Three circuits for turning on a bipolar transistor. Bipolar Transistor Switching Circuits

Which have no less than three conclusions. In certain situations, they are able to amplify power, generate oscillations, or transform a signal. There are a lot of different designs of these devices, and among them is a pnp transistor.

Transistors are classified by semiconductor material. They come from silicon, germanium, etc.

If a two-region transistor has two hole conductivity, it is called a “direct conductivity transistor,” or “a pnp junction transistor." A device in which two regions have electronic conductivity is called a reverse conductivity transistor, or with an npn junction. Both transistors work the same way, and the difference lies solely in polarity.

Where is the pnp transistor used?

Depending on what characteristics the transformer has, it can be used for a variety of purposes. As already mentioned, a transistor is used to generate, convert and amplify electrical signals. Due to the fact that the input voltage or current changes, the current of the input circuit is controlled. Small changes in the input parameters lead to an even larger change in the output current and voltage. This gain property is used in analog technology (radio, communications, etc.).

Nowadays, analog technology is used. But another, very important industry - digital technology - almost abandoned it and uses only field technology. appeared much earlier than the field, because in everyday life it is simply called a transistor.

Execution and parameters of transistors

Transistors are structurally manufactured in plastic and metal cases. Given the different purpose of the transistors, these devices are selected according to certain parameters. For example, if you need a transistor to amplify high frequencies, it must have a high signal amplification frequency. And if the pnp transistor is used, it must have a high collector operating current.

Reference literature contains the main characteristics of transistors:

Ik - working (maximum allowable) collector current;
h21e is the gain;
Fgr - maximum gain frequency;
Pk is the collector power dissipation.

Phototransistors

A phototransistor is a sensitive device that irradiates it. In the sealed case of such a transistor, a window is made, for example, of transparent plastic or glass. Radiation through it falls into the base zone of the phototransistor. If the base is irradiated, then charge carriers are generated. The phototransistor will open when the charge carriers go into the collector junction, and the more the base is illuminated, the collector current will become more significant.

Without transistors, modern electronics cannot be imagined. Almost no serious device can do without them. Over the years of application and improvement, transistors have changed significantly, but the principle of their operation remains the same.

Transistors are divided into bipolar and field. Each of these types has its own principle of operation and design, however, the presence of semiconductor p-n structures is common to them.

Conventional graphic designations (UGO) of transistors are given in the table:

Device type
Bipolar	Bipolar pnp type
Bipolar	Bipolar n-p-n type
Field	With manager p-n junction	With p-type channel
	With manager p-n junction	With n-type channel
	With isolated shutter MOS transistors	With integrated channel	Built-in channel p-type
		With integrated channel	Built-in channel n-type
		With induced channel	Induced channel p-type
		With induced channel	Induced channel n-type

Bipolar transistors

The definition of "bipolar" indicates that the operation of the transistor is associated with processes in which two types of charge carriers participate - electrons and holes.

A transistor is a semiconductor device with two electron-hole transitions, designed to amplify and generate electrical signals. The transistor uses both types of carriers - basic and non-basic, so it is called bipolar.

A bipolar transistor consists of three regions of a single-crystal semiconductor with different types of conductivity: emitter, base, and collector.

E - emitter,
B - base
K - collector
EP - emitter junction,
KP - collector junction,
W is the thickness of the base.

Each of the transitions of the transistor can be turned on either in the forward or in the opposite direction. Depending on this, three modes of operation of the transistor are distinguished:

Cutoff mode - both pn junctions are closed, while a relatively small current usually goes through the transistor
Saturation mode - both pn junctions are open
Active mode - one of the p-n junctions is open and the other is closed

In clipping mode and saturation mode, transistor control is not possible. Effective transistor control is carried out only in the active mode. This mode is basic. If the voltage at the emitter junction is direct, and the reverse at the collector junction, then the inclusion of the transistor is considered normal, with the opposite polarity - inverse.

In normal mode, the collector pn junction is closed, the emitter junction is open. The collector current is proportional to the base current.

The movement of charge carriers in an n-p-n type transistor is shown in the figure:

When the emitter is connected to the negative terminal of the power source, an emitter current Ie arises. Since an external voltage is applied in the forward direction to the emitter junction, the electrons overcome the junction and fall into the base region. The base is made of a p-semiconductor, therefore electrons are minority carriers for it.

Electrons entering the base region partially recombine with the base holes. However, the base is usually made very thin from a p-conductor with a high resistivity (low impurity content), therefore, the concentration of holes in the base is low and only a few electrons entering the base recombine with its holes, forming the base current Ib. Most electrons, due to thermal motion (diffusion) and under the action of the collector field (drift), reach the collector, forming the collector current component Iк.

The relationship between the increments of the emitter and collector currents is characterized by the current transfer coefficient

As follows from a qualitative examination of the processes occurring in a bipolar transistor, the current transfer coefficient is always less than unity. For modern bipolar transistors, α \u003d 0.9 ÷ 0.95

When Ie ≠ 0, the collector current of the transistor is equal to:

In the considered switching circuit, the base electrode is common for the emitter and collector circuits. Such a switching circuit of a bipolar transistor is called a common base circuit, while the emitter circuit is called the input circuit, and the collector circuit is called the output circuit. However, such a circuit for switching on a bipolar transistor is used very rarely.

Three circuits for turning on a bipolar transistor

There is a switching circuit with a common base, a common emitter, a common collector. The circuits for the p-n-p transistor are shown in figures a, b, c:

In a circuit with a common base (Fig. A), the electrode base is common for the input and output circuits, in a circuit with a common emitter (Fig. B), the emitter is common, in a circuit with a common collector (Fig. C), the collector is common.

The figure shows: E1 - input circuit power, E2 - output circuit power, Uin - source of the amplified signal.

The main circuit is a switching circuit in which the common electrode for the input and output circuit is an emitter (switching circuit of a bipolar transistor with a common emitter). For such a circuit, the input circuit passes through the base-emitter junction and the base current appears in it:

The low value of the base current in the input circuit led to the widespread use of the circuit with a common emitter.

Bipolar transistor in a common emitter (OE) circuit

In a transistor included in the OE scheme, the relationship between current and voltage in the input circuit of the transistor Ib \u003d f1 (Ube) is called the input or base current-voltage characteristic (CVC) of the transistor. The dependence of the collector current on the voltage between the collector and the emitter at fixed base current values \u200b\u200bIk \u003d f2 (Uke), Ib - const is called the family of output (collector) characteristics of the transistor.

The input and output current-voltage characteristics of a medium power bipolar transistor of the n-p-n type are shown in the figure:

As can be seen from the figure, the input characteristic is practically independent of the voltage Uke. The output characteristics are approximately equidistant from each other and almost rectilinear over a wide range of voltage changes Uke.

The dependence Ib \u003d f (Ube) is an exponential dependence characteristic of the current of a bias pn junction. Since the base current is recombination, its Ib value is β times smaller than the injected emitter current Ie. With an increase in the collector voltage Uk, the input characteristic shifts to the region of high voltages Ub. This is due to the fact that due to the modulation of the base width (Earley effect), the fraction of the recombination current in the base of the bipolar transistor decreases. The voltage Ube does not exceed 0.6 ... 0.8 V. Exceeding this value will lead to a sharp increase in the current flowing through an open emitter junction.

The dependence Ik \u003d f (Uke) shows that the collector current is directly proportional to the base current: Ik \u003d B · Ib

Bipolar transistor parameters

Representation of a transistor in a low-signal mode of operation by a four-terminal network

In the low-signal operation mode, the transistor can be represented by a four-terminal device. When voltages u1, u2 and currents i1, i2 change according to a sinusoidal law, the connection between voltages and currents is established using Z, Y, h parameters.

Potentials 1 ", 2", 3 are the same. The transistor is conveniently described using h-parameters.

The electrical state of the transistor, connected according to the circuit with a common emitter, is characterized by four values: Ib, Ube, Ik and Uke. Two of these quantities can be considered independent, and the other two can be expressed through them. For practical reasons, it is convenient to choose the values \u200b\u200bof Ib and Uke as independent. Then Ube \u003d f1 (Ib, Uke) and Ik \u003d f2 (Ib, Uke).

In amplifying devices, input signals are increments of input voltages and currents. Within the linear part of the characteristics for the increments Ube and Ik the equalities are true:

The physical meaning of the parameters:

For a scheme with OE, the coefficients are written with the index E: h11e, h12e, h21e, h22e.

In the passport data indicate h21e \u003d β, h21b \u003d α. These parameters characterize the quality of the transistor. To increase the value of h21, one must either reduce the width of the base W or increase the diffusion length, which is rather difficult.

Compound transistors

To increase the value of h21, bipolar transistors are connected according to the Darlington circuit:

In a composite transistor having characteristics as one, the base VT1 is connected to the emitter VT2 and ΔIe2 \u003d ΔIb1. The collectors of both transistors are connected and this output is the output of a composite transistor. The base VT2 plays the role of the base of the composite transistor ΔIb \u003d ΔIb2, and the emitter VT1 plays the role of the emitter of the composite transistor ΔIe \u003d ΔI1.

We obtain the expression for the current gain β for the Darlington circuit. Let us express the relationship between the change in the base current dIб and the resulting change in the collector current dIк of the composite transistor as follows:

Since for bipolar transistors the current gain is usually several tens (β1, β2 \u003e\u003e 1), the total gain of the composite transistor will be determined by the product of the gain of each of the transistors βΣ \u003d β1 · β2 and can be quite large in magnitude.

Note the features of the operation mode of such transistors. Since the emitter current VT2 Ie2 is the base current VT1 dIб1, then, therefore, the transistor VT2 must operate in micropower mode, and the transistor VT1 should operate in large injection mode, their emitter currents differ by 1-2 orders of magnitude. With such a non-optimal choice of the operating characteristics of the bipolar transistors VT1 and VT2, it is not possible to achieve high current amplification values \u200b\u200bin each of them. Nevertheless, even with values \u200b\u200bof the gain β1, β2 ≈ 30, the total gain βΣ will be βΣ ≈ 1000.

High gain values \u200b\u200bin composite transistors are implemented only in the statistical mode, so composite transistors are widely used in the input stages of operational amplifiers. In circuits at high frequencies, composite transistors no longer have such advantages, on the contrary, both the limiting current amplification frequency and the speed of composite transistors are less than the same parameters for each of the transistors VT1, VT2 separately.

Frequency properties of bipolar transistors

The process of propagation of minority carriers injected into the base from the emitter to the collector junction proceeds via the diffusion path. This process is rather slow, and the carriers injected from the emitter reached the collector no earlier than during the diffusion of carriers through the base. Such a delay will lead to a phase shift between the current Ie and the current Ik. At low frequencies, the phases of the currents Ie, Ik and Ib coincide.

The frequency of the input signal, at which the gain modulus decreases by a factor compared to the static value β0, is called the limiting current amplification frequency of the bipolar transistor in a circuit with a common emitter

Fβ - limit frequency (cutoff frequency)
fgr - cutoff frequency (unit gain frequency)

Field effect transistors

Field, or unipolar, transistors use the field effect as the main physical principle. Unlike bipolar transistors, in which both types of carriers, both main and minor, are responsible for the transistor effect, in field effect transistors only one type of carrier is used to realize the transistor effect. For this reason, field effect transistors are called unipolar. Depending on the conditions for the implementation of the field effect, field-effect transistors are divided into two classes: field-effect transistors with an isolated gate and field-effect transistors with a p-n junction control.

Field effect transistors with p-n junction control

Schematically, a field-effect transistor with a p-n junction control can be represented in the form of a plate, to the ends of which electrodes, a source and a drain are connected. In fig. shows the structure and circuit of the field effect transistor with an n-type channel:

In a transistor with an n-channel, the main charge carriers in the channel are electrons that move along the channel from the source with a low potential to the drain with a higher potential, forming a drain current Ic. Between the gate and the source, a voltage is applied to block the p-n junction formed by the n-region of the channel and the p-region of the gate.

When a blocking voltage is applied to the pn junction Uzi at the channel boundaries, a uniform layer appears, depleted in charge carriers and having a high resistivity. This leads to a decrease in the conductive channel width.

By changing the magnitude of this voltage, you can change the cross section of the channel and, therefore, change the magnitude of the electrical resistance of the channel. For an n-channel field effect transistor, the drain potential is positive with respect to the source potential. With a grounded gate, current flows from the drain to the source. Therefore, to stop the current on the gate, you need to apply a reverse voltage of several volts.

The value of the voltage Uzi, at which the current through the channel becomes almost zero, is called the cutoff voltage Uap

Thus, a field effect transistor with a gate in the form of a p-n junction is a resistance, the value of which is regulated by an external voltage.

The field effect transistor is characterized by the following CVC:

Here, the dependences of the drain current Is on voltage at a constant voltage across the gate Uzi determine the output, or stock, characteristics of the field-effect transistor. In the initial section of the characteristics Usi + | Usi |< Uзап ток стока Iс возрастает с увеличением Uси . При повышении напряжения сток - исток до Uси = Uзап - |Uзи | происходит перекрытие канала и дальнейший рост тока Iс прекращается (участок насыщения). Отрицательное напряжение Uзи между затвором и истоком смещает момент перекрытия канала в сторону меньших значений напряжения Uси и тока стока Iс . Участок насыщения является рабочей областью выходных характеристик полевого транзистора. Дальнейшее увеличение напряжения Uси приводит к пробою р-n-перехода между затвором и каналом и выводит транзистор из строя.

The I – V characteristic Ic \u003d f (Uzi) shows the voltage Uap. Since the Uzi ≤ 0 pn junction is closed and the gate current is very small, of the order of 10 -8 ... 10-9 A, therefore, the main advantages of a field-effect transistor, compared with a bipolar, is a high input impedance, of the order of 10 10 ... 1013 Ohm. In addition, they are distinguished by low noise and manufacturability.

Two main switching schemes have practical applications. A circuit with a common source (Fig. A) and a circuit with a common drain (Fig. B), which are shown in the figure:

Insulated Gate Field Effect Transistors
(MOSFETs)

The term "MOS transistor" is used to refer to field-effect transistors in which the control electrode - the gate - is separated from the active area of \u200b\u200bthe field-effect transistor by a dielectric layer - an insulator. The main element for these transistors is the metal-dielectric-semiconductor (M-D-P) structure.

The technology of an MOS transistor with an integrated gate is shown in the figure:

The original semiconductor on which the MIS transistor is made is called the substrate (pin P). Two heavily doped n + regions are called source (I) and drain (C). The area of \u200b\u200bthe substrate under the shutter (3) is called the built-in channel (n-channel).

The physical basis for the operation of a field effect transistor with a metal-insulator-semiconductor structure is the field effect. The field effect consists in the fact that under the influence of an external electric field the concentration of free charge carriers in the near-surface region of the semiconductor changes. In field devices with a MIS structure, the external field is caused by the applied voltage to the metal gate electrode. Depending on the sign and magnitude of the applied voltage, there can be two states of the space charge region (SCR) in the channel — enrichment and depletion.

The depletion mode corresponds to a negative voltage Uz, at which the electron concentration in the channel decreases, which leads to a decrease in the drain current. The enrichment mode corresponds to a positive voltage Uzi and an increase in the drain current.

The CVC is presented in the figure:

The topology of an MOS transistor with an induced (induced) p-type channel is shown in the figure:

When Uzi \u003d 0, the channel is absent and Ic \u003d 0. The transistor can only work in the Uzi enrichment mode< 0. Если отрицательное напряжение Uзи превысит пороговое Uзи.пор , то происходит формирование инверсионного канала. Изменяя величину напряжения на затворе Uзи в области выше порогового Uзи.пор , можно менять концентрацию свободных носителей в инверсионном канале и сопротивление канала. Источник напряжения в стоковой цепи Uси вызовет ток стока Iс .

The CVC is presented in the figure:

In MOS transistors, the gate is separated from the semiconductor by a layer of SiO2 oxide. Therefore, the input impedance of such transistors is of the order of 1013 ... 1015 Ohms.

The main parameters of field effect transistors include:

The steepness of the characteristic at Us \u003d const, Up \u003d const. Typical parameter values \u200b\u200bare (0.1 ... 500) mA / V;
The steepness of the characteristic on the substrate at Us \u003d const, Us \u003d const. Typical parameter values \u200b\u200bare (0.1 ... 1) mA / V;
Initial drain current I.s. - drain current at zero voltage value Uзи. Typical parameter values: (0.2 ... 600) mA - for transistors with a p-n junction control channel; (0.1 ... 100) mA - for transistors with an integrated channel; (0.01 ... 0.5) μA - for transistors with an induced channel;
Cutoff voltage . Typical values \u200b\u200b(0.2 ... 10) V; threshold voltage Uп. Typical values \u200b\u200b(1 ... 6) V;
Resistance drain-source in the open state. Typical Values \u200b\u200b(2..300) Ohm
Differential resistance (internal): with Us \u003d const;
Statistical gain: μ \u003d S · ri

Thyristors

A thyristor is a semiconductor device with three or more electron-hole p-n junctions. They are mainly used as electronic keys. Depending on the number of external terminals, they are divided into thyristors with two external terminals - dinistors and thyristors with three terminals - trinistors. To indicate thyristors, the letter symbol VS is adopted.

The device and principle of operation of the dinistor

The structure, UGO and I-V characteristics of the dinistor are shown in the figure:

The outer p-region is called the anode (A), the outer n-region is called the cathode (K). Three p-n junctions are indicated by the numbers 1, 2, 3. The structure of the dinistor is 4-layer - p-n-p-n.

The supply voltage E is supplied to the dinistor in such a way that 1 of 3 transitions are open and their resistance is negligible, and transition 2 is closed and all supply voltage Upr is applied to it. A small reverse current flows through the dinistor, the load R is disconnected from the power supply current E.

Upon reaching a critical voltage equal to the on-voltage U on, transition 2 opens, and all three transitions 1, 2, 3 will be in the open (on) state. The resistance of the dinistor drops to tenths of Ohm.

The switching voltage is a few hundred volts. The dinistor opens, and significant currents flow through it. The voltage drop across the dynistor in the open state is 1-2 volts and little depends on the magnitude of the flowing current, the value of which is τa ≈ E / R, and UR ≈ E, i.e. the load is connected to a power source E. The voltage at the dynistor, corresponding to the maximum permissible point I open max, is called the open state voltage Uoc. The maximum permissible current is from hundreds of mA to hundreds of A. The dinistor is in the open state until the current flowing through it becomes less than the holding current Iud. The dinistor closes when the external voltage decreases to a value of the order of 1V or when the polarity of the external source changes. Therefore, such a device is used in transient current circuits. Points B and D correspond to the boundary values \u200b\u200bof the currents and voltages of the dinistor. The recovery time of the resistance of transition 2 after removing the supply voltage is about 10-30 μs.

Dinistors are by their principle devices of key action. In the on state (BV section) it is similar to a closed key, and in the off state (exhaust gas section) it is like an open key.

The device and principle of operation of the thyristor (trinistor)

The trinistor is a controlled device. It contains a control electrode (RE) connected to the p-type semiconductor or the n-type semiconductor of the middle transition 2.

The structure, UGO, and I – V characteristics of a trinistor (usually called a thyristor) are shown in the figure:

The voltage U off, at which an avalanche-like increase in current begins, can be reduced by introducing minority charge carriers into any of the layers adjacent to transition 2. To what extent U on decreases is shown in the I – V characteristic. An important parameter is the control trigger current Iу.ot, which ensures the thyristor switches to the open state at voltages lower than the voltage Uin. The figure shows three values \u200b\u200bof the voltage on UI on< Un вкл < Um вкл соответствует трем значениям управляющего тока UI у.от > Un.ot\u003e Um.ot.

Consider the simplest circuit with a thyristor loaded on a resistor load Rн

Ia - anode current (power current in the circuit of the anode-cathode of the thyristor);
Uak is the voltage between the anode and cathode;
Iу is the current of the control electrode (current pulses use current pulses);
Uuk is the voltage between the control electrode and the cathode;
Upit - supply voltage.

To transfer the thyristor to the open state, a non-controlling electrode is supplied from the pulse generation circuit by a short-term (of the order of several microseconds) control pulse.

A characteristic feature of this non-lockable thyristor, which is very widely used in practice, is that it cannot be turned off using the control current.

To turn off the thyristor in practice, a reverse voltage Uak is applied to it< 0 и поддерживают это напряжение в течении времени, большего так называемого времени выключения tвыкл . Оно обычно составляет единицы или десятки микросекунд.

The device and principle of operation of the triac

Widely used are the so-called symmetric thyristors (triacs, triacs). Each triac is similar to a pair of thyristors considered, connected in parallel. Symmetric trinistors are a controlled device with a symmetrical current-voltage characteristic. To obtain a symmetrical characteristic, double-sided semiconductor structures of the p-n-p-n-p type are used.

The structure of the triac, its UGO and CVC are shown in the figure:

The triac (triac) contains two thyristors p1-n1-p2-n2 and p2-n2-p1-n4 connected in opposite-parallel. The triac contains 5 transitions P1-P2-P3-P4-P5. In the absence of a control electron UE, the triac is called a diac.

With a positive polarity, the thyristor effect in p1-n1-p2-n2 is realized on the electrode E1, and with the opposite polarity in p2-n1-p1-n4.

When a control voltage is supplied to the RE, depending on its polarity and magnitude, the voltage of the switch U on

Thyristors (dinistors, trinistors, triacs) are the main elements in power devices of electronics. There are thyristors for which the switching voltage is more than 1 kV, and the maximum allowable current is more than 1 kA

Electronic keys

To increase the efficiency of power electronics devices, pulsed operation of diodes, transistors and thyristors is widely used. The pulse mode is characterized by sharp changes in currents and voltages. In pulse mode, diodes, transistors and thyristors are used as keys.

Using electronic keys, electronic circuits are switched: connecting / disconnecting the circuit to / from sources (s) of electric energy or signal, connecting or disconnecting circuit elements, changing the parameters of circuit elements, changing the type of the acting signal source.

UGO ideal keys are shown in the figure:

Keys working for closing and opening, respectively.

The key mode is characterized by two states: on / off.

Ideal keys are characterized by an instantaneous change in resistance, which can take the value 0 or ∞. The voltage drop on an ideal closed key is 0. When the key is open, the current is 0.

Real keys are also characterized by two extreme resistance values \u200b\u200bRmax and Rmin. The transition from one resistance value to another in real keys occurs in a finite time. The voltage drop on a real closed key is not equal to zero.

Keys are divided into keys used in low-power circuits, and keys used in power circuits. Each of these classes has its own characteristics.

The keys used in low-power circuits are characterized by:

Key resistances in open and closed states;
Performance - the time the key transitions from one state to another;
Voltage drop on the closed key and leakage current of the open key;
Immunity - the ability of the key to remain in one of the states when exposed to interference;
Key sensitivity - the value of the control signal that transfers the key from one state to another;
The threshold voltage is the value of the control voltage, in the vicinity of which there is a sharp change in the resistance of the electronic switch.

Diode electronic keys

The simplest type of electronic key is diode key. The diode switch circuit, the static transfer characteristic, the current-voltage characteristic and the dependence of the differential resistance on the voltage on the diode are shown in the figure:

The principle of operation of the diode electronic key is based on a change in the differential resistance of the semiconductor diode in the vicinity of the threshold voltage value on the diode Uпор. Figure "c" shows the current-voltage characteristic of a semiconductor diode, which shows the value of Upor. This value is at the intersection of the stress axis with the tangent drawn to the ascending participant of the current-voltage characteristic.

Figure "d" shows the dependence of the differential resistance on the voltage on the diode. It follows from the figure that in the vicinity of the threshold voltage of 0.3 V, a sharp change in the differential resistance of the diode occurs with extreme values \u200b\u200bof 900 and 35 Ohms (Rmin \u003d 35 Ohms, Rmax \u003d 900 Ohms).

In the “on” state, the diode is open and, Uout ≈ Uin.

In the off state, the diode is closed and, Uout ≈ Uin · Rn / Rmax<

In order to reduce the switching time, the diodes used with a small junction capacitance of the order of 0.5-2 pF, while providing a turn-off time of the order of 0.5-0.05 μs.

Diode keys do not allow to electrically separate the control and controlled circuits, which is often required in practical circuits.

Transistor keys

Most circuits used in computers, telecontrol devices, automatic control systems, etc., are based on transistor switches.

The key circuits on the bipolar transistor and the CVC are shown in the figure:

The first state is “off” (the transistor is closed) is determined by point A1 on the output characteristics of the transistor; it is called a cutoff mode. In the cutoff mode, the base current is Ib \u003d 0, the collector current Ik1 is equal to the initial collector current, and the collector voltage is Uк \u003d Uк1 ≈ Ek. The cutoff mode is realized at Uin \u003d 0 or at negative base potentials. In this state, the key resistance reaches its maximum value: Rmax \u003d, where RT is the transistor resistance in the closed state, more than 1 MΩ.

The second state is “on” (the transistor is open) is determined by point A2 on the I – V characteristic and is called the saturation mode. From the cutoff mode (A1) in the saturation mode (A2), the transistor is transferred by the positive input voltage Uin. In this case, the voltage Uout takes a minimum value of Uк2 \u003d Uк.э. to us of the order of 0.2-1.0 V, the collector current Iк2 \u003d Iк.нас ≈ Ec / Rк. The base current in saturation mode is determined from the condition: Ib\u003e Ib.nas \u003d Ik.nas / h21.

The input voltage required to transfer the transistor to the open state is determined from the condition: U in\u003e IB.s.Rb + U.s.

Good noise immunity and low power dissipated in the transistor is explained by the fact that the transistor most of the time is either saturated (A2) or closed (A1), and the transition time from one state to another is a small part of the duration of these states. The switching time of the keys on bipolar transistors is determined by the barrier capacitances of the pn junctions and the processes of accumulation and resorption of minority charge carriers in the base.

To increase the speed and input resistance, keys on field-effect transistors are used.

Key circuits on field effect transistors with a pn junction control and with an induced channel with a common source and a common drain are shown in the figure:

For any key on the field effect transistor Rн\u003e 10-100 kOhm.

The control signal Uin on the gate is of the order of 10-15 V. The resistance of the field-effect transistor in the closed state is large, of the order of 108 -109 Ohms.

The resistance of the field effect transistor in the open state can be 7-30 ohms. The resistance of the field effect transistor in the control circuit can be 108 -109 Ohms. (circuit "a" and "b") and 1012-1014 ohms (circuit "c" and "g").

Power (powerful) semiconductor devices

Powerful semiconductor devices are used in power electronics, the most intensively developing and promising field of technology. They are designed to control currents of tens, hundreds of amperes, voltages of tens, hundreds of volts.

Powerful semiconductor devices include thyristors (dinistors, trinistors, triacs), transistors (bipolar and field) and bipolar statically induced transistors (IGBT). They are used as electronic keys for switching electronic circuits. Their characteristics are trying to bring closer to the characteristics of ideal keys.

By the principle of operation, characteristics and parameters, powerful transistors are similar to low-power ones, however, there are certain features.

Power Field Transistors

Currently, the field effect transistor is one of the most promising power devices. The most widely used transistors with insulated gate and induced channel. To reduce the resistance of the channel, reduce its length. To increase the drain current, hundreds and thousands of channels are performed in the transistor, and the channels are connected in parallel. The probability of self-heating of the field effect transistor is small, because channel resistance increases with increasing temperature.

Power field effect transistors have a vertical structure. Channels can be located both vertically and horizontally.

DMDP transistor

This MIS type transistor made by double diffusion has a horizontal channel. The figure shows a structure element containing a channel.

VMDP transistor

This V-shaped MOS transistor has a vertical channel. The figure shows one structural element containing two channels.

It is easy to see that the structures of the VMDP transistor and the DMDP transistor are similar.

IGBT Transistor

IGBT is a hybrid semiconductor device. It combines two methods of controlling electric current, one of which is characteristic for field-effect transistors (control of an electric field), and the second for bipolar (control of injection of carriers of electricity).

Typically, an IGBT uses an n-type MOSFET transistor structure. The structure of this transistor differs from the structure of the DMDP transistor by an additional p-type semiconductor layer.

Let us pay attention to the fact that the terms "emitter", "collector" and "gate" are commonly used to designate IGBT electrodes.

The addition of a p-type layer leads to the formation of a second structure of a bipolar transistor (p-n-p type). Thus, there are two bipolar structures in the IGBT — n-p-n type and p-n-p type.

The UGO and IGBT shutdown circuit are shown in the figure:

A typical view of the output characteristics is shown in the figure:

SIT Transistor

SIT is a field effect transistor with a pn junction control with static induction. It is multi-channel and has a vertical structure. The schematic representation of the SIT and the switching circuit with a common source are shown in the figure:

Regions of the p-type semiconductor are in the form of cylinders, the diameter of which is units of micrometers or more. This cylinder system acts as a shutter. Each cylinder is connected to the shutter electrode (the shutter electrode is conventionally not shown in figure "a").

Dotted lines indicate the regions of pn junctions. The actual number of channels can be thousands. Typically, SIT is used in common source circuits.

Each of the considered devices has its own field of application. The keys on the thyristors are used in devices operating at low frequencies (kilohertz and below). The main disadvantage of such keys is their low performance.

The main field of application of thyristors is low-frequency devices with large switched power up to several megawatts, which do not impose serious performance requirements.

Powerful bipolar transistors are used as high-voltage switches in devices with a switching or conversion frequency in the range of 10-100 kHz, with an output power level from units of W to several kW. The optimum range of switching voltages is 200-2000 V.

Field effect transistors (MOSFETs) are used as electronic keys for switching low-voltage high-frequency devices. Optimum values \u200b\u200bof switching voltages do not exceed 200 V (maximum value up to 1000 V), while the switching frequency can range from units of kHz to 105 kHz. The range of switched currents is 1.5-100 A. The positive properties of this device are controllability by voltage, not current, and less dependence on temperature in comparison with other devices.

Insulated Gate Bipolar Transistors (IGBTs) are used at frequencies of less than 20 kHz (some types of devices are used at frequencies of more than 100 kHz) with switched powers above 1 kW. Switching voltages are not lower than 300-400 V. Optimum values \u200b\u200bof switching voltages above 2000 V. IGBT and MOSFET require a voltage of not higher than 12-15 V to fully turn on, to close the devices it is not necessary to supply a negative voltage. They are characterized by high switching speeds.

Material for preparation for certification

So, the third and final part of the story about bipolar transistors on our site \u003d) Today we will talk about using these wonderful devices as amplifiers, consider possible bipolar transistor and their main advantages and disadvantages. Let's get started!

This circuit is very good when using high frequency signals. In principle, for this, such a turn on of the transistor is used primarily. Very big disadvantages are the low input impedance and, of course, the lack of current gain. See for yourself, at the input we have emitter current, at the output.

That is, the emitter current is greater than the collector current by a small amount of base current. And this means that the current gain is not just absent, moreover, the output current is slightly less than the input current. Although, on the other hand, this circuit has a rather large transmission coefficient for voltage) These are the advantages and disadvantages, we continue ...

Common Collector Bipolar Transistor

This is how the switching circuit of a bipolar transistor with a common collector looks like. Doesn’t resemble anything?) If you look at the circuit from a slightly different angle, we will recognize our old friend here - the emitter repeater. There was almost an entire article about him (), so we already examined everything related to this scheme there. And in the meantime, we are waiting for the most commonly used circuit - with a common emitter.

Circuit for switching a bipolar transistor with a common emitter.

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

We got acquainted with all the circuits, now let us consider in more detail the last (but not least) circuit of the amplifier using a bipolar transistor (with a common emitter). To start, let's portray it a little differently:

There is one minus - a grounded emitter. With such a turn on of the transistor, the output contains non-linear distortions, which, of course, must be fought. Non-linearity occurs due to the influence of the input voltage on the emitter-base junction voltage. Indeed, there is nothing "superfluous" in the emitter circuit; all the input voltage turns out to be applied precisely to the base-emitter junction. To cope with this phenomenon, we add a resistor to the emitter circuit. So we get negative feedback.

But what is it?

In short, then negative back principleth communication lies in the fact that some part of the output voltage is transmitted to the input and subtracted from the input signal. Naturally, this leads to a decrease in the gain, since the input voltage of the transistor will receive a lower voltage value than in the absence of feedback.

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

So, even if there is no feedback, an increase in the input signal by 0.5 V leads to the same increase. Everything is clear here 😉 And now we add feedback! And in the same way, we increase the input voltage by 0.5 V. Following this, it increases, which leads to an increase in the emitter current. And growth leads to an increase in voltage on the feedback resistor. It would seem that this is so? But this voltage is subtracted from the input! See what happened:

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

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

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

There is another problem in our amplifier. If a negative voltage value appears at the input, the transistor will immediately close (the base voltage will become lower 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 the divider as follows:

Got such 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, for example, -4V, comes to the input, then the base potential will be + 2V, that is, the value is positive and does not interfere with the normal operation of the transistor. Here's how useful it is to create an offset in the base chain)

What else would 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 but a useful amplified signal at the input. How to provide this? 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 the transistor itself, was overgrown with additional elements 😉 Perhaps we will stop there, soon there will be an article devoted to the practical calculation of an amplifier based on a bipolar transistor. In it, we will not only compose amplifier circuit diagram, but we also calculate the values \u200b\u200bof all the elements, and at the same time choose a transistor suitable for our purposes. See you soon! \u003d)

There are three main circuits for turning on transistors. In this case, one of the electrodes of the transistor is a common point of entry and exit of the cascade. It must be remembered that under the input (output) we mean the points between which the input (output) alternating voltage acts. The main switching circuits are called circuits with a common emitter (OE), a common base (OB) and a common collector (OK).

Circuit with a common emitter (OE). Such a circuit is shown in Figure 1. In all the books it is written that this circuit is the most common, because it gives the greatest power gain.

Fig. 1 - Connection diagram of a transistor with a common emitter

The enhancing properties of the transistor are characterized by one of its main parameters - the static current transfer coefficient of the base or the static current gain ?. Since it should characterize only the transistor itself, it is determined in the no-load mode (R k \u003d 0). Numerically, it is equal to:

when U k-e \u003d const

This coefficient can be equal to tens or hundreds, but the real coefficient k i is always less than?, Because when the load is turned on, the collector current decreases.

The voltage gain stage cascade k u is equal to the ratio of the amplitude or current values \u200b\u200bof the output and input alternating voltage. The input voltage is alternating voltage u bee, and the output voltage is alternating voltage across the resistor, or the same thing, collector-emitter voltage. The base-emitter voltage does not exceed tenths of a volt, and the output reaches unity and tens of volts (with sufficient load resistance and source voltage E 2). It follows that the gain of the cascade in power is hundreds, thousands, and sometimes tens of thousands.

An important characteristic is the input resistance R I, which is determined by Ohm's law:

and usually ranges from hundreds of ohms to units of kilo-ohms. The input impedance of the transistor when turned on according to the OE scheme, as can be seen, turns out to be relatively small, which is a significant drawback. It is also important to note that the cascade according to the OE scheme reverses the voltage phase by 180 °

The advantages of the OE circuit include the convenience of supplying it from a single source, since the supply voltage of the same sign is applied to the base and collector. The disadvantages include the worst frequency and temperature properties (for example, in comparison with the OB scheme). With increasing frequency, the gain in the OE circuit decreases. In addition, the cascade according to the OE scheme during amplification introduces significant distortions.

Scheme with a common base (OB). The OB scheme is shown in Figure 2.

Fig. 2 - Connection diagram of a transistor with a common base

Such a switching circuit does not give significant gain, but has good frequency and temperature properties. It is not used as often as the MA scheme.

The current gain of the OB circuit is always slightly less than unity:

since the collector current is always only slightly less than the emitter current.

Is the static current transfer coefficient for the OB circuit indicated? and is determined:

when u b \u003d const

This coefficient is always less than 1 and the closer it is to 1, the better the transistor. The voltage gain is the same as in the OE circuit. The input impedance of the OB circuit is ten times lower than in the OE circuit.

For the OB circuit, there is no phase shift between the input and output voltage, that is, the phase of the voltage does not invert during amplification. In addition, with amplification, the OB scheme introduces much less distortion than the OE scheme.

Circuit with a common collector (OK). The switching circuit with a common collector is shown in Figure 3. Such a circuit is more often called an emitter follower.

Fig. 3 - Connection diagram of a transistor with a common collector

The peculiarity of this circuit is that the input voltage is completely transmitted back to the input, i.e., negative feedback is very strong. The current gain is almost the same as in the OE circuit. The voltage gain is close to unity, but always less than it. As a result, the power gain is approximately equal to k i, i.e., several tens.

In the OK circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, the output voltage in phase and amplitude coincides with the input, 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.

The input resistance of the OK circuit is quite high (tens of kilo-ohms), and the output resistance is relatively small. This is an important advantage of the scheme.