in the output circuit (in the collector or emitter circuit) of the bipolar transistor is carried out using current in the control electrode circuit - base. Base called average layer in the structure of the transistor. The outermost layers are called emitter (emit, ejaculate) and collector (gather). The concentration of impurities (and, consequently, of the majority charge carriers) in the emitter is significantly higher than in the base and higher than in the collector. Therefore, the emitter region is the most low impedance.

To illustrate the physical processes in the transistor, we will use the simplified structure of the transistor p-p-p- of the type shown in Figure 1.24. To understand the principle of operation of the transistor, it is extremely important to take into account that pn- the transitions of the transistor interact strongly with each other. This means that the current of one junction strongly influences the current of the other, and vice versa.

In active mode (when the transistor works as an amplifying element), two power supplies are connected to the transistor in such a way that emitter transition has been shifted forward direction, a collector - in the opposite(Figure 1.24). Under the influence of the electric field of the source E BE through the emitter junction, a sufficiently large forward current flows I E, which is provided mainly by injection electrons from the emitter to the base The injection of holes from the base to the emitter will be insignificant due to the above difference in the concentrations of impurity atoms.

Figure 1.24 - Physical processes in a bipolar transistor

Electron flow providing current I E through the transition emitter - base is shown in Figure 1.24 with a wide arrow. Part of electrons injected into the region of the base (1 ... 5%) recombine with the main charge carriers for this region - holes, forming a current in the external circuit of the base I B. Due to the large difference in the concentrations of the majority charge carriers in the emitter and base, uncompensated electrons injected into the base move deep into the base towards the collector.

Near the collector p-p- transition electrons are subject to an accelerating electric field this reverse biased transition. And since they are minor carriers in the database, then there is retraction (extraction ) electrons to the collector region. In the collector, electrons become the main charge carriers and easily reach the collector terminal, creating a current in the external circuit of the transistor.

In this way, the current through the base terminal of the transistor is determined by two oppositely directed current components... If there were no recombination processes in the base, then these currents would be equal to each other, and the resulting base current would be zero. But since there are recombination processes in any real transistor, the emitter current p-n- the transition is slightly more than the collector current p-n-transition.

For the collector current, the following equality can be written

, (1.9)

where a st- static transmission coefficient of the emitter current;

I KBO- reverse current of the collector junction (thermal current) (for low-power transistors at normal temperature it is 0.015 ... 1 μA).

In practice, the static emitter current transfer coefficient a st, depending on the type of transistor, can take values ​​in the range 0.95 ... 0.998.

The emitter current in the transistor is numerically the largest and is equal to

, (1.11)

where is the static current transfer coefficient of the base in a circuit with a common emitter (in the reference literature, the notation is used h 21E, usually takes the value b st= 20 ... 1000 depending on the type and power of the transistor).

From the above, it follows that the transistor is a controlled element, since the value of its collector (output) current depends on the values ​​of the emitter and base currents.

Concluding the consideration of the principle of operation of a bipolar transistor, it should be noted that the resistance of the reverse-biased collector junction (when a reverse voltage is applied to it) is very high (hundreds of kilo-ohms). So load resistors with very high resistances can be included in the collector circuit, thereby practically not changing the value of the collector current. Accordingly, significant power will be allocated in the load circuit.

The resistance of the forward-biased emitter junction, on the other hand, is very small (tens - hundreds of ohms). Therefore, with almost identical values ​​of the emitter and collector currents, the power consumed in the emitter circuit is significantly less than the power released in the load circuit. This indicates that a transistor is a semiconductor device that amplifies power.

The manufacturing technology of bipolar transistors can be different: fusion, diffusion , epitaxy... This largely determines the characteristics of the device. Typical structures of bipolar transistors manufactured by various methods are shown in Figure 1.25. In particular, in Figure 1.25, a shows the structure floating, in Figure 1.25, b - epitaxially-diffusion, in Figure 1.25, v - planar, in Figure 1.25, G - mesplanar transistors.

Figure 1.25 - Methods of manufacturing bipolar transistors

Modes of operation and circuits for switching on the transistor

For every p-p- The junction of the transistor can be supplied with both forward and reverse voltage. In accordance with this, four modes of operation of the bipolar transistor are distinguished: mode cutoffs, mode saturation, active regime and inverse mode.

Active the mode is ensured by applying a forward voltage to the emitter junction, and the reverse voltage to the collector junction (the main operating mode of the transistor). This mode corresponds to the maximum value of the emitter current transfer coefficient and provides minimal distortion of the amplified signal.

V inverse mode, a forward voltage is applied to the collector junction, and a reverse voltage is applied to the emitter junction (a st® min; used very rarely).

In the mode saturation both transitions are under forward bias. In this case, the output current does not depend on the input current and is determined only by the load parameters.

In the mode cutoffs both transitions are biased in opposite directions. The output current is close to zero.

Saturation and cutoff modes are used simultaneously in key schemes(when the transistor is in the key mode).

When using a transistor in electronic devices, two leads are needed to supply the input signal and two leads to connect the load (remove the output signal). Since the transistor has only three pins, one of them must be common for the input and output signals.

Depending on which terminal of the transistor is common when the signal source and load are connected, three transistor switching circuits are distinguished: common base(OB) (Figure 1.26, a); With common emitter(OE) (Figure 1.26, b); With common collector(OK) (Figure 1.26, v).

In these circuits, constant voltage sources and resistors provide the operating modes of the transistors for direct current, that is, the required values ​​of voltages and initial currents. AC input signals are generated by sources and in. They change the emitter (base) current of the transistor, and, accordingly, the collector current. Collector current increments (Figure 1.26, a, b) and emitter current (Figure 1.26, v) will create, respectively, on the resistors R K and R E voltage increments, which are the output signals and out.

a B C

Figure 1.26 - Transistor switching circuits

When determining the switching circuit of the transistor, it is necessary to take into account the fact that the resistance of the constant voltage source for alternating current is close to zero.

Current-voltage characteristics of the transistor

The properties of a bipolar transistor are most fully described using static current-voltage characteristics. In this case, the input and output I - V characteristics of the transistor are distinguished. Since all three currents (base, collector and emitter) in the transistor are closely interconnected, when analyzing the operation of the transistor, it is necessary to use simultaneously the input and output I – V characteristics.

Each transistor switching circuit has its own current-voltage characteristics, which are the functional dependence of the currents through the transistor on the applied voltages. Due to the non-linear nature of these dependencies, they are usually presented in graphical form.

A transistor, like a four-pole, is characterized by input and weekend static I - V characteristics, showing, respectively, the dependence of the input current on the input voltage (at a constant value of the output voltage of the transistor) and the output current on the output voltage (at a constant input current of the transistor).

Figure 1.27 shows the static I - V characteristics p-p-p- a transistor connected according to a circuit with an OE (most often used in practice).

a b

Figure 1.27 - Static I - V characteristics of a bipolar transistor connected according to the circuit with OE

Input I - V characteristic (Figure 1.27, a) is similar to the direct branch of the I - V characteristic of a diode. It represents the dependence of the current I B from stress U BE U CE, that is, a dependence of the form

. (1.12)

From Figure 1.27, a it can be seen: the greater the voltage U CE, the more to the right the branch of the input I - V characteristic is displaced. This is due to the fact that with an increase in the reverse bias voltage U CE there is an increase in the height of the potential barrier of the collector R-P-transition. And since in the transistor the collector and emitter R-P-transitions interact strongly, this, in turn, leads to a decrease in the base current at a constant voltage U BE.

Static I - V characteristics, presented in Figure 1.27, a, filmed at normal temperature (20 ° C). With an increase in temperature, these characteristics will shift to the left, and with a decrease, to the right. This is due to the fact that as the temperature rises, the intrinsic electrical conductivity of semiconductors increases.

For the output circuit of the transistor connected according to the circuit with the OE, a family of output I - V characteristics is built (Figure 1.27, b). This is due to the fact that the collector current of the transistor depends not only (and not so much, as can be seen from the figure) on the voltage applied to the collector junction, but also on the base current. Thus, the output current-voltage characteristic for a circuit with an OE is called the current dependence I K from stress U CE at a fixed current I B, that is, a dependence of the form

. (1.13)

Each of the output I - V characteristics of a bipolar transistor is characterized at the beginning by a sharp increase in the output current I K with increasing output voltage U CE, and then, as the voltage further increases, a slight change in current.

On the output current-voltage characteristic of the transistor, three regions can be distinguished, corresponding to different modes of operation of the transistor: the region saturation, area cutoffs and area active work(gain) , corresponding to the active state of the transistor when ½ U BE½> 0 and ½ U CE½> 0.

Input and output static I - V characteristics of transistors are used for graphical-analytical calculation of stages containing transistors.

Static input and output I - V characteristics of a bipolar transistor R-P-R-type for the switching circuit with OB are shown in Figure 1.28, a and 1.28, b respectively.

a b

Figure 1.28 - Static I - V characteristics of a bipolar transistor for a switching circuit with OB

For a circuit with OB input static I - V characteristic, the current dependence is called I e from stress U EB at a fixed voltage value U KB, that is, a dependence of the form

. (1.14)

The output static I - V characteristic for a circuit with OB is called the current dependence I K from stress U KB at a fixed current I e, that is, a dependence of the form

. (1.15)

In Figure 1.28, b two areas can be distinguished, corresponding to two modes of operation of the transistor: active mode ( U KB< 0 и коллекторный переход смещен в обратном направлении); режим saturation(U KB> 0 and the collector junction is forward biased).

Mathematical model of a bipolar transistor

By now, many electrical models of bipolar transistors are known. In design automation systems (CAD) of radioelectronic facilities, the following are most often used: Ebers-Moll models, generalized model of Gummel-Pune charge control, Linville's model, as well as local P- and T-shaped models of linear increments of Giacolleto.

Let us consider, as an example, one of the variants of the Ebers-Moll model (Figure 1.29), which reflects the properties of the transistor structure in the linear mode of operation and in the cut-off mode.

Figure 1.29 - Equivalent circuit of a bipolar transistor (Ebers-Moll model)

Figure 1.29 uses the following notation: r e, r b, r to- resistance, respectively, of the emitter, base and collector regions of the transistor and contacts to them; I b , I to - voltage controlled and n at the input junction, current sources reflecting the transfer of current through the transistor; R eb- Leakage resistance of the base-emitter junction; R kb - Leakage resistance of base-collector junction. Source current I b is related to the voltage across the junction by the relation

, (1.15)

where I BO- saturation current of the base-emitter junction (reverse current);

y To= (0.3 ... 1.2) V - contact potential difference (depends on the type of semiconductor material);

T- empirical coefficient.

Parallel to the base-emitter junction included barrier capacity With bae and diffusion capacity With de transition. The magnitude With bae determined reverse voltage at the crossing and n and depends on him by law

, (1.16)

where C 0 b - junction capacity at and n = 0;

g = 0.3 ... 0.5 - coefficient depending on the distribution of impurities in the base region of the transistor.

Diffusion capacity is a function of current I b flowing through the transition and is determined by the expression

where A - coefficient depending on the properties of the transition and its temperature.

The collector-base junction is modeled in a similar way, the only difference is that only the barrier capacitance of the junction is taken into account.

, (1.18)

since when the transistor operates in the linear mode and the collector current cutoff mode, this junction is closed. Expression for current controlled collector current source, which simulates the amplifying properties of the transistor, has the form

, (1.19)

where b st- static current transfer coefficient of the base of the transistor in a circuit with a common emitter.

The parameters of the Ebers-Moll model can be obtained either by calculation based on the analysis of the physical-topological model of the transistor, or measured experimentally. The static parameters of the DC model are determined most easily.

The global the electrical model of a discrete bipolar transistor, taking into account the inductance and capacitance of its terminals, is shown in Figure 1.30.

Figure 1.30 - Global model of a bipolar transistor

Basic parameters of bipolar transistor

When determining the alternating components of currents and voltages (that is, when analyzing electrical circuits on alternating current) and provided that the transistor operates in an active mode, it is often represented as a linear four-pole (Figure 1.31, a). The names (physical essence) of the input and output currents and voltages of such a two-port network depend on the switching circuit of the transistor.

a b

Figure 1.31 - Representation of a bipolar transistor with a linear four-pole

For the switching circuit of a transistor with a common emitter, the currents and voltages of the four-pole system (Figure 1.31, b) correspond to the following currents and voltages of the transistor:

- i 1 - variable component of the base current;

- u 1 - variable component of the voltage between the base and the emitter;

- i 2 - alternating component of the collector current;

- u 2 - the variable component of the voltage between the collector and the emitter.

It is convenient to describe the transistor using the so-called h-parameters. In this case, the system of equations of a four-port network in matrix form will take the form

. (1.20)

Odds h ij(that is h-parameters) are determined empirically, using alternately the short-circuit and no-load modes at the input and output of the four-pole.

The essence h-parameters for the switching circuit of the transistor with the OE are as follows:

- - input impedance of the transistor for an alternating signal with a short circuit at the output;

- r b is the ohmic resistance of the base body. In real transistors, it reaches values ​​of 100 ... 200 Ohm;

- r e- resistance R-P- a transition, the value of which depends on the operating mode of the transistor and changes in the active mode within fractions - tens of Ohms;

B is the differential coefficient of transfer of the base current, determined from the expression

; (1.25)

Collector area resistance, determined from the expression

, (1.26)

where r to- the differential resistance of the collector junction (usually within a fraction - tens of megohms), determined from the expression

(1.27)

Good afternoon friends!

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

Sit back - 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" (transistor) is formed 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 quite a 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.

The 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 indicated in a certain way (see figure). We see that the transistor is essentially a pn junction connected in series.

Backfill question - why can't you replace the transistor with two diodes? After all, each of them has a pn junction, right? I switched on two diodes in series - and it's in the bag!

Not! The fact is that the base in the transistor is made very thin during manufacture, which cannot be achieved by connecting two separate diodes.

The principle of operation of a bipolar transistor

The basic principle of operation of the transistor is that a small base current can drive much higher collector current - in the range practically from zero to a certain maximum possible value.

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

It is interesting to note that low-power transistors have more of it than high-power ones (and not vice versa, as one might think).

The difference is that, in contrast to the DC gate, during control, the base current is always present, i.e. some kind of power is always spent on control.

The higher the voltage between the emitter and base, the greater the base current and, accordingly, the greater the collector current. However, any transistor has a maximum allowable voltage between emitter and base and between emitter and collector. For exceeding these parameters, you will have to pay with a new transistor.

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

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

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

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

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

The lamp will go out.

How to check a bipolar transistor?

Since a bipolar transistor consists of two pn junctions, it is quite easy to test it with a digital tester.

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

In fact, we just sequentially check 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 across the emitter-base and collector-base junctions 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 the operating mode, most often one of the transitions of the transistor is open, and the second is closed.

Measuring the current transfer coefficient of a bipolar transistor

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

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

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

Pinout (pin names) 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) ratio, 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 less - a few units or tens.

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

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 ratio is the product of the ratio of the first and second transistors.

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

Built-in diode protection

Some transistors (high-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 the diode test mode, then it will show the same 0.6 - 0.7 V (if the diode is forward biased) or a "locked diode" (if the diode is biased in the reverse direction) ...

If the tester shows some slight voltage, and even in both directions, then the transistor is definitely broken and must be replaced... A short circuit can also be determined in the resistance measurement mode - the tester will show a low resistance.

Occurs (fortunately, quite rarely) "vile" malfunction of transistors. This is when it initially works, and after some time (or after warming up) it changes its parameters or fails altogether.

If such a transistor is evaporated and checked with a tester, then it will have time to cool down before connecting the probes, and the tester will show that it is normal. It is best to verify this by replacing the "suspicious" transistor in the device.

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

In fact, the work of a bipolar transistor is described by many formulas, there are many varieties of them, but this is a complex science. For those wishing to dig deeper, I can recommend Horowitz and Hill's wonderful book The Art of Circuitry.

You can buy transistors for your experiments

See you on the blog!

Transistor

A transistor is a semiconductor device that allows a stronger signal to be controlled with a weak signal. Because of this property, it is often said about the ability of a transistor to amplify a signal. Although in fact, it does not amplify anything, but simply allows you to turn on and off a large current with much weaker currents. Transistors are very common in electronics, because the output of any controller can rarely deliver a current of more than 40 mA, therefore, even 2-3 low-power LEDs can no longer be powered directly from the microcontroller. This is where transistors come to the rescue. The article discusses the main types of transistors, the differences between P-N-P and N-P-N bipolar transistors, P-channel from N-channel field-effect transistors, discusses the main subtleties of connecting transistors and discloses their areas of application.

Do not confuse a transistor with a relay. A relay is a simple switch. The essence of its work is to close and open metal contacts. The transistor is more complex and its operation is based on an electron-hole junction. If you are interested in learning more about this, you can watch an excellent video that describes how the transistor works from simple to complex. Do not be confused by the year the video was produced - the laws of physics have not changed since then, and a newer video, in which the material is presented in such a high quality, could not be found:

Types of transistors

Bipolar transistor

The bipolar transistor is designed to control light loads (for example, low-power motors and servos). He always has three conclusions:

Collector - a high voltage is supplied, which the transistor controls

• Base (English base) - current is supplied or disconnected to open or close the transistor

Emitter (English emitter) - "outlet" output of the transistor. Through it, current flows from the collector and base.

The bipolar transistor is current driven. The more current is applied to the base, the more current will flow from the collector to the emitter. The ratio of the current passing from the emitter to the collector to the current at the base of the transistor is called the gain. Denoted as h fe (in English literature called gain).

For example, if h fe= 150, and 0.2 mA passes through the base, then the transistor will pass a maximum of 30 mA through itself. If a component is connected that consumes 25mA (such as an LED), it will be provided with 25mA. If a component is connected that draws 150 mA, only the maximum 30 mA will be provided. The documentation for the contact indicates the maximum permissible values ​​of currents and voltages base-> emitter and collector -> emitter ... Exceeding these values ​​leads to overheating and failure of the transistor.

Funny pictures:

NPN and PNP bipolar transistors

There are 2 types of polarized transistors: NPN and PNP... They differ in the alternation of layers. N (from negative - negative) is a layer with an excess of negative charge carriers (electrons), P (from positive - positive) is a layer with an excess of positive charge carriers (holes). For more information on electrons and holes, see the video above.

The behavior of transistors depends on the alternation of layers. The animation above shows NPN transistor. V PNP control of the transistor is arranged the other way around - current flows through the transistor when the base is grounded and is blocked when current is passed through the base. In the display on the diagram PNP and NPN differ in the direction of the arrow. The arrow always indicates the transition from N To P:

Designation of NPN (left) and PNP (right) transistors in the diagram

NPN transistors are more common in electronics because they are more efficient.

Field effect transistor

Field-effect transistors differ from bipolar transistors in their internal structure. Most common in hobby electronics are MOSFETs. MOS is an abbreviation for Metal Oxide Conductor. Same in English: Metal-Oxide-Semiconductor Field Effect Transistor, abbreviated as MOSFET. MOS transistors allow you to control high powers with a relatively small size of the transistor itself. The transistor is controlled by voltage, not current. Since the transistor is controlled by an electrical field, the transistor got its name - field howl.

Field-effect transistors have at least 3 pins:

Drain - high voltage is supplied to it, which you want to control

Gate (English gate) - voltage is applied to it to control the transistor

Source (English source) - current from the drain passes through it when the transistor is "open"

There should be an animation with a field effect transistor, but it will be no different from a bipolar one, except for a schematic representation of the transistors themselves, so there will be no animation.

N channel and P channel field effect transistors

Field-effect transistors are also divided into 2 types depending on the device and behavior. N channel(N channel) opens when voltage is applied to the gate and closes. when there is no tension. P channel(P channel) works the other way around: while there is no voltage at the gate, current flows through the transistor. When voltage is applied to the gate, the current stops. On the diagram, field-effect transistors are depicted in a slightly different way:

By analogy with bipolar transistors, field-effect transistors differ in polarity. The N-Channel transistor was described above. They are the most common.

The P-Channel, when labeled, differs in the direction of the arrow and, again, has an "inverted" behavior.

There is a misconception that a field effect transistor can drive alternating current. This is not true. To control AC current, use a relay.

Darlington transistor

The Darlington transistor is not entirely correct to refer to a separate type of transistor. However, it is impossible not to mention them in this article. The Darlington transistor is most often found in the form of a microcircuit, which includes several transistors. For example ULN2003. The Darlington transistor is characterized by the ability to quickly open and close (which means it can work with) and at the same time withstands high currents. It is a kind of composite transistor and is a cascade connection of two or, rarely, more transistors connected in such a way that the load in the emitter of the previous stage is the base-emitter transition of the transistor of the next stage, that is, the transistors are connected by collectors, and the emitter of the input transistor is connected to the base weekend. In addition, the resistive load of the emitter of the previous transistor can be used as part of the circuit to accelerate the closing. Such a connection is generally considered as a single transistor, the current gain of which, when the transistors are in active mode, is approximately equal to the product of the gains of all transistors.

Transistor connection

It's no secret that the Arduino board is capable of supplying the output with a voltage of 5 V with a maximum current of up to 40 mA. This current is not enough to connect a powerful load. For example, if you try to connect an LED strip or a motor directly to an output, you are guaranteed to damage the Arduino output. It is possible that the entire board will fail. In addition, some connected components may require more than 5V to operate. The transistor solves both of these problems. It will help with the help of a small current from the Arduino output to control a powerful current from a separate power supply or with a voltage of 5 V to control a large voltage (even the weakest transistors rarely have a voltage limit below 50 V). As an example, consider connecting a motor:

In the above diagram, the motor is connected to a separate power source. We put a transistor between the motor pin and the power supply for the motor, which will be controlled by any digital pin on the Arduino. When applying a HIGH signal to the controller output from the controller output, we will take a very small current to open the transistor, and a large current will flow through the transistor and will not damage the controller. Pay attention to the resistor installed between the Arduino pin and the base of the transistor. It is needed to limit the current flowing along the path of the microcontroller - transistor - ground and to prevent a short circuit. As mentioned earlier, the maximum current that can be drawn from the Arduino pin is 40mA. Therefore, we need a resistor of at least 125 Ohm (5V / 0.04A = 125 Ohm). You can safely use a 220 ohm resistor. In fact, the resistor should be selected taking into account the current that must be supplied to the base to obtain the required current through the transistor. For the correct selection of the resistor, you need to take into account the gain ( h fe).

IMPORTANT!! If you connect a powerful load from a separate power supply, then you need to physically connect the ground ("minus") of the load power supply and the ground (pin "GND") of the Arduino. Otherwise, it will not work to control the transistor.

When using a field effect transistor, a current limiting resistor at the gate is not needed. The transistor is controlled solely by voltage and no current flows through the gate.