Bipolar transistors have the following electrodes. Switching circuits for bipolar transistors

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The design and principle of operation of a bipolar transistor

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

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

Operating modes of a bipolar transistor

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

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

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

Under direct voltage UEB holes are injected from the emitter into the base. Once in the n-type base, holes become minority charge carriers in it and, under the influence of diffusion forces, move (diffuse) to the collector p-n junction. Some of the holes in the base are filled (recombined) with the free electrons present in it. However, the width of the base is small - from several units to 10 microns. Therefore, the main part of the holes reaches the collector p-n junction and is transferred by its electric field to the collector. Obviously, the collector current I K p there cannot be more emitter current, since some of the holes recombine in the base. That's why I K p = h 21B I uh
Magnitude h 21B is called the static transfer coefficient of the emitter current. For modern transistors h 21B= 0.90...0.998. Since the collector junction is switched in the opposite direction (often said - biased in the opposite direction), reverse current also flows through it I BWC , formed by minority carriers of the base (holes) and collector (electrons). Therefore, the total collector current of a transistor connected according to a circuit with a common base

ITo = h 21B I uh +IBWC
Holes that did not reach the collector junction and recombined (filled) in the base give it a positive charge. To restore the electrical neutrality of the base, the same number of electrons is supplied to it from the external circuit. The movement of electrons from the external circuit to the base creates a recombination current in it I B.rec. In addition to the recombination current, the reverse collector current flows through the base in the opposite direction and the full base current
I B = I B.rek - I KBO
In active mode, the base current is tens and hundreds of times less than the collector current and emitter current.

Bipolar transistor connection circuits

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

The following figure shows a circuit in which the emitter is the common electrode for the input and output circuits. This is a common emitter circuit, or "OE diagram".

In it, the output current, as in the OB circuit, is the collector current I K, slightly different from the emitter current I e, and the input is the base current I B, significantly less than the collector current. Communication between currents I B And I K in the OE scheme is determined by the equation: I K= h 21 E I B + I KEO
Proportionality factor h 21 E is called the static base current transfer coefficient. It can be expressed in terms of the static transfer coefficient of the emitter current h 21B
h 21 E = h 21B / (1 —h 21B )
If h 21B is within the range of 0.9...0.998, the corresponding values h 21 E will be within 9...499.
Component I keo is called the reverse collector current in the OE circuit. Its value is 1+ h 21 E times more than I BWC, i.e. I KEO =(1+ h 21 E ) I KBO. Reverse currents I BWC and I CEOs do not depend on input voltages U EB And U BE and as a result are called uncontrolled components of the collector current. These currents strongly depend on the ambient temperature and determine the thermal properties of the transistor. It has been established that the reverse current value I The BER doubles with a temperature increase of 10 °C for germanium and 8 °C for silicon transistors. In the OE circuit, temperature changes in the uncontrolled reverse current I KEO can be tens and hundreds of times higher than the temperature changes of the uncontrolled reverse current I BWC and completely disrupt the operation of the transistor. Therefore, in transistor circuits, special measures are used for thermal stabilization of transistor cascades, helping to reduce the influence of temperature changes in currents on the operation of the transistor.
In practice, there are often circuits in which the common electrode for the input and output circuits of the transistor is the collector. This is a connection circuit with a common collector, or “OK circuit” (emitter follower) .

If we consider mechanical analogues, the operation of transistors resembles the principle of operation of a hydraulic power steering in a car. But the similarity is only valid at a first approximation, since transistors do not have valves. In this article we will separately consider the operation of a bipolar transistor.

Bipolar transistor device

The basis of the bipolar transistor device is a semiconductor material. The first semiconductor crystals for transistors were made from germanium; today silicon and gallium arsenide are more often used. First, a pure semiconductor material with a well-ordered crystal lattice is produced. Then the crystal is given the required shape and a special impurity is introduced into its composition (the material is doped), which gives it certain properties of electrical conductivity. If conductivity is due to the movement of excess electrons, it is defined as n-type donor (electronic). If the conductivity of a semiconductor is due to the sequential replacement of vacant places, so-called holes, by electrons, then such conductivity is called acceptor (hole) and is designated p-type conductivity.

Figure 1.

The transistor crystal consists of three parts (layers) with sequential alternation of conductivity type (n-p-n or p-n-p). Transitions from one layer to another form potential barriers. The transition from base to emitter is called emitter(EP), to the collector – collector(KP). In Figure 1, the transistor structure is shown as symmetrical, idealized. In practice, during production, the sizes of the areas are significantly asymmetrical, approximately as shown in Figure 2. The area of ​​the collector junction is significantly larger than the emitter junction. The base layer is very thin, on the order of several microns.

Figure 2.

Operating principle of a bipolar transistor

Any p-n junction of a transistor works similarly. When a potential difference is applied to its poles, it is “displaced.” If the applied potential difference is conditionally positive, and the pn junction opens, the junction is said to be forward biased. When a conditionally negative potential difference is applied, a reverse bias of the junction occurs, at which it is locked. A feature of the operation of the transistor is that with a positive bias of at least one transition, the general area, called the base, is saturated with electrons or electron vacancies (depending on the type of conductivity of the base material), which causes a significant reduction in the potential barrier of the second transition and, as a consequence, its conductivity under reverse bias.

Operating modes

All transistor connection circuits can be divided into two types: normal And inverse.

Figure 3.

Normal transistor switching circuit involves changing the electrical conductivity of the collector junction by controlling the bias of the emitter junction.

Inverse scheme, as opposed to normal, allows you to control the conductivity of the emitter junction by controlling the bias of the collector junction. The inverse circuit is a symmetrical analogue of the normal one, but due to the structural asymmetry of the bipolar transistor, it is ineffective for use, has more stringent restrictions on the maximum permissible parameters and is practically not used.

With any switching circuit, the transistor can operate in three modes: Cut-off mode, active mode And saturation mode.

To describe the work, the direction of the electric current in this article is conventionally taken to be the direction of the electrons, i.e. from the negative pole of the power supply to the positive pole. Let's use the diagram in Figure 4 for this.

Figure 4.

Cut-off mode

For a pn junction, there is a minimum forward bias voltage at which electrons are able to overcome the potential barrier of this junction. That is, at a forward bias voltage up to this threshold value, no current can flow through the junction. For silicon transistors, the value of this threshold is approximately 0.6 V. Thus, with a normal switching circuit, when the forward bias of the emitter junction does not exceed 0.6 V (for silicon transistors), no current flows through the base, it is not saturated with electrons, and as a result, there is no emission of base electrons into the collector region, i.e. There is no collector current (zero).

Thus, for the cutoff mode the necessary conditions are the identities:

U BE<0,6 В

I B =0

Active mode

In the active mode, the emitter junction is biased in the forward direction until the moment of unlocking (the current begins to flow) with a voltage greater than 0.6 V (for silicon transistors), and the collector junction in the reverse direction. If the base has p-type conductivity, electrons are transferred (injected) from the emitter into the base, which are instantly distributed in a thin layer of the base and almost all reach the collector boundary. Saturation of the base with electrons leads to a significant reduction in the size of the collector junction, through which electrons, under the influence of a negative potential from the emitter and base, are forced into the collector area, flowing through the collector terminal, thereby causing the collector current. The very thin layer of the base limits its maximum current passing through a very small cross section in the direction of the base exit. But this small thickness of the base causes its rapid saturation with electrons. The junction area is significant, which creates conditions for the flow of significant emitter-collector current, tens and hundreds of times greater than the base current. Thus, by passing small currents through the base, we can create conditions for much larger currents to pass through the collector. The greater the base current, the greater its saturation, and the greater the collector current. This mode allows you to smoothly control (regulate) the conductivity of the collector junction by correspondingly changing (regulating) the base current. This property of the active mode of the transistor is used in various amplifier circuits.

In active mode, the emitter current of the transistor is the sum of the base and collector current:

I E = I K + I B

The collector current can be expressed as:

I K = α I E

where α is the emitter current transfer coefficient

From the above equalities we can obtain the following:

where β is the base current amplification factor.

Saturation mode

The limit for increasing the base current until the moment when the collector current remains unchanged determines the point of maximum saturation of the base with electrons. A further increase in the base current will not change the degree of its saturation, and will not affect the collector current in any way; it can lead to overheating of the material in the base contact area and failure of the transistor. The reference data for transistors can indicate the values ​​of the saturation current and the maximum permissible base current, or the emitter-base saturation voltage and the maximum permissible emitter-base voltage. These limits determine the saturation mode of the transistor under normal operating conditions.

The cutoff mode and saturation mode are effective when transistors operate as electronic switches for switching signal and power circuits.

The difference in the principle of operation of transistors with different structures

The case of operation of an n-p-n transistor was considered above. Transistors of pnp structures work similarly, but there are fundamental differences that you should know. A semiconductor material with p-type acceptor conductivity has a relatively low electron throughput, since it is based on the principle of electron transition from one vacant site (hole) to another. When all vacancies are replaced by electrons, their movement is possible only as vacancies appear in the direction of movement. With a significant area of ​​such material, it will have significant electrical resistance, which leads to greater problems when used as the most massive collector and emitter of p-n-p bipolar transistors than when used in a very thin base layer of n-p-n transistors. A semiconductor material with n-type donor conductivity has the electrical properties of conductive metals, making it more advantageous for use as an emitter and collector, as in n-p-n transistors.

This distinctive feature of different bipolar transistor structures leads to great difficulties in producing pairs of components with different structures and electrical characteristics similar to each other. If you pay attention to the reference data for the characteristics of pairs of transistors, you will notice that when the same characteristics are achieved for two transistors of different types, for example KT315A and KT361A, despite their identical collector power (150 mW) and approximately the same current gain (20-90) , they have different maximum permissible collector currents, emitter-base voltages, etc.

P.S. This description of the principle of operation of the transistor was interpreted from the position of Russian Theory, therefore there is no description of the action of electric fields on fictitious positive and negative charges. Russian Physics makes it possible to use simpler, understandable mechanical models that are closer to reality than abstractions in the form of electric and magnetic fields, positive and electric charges, which the traditional school treacherously palms off on us. For this reason, I do not recommend using the stated theory without preliminary analysis and comprehension when preparing to take tests, coursework and other types of work; your teachers may simply not accept dissent, even competitive and quite consistent from the point of view of common sense and logic. In addition, on my part, this is the first attempt to describe the operation of a semiconductor device from the position of Russian Physics, it can be refined and supplemented in the future.

Device and principle of operation

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

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

Operating modes of a bipolar transistor

Normal active mode

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

Inverse active mode

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

Saturation mode

Both pn junctions are forward biased (both open). If the emitter and collector pn junctions are connected to external sources in the forward direction, the transistor will be in saturation mode. The diffusion electric field of the emitter and collector junctions will be partially weakened by the electric field created by external sources Ueb and Ukb. As a result, the potential barrier that limited the diffusion of the main charge carriers will decrease, and the penetration (injection) of holes from the emitter and collector into the base will begin, that is, currents called saturation currents of the emitter (IE.sat) and collector (IC) will flow through the emitter and collector of the transistor. us).

Cut-off mode

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

Barrier mode

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

Connection schemes

Any transistor connection circuit is characterized by two main indicators:

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

Connection diagram with a common base

Common base amplifier.

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

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

Advantages:

• Good temperature and frequency properties.
• High permissible voltage

Disadvantages of a common base scheme:

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

Connection circuit with common emitter

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

Advantages:

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

Flaws:

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

Common collector circuit

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

Advantages:

• High input impedance
• Low output impedance

Flaws:

• The voltage gain is less than 1.

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

Basic parameters

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

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

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

Equivalent circuit of a bipolar transistor using h-parameters

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

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

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

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

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

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

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

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

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

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

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

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

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

H 21e = I mк /I mb = β.

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

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

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

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

Transistor manufacturing technology

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

Application of transistors

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

See also

Literature

Notes

Transistors are divided into bipolar and field-effect. Each of these types has its own operating principle and design, however, what they have in common is the presence of semiconductor p-n structures.

Symbols of transistors are given in the table:

Device type				Conventional graphic symbol (UGO)
Bipolar	Bipolar pnp type
	Bipolar n-p-n type
Field	With the manager p-n junction	With p-type channel
		With n-type channel
	With isolated shutter MOSFET transistors	With built-in channel	Built-in channel p-type
			Built-in channel n-type
		With induced channel	Induced channel p-type
			Induced channel n-type

Bipolar transistors

The definition of “bipolar” indicates that the operation of a transistor is associated with processes in which charge carriers of two types take part - electrons and holes.

A transistor is a semiconductor device with two electron-hole junctions, designed to amplify and generate electrical signals. A transistor uses both types of carriers - major and minor, which is why it is called bipolar.

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

• E - emitter,
• B - base,
• K - collector,
• EP - emitter junction,
• KP - collector junction,
• W - base thickness.

Each of the transitions of the transistor can be turned on either in the forward or reverse direction. Depending on this, there are three operating modes of the transistor:

1. Cut-off mode - both p-n junctions are closed, while a relatively small current usually flows through the transistor
2. Saturation mode - both p-n junctions are open
3. Active mode - one of the p-n junctions is open and the other is closed

In cutoff mode and saturation mode, the transistor cannot be controlled. Effective control of the transistor is carried out only in active mode. This mode is the main one. If the voltage at the emitter junction is direct, and at the collector junction it is reverse, then the switching on of the transistor is considered normal; if the polarity is opposite, it is inverse.

In normal mode, the collector p-n 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 transistor is shown in the figure:

When the emitter is connected to the negative terminal of the power source, an emitter current Ie occurs. Since an external voltage is applied to the emitter junction in the forward direction, the electrons cross the junction and enter the base region. The base is made of a p-semiconductor, so electrons are minority charge carriers for it.

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

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

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

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

In the considered connection circuit, the base electrode is common to the emitter and collector circuits. This circuit for connecting a bipolar transistor is called a circuit with a common base, 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 switching on a bipolar transistor

There are switching circuits with a common base, a common emitter, and a common collector. Circuits for a pnp transistor are shown in figures a, b, c:

In a circuit with a common base (Fig. a), the base electrode is common to 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 – power supply of the input circuit, E2 – power supply of the output circuit, Uin – source of the amplified signal.

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

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

Bipolar transistor in a common emitter (CE) circuit

In a transistor connected according to the OE circuit, the relationship between current and voltage in the input circuit of the transistor Ib = f1 (Ube) is called the input or basic current-voltage characteristic (VC) of the transistor. The dependence of the collector current on the voltage between the collector and the emitter at fixed values ​​of the base current Iк = 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 linear over a wide range of voltage changes Uke.

The dependence Ib = f(Ube) is an exponential dependence characteristic of the current of a forward-biased p-n junction. Since the base current is recombination, its value Ib is β times less than the injected emitter current Ie. As the collector voltage Uк increases, the input characteristic shifts to the region of higher voltages Ub. This is due to the fact that due to modulation of the base width (Early effect), the proportion of 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 the open emitter junction.

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

Bipolar transistor parameters

Representation of a transistor in a small-signal mode of operation as a four-terminal network

In a small-signal operating mode, the transistor can be represented by a four-terminal network. 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. It is convenient to describe a transistor using h-parameters.

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

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

Physical meaning of the parameters:

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

The passport data indicates h21е = β, h21b = α. These parameters characterize the quality of the transistor. To increase the value of h21, you need to either reduce the base width W or increase the diffusion length, which is quite difficult.

Composite transistors

To increase the value of h21, bipolar transistors are connected using a Darlington circuit:

In a composite transistor that has the same characteristics as one, the base VT1 is connected to the emitter VT2 and ΔIе2 = ΔIb1. The collectors of both transistors are connected and this terminal is the terminal of the composite transistor. The base VT2 plays the role of the base of the composite transistor ΔIb = ΔIb2, and the emitter VT1 plays the role of the emitter of the composite transistor ΔIe = ΔI1.

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

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

Let us note the features of the operating mode of such transistors. Since the emitter current VT2 Ie2 is the base current VT1 dIb1, then, therefore, transistor VT2 should operate in micropower mode, and transistor VT1 - in high injection mode, their emitter currents differ by 1-2 orders of magnitude. With such a suboptimal choice of operating characteristics of bipolar transistors VT1 and VT2, it is not possible to achieve high current gain values ​​in each of them. Nevertheless, even with gain values ​​β1, β2 ≈ 30, the total gain βΣ will be βΣ ≈ 1000.

High gain values ​​in composite transistors are realized only in 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 cutoff frequency of current amplification and the operating 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 charge carriers injected into the base from the emitter to the collector junction proceeds by diffusion. This process is quite slow, and the carriers injected from the emitter will reach 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 modulus of the gain decreases by a factor of compared to the static value β0 is called the limiting frequency of current amplification of a bipolar transistor in a common-emitter circuit

Fβ – limiting frequency (cutoff frequency)
fgr – cut-off frequency (unity gain frequency)

Field effect transistors

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

Field-effect transistors with control p-n junction

Schematically, a field-effect transistor with a control pn junction can be represented as a plate, to the ends of which electrodes, a source and a drain are connected. In Fig. shows the structure and connection diagram of a field-effect transistor with an n-type channel:

In an n-channel transistor, the majority charge carriers in the channel are electrons, which move along the channel from a low-potential source to a higher-potential drain, producing a drain current Ic. A voltage is applied between the gate and the source, blocking 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 p-n junction Uzi, a uniform layer appears at the channel boundaries, depleted of charge carriers and having a high resistivity. This leads to a decrease in the conductive width of the channel.

By changing the value of this voltage, it is possible to change the cross-section of the channel and, consequently, change the value 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. When the gate is grounded, current flows from drain to source. Therefore, to stop the current, a reverse voltage of several volts must be applied to the gate.

The voltage value Uzi, at which the current through the channel becomes almost equal to zero, is called the cut-off voltage Uzap

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

The field-effect transistor is characterized by the following current-voltage characteristic:

Here, the dependence of the drain current Ic on the voltage at a constant voltage at the gate Uzi determines the output, or drain, characteristics of the field-effect transistor. At the initial section of the characteristics Usi + |Uzi |< 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 current-voltage characteristic Ic = f(Uzi) shows the voltage Uzap. Since Uzi ≤ 0 the p-n junction is closed and the gate current is very small, about 10 -8…10-9 A, therefore, the main advantages of a field-effect transistor, compared to a bipolar transistor, include a high input resistance, about 10 10…1013 Ohm. In addition, they are distinguished by low noise and manufacturability.

There are two main switching schemes that have practical application. 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
(MOS transistors)

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

The technology of an MOS transistor with a built-in gate is shown in the figure:

The original semiconductor on which the MOS transistor is made is called the substrate (pin P). The two heavily doped n+ regions are called source (I) and drain (C). The area of ​​the substrate under the gate (3) is called the embedded 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 is 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 an 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, depletion.

The depletion mode corresponds to a negative voltage Uzi, 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 drain current.

The current-voltage characteristic is shown in the figure:

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

When Uzi = 0 there is no channel and Ic = 0. The transistor can only operate in 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 current-voltage characteristic is shown in the figure:

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

The main parameters of field-effect transistors include:

• The slope of the characteristic at Usp = const, Upi = const. Typical parameter values ​​are (0.1...500) mA/V;
• The slope of the characteristic along the substrate at Usp = const, Uzi = const. Typical parameter values ​​(0.1...1) mA/V;
• Initial drain current Is.init. – drain current at zero voltage value Uzi. Typical parameter values: (0.2...600) mA – for transistors with a control channel p-n junction; (0.1...100) mA – for transistors with a built-in channel; (0.01...0.5) µA – for transistors with an induced channel;
• Cut-off voltage Uzi.ots. . Typical values ​​(0.2...10) V; threshold voltage Up. Typical values ​​(1...6) V;
• Drain-source resistance in open state. Typical values ​​(2..300) Ohm
• Differential resistance (internal): at Uzi = const;
• Statistical gain: μ = 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 - thyristors. The letter symbol VS is used to designate thyristors.

Design and principle of operation of the dinistor

The structure, UGO and current-voltage 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 designated by 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 the 3 junctions is open and their resistance is insignificant, and transition 2 is closed and all the supply voltage Upr is applied to it. A small reverse current flows through the dinistor, the load R is disconnected from the power source E.

When a critical voltage is reached equal to the switch-on voltage Uon, transition 2 opens, while all three transitions 1, 2, 3 will be in the open (on) state. The resistance of the dinistor drops to tenths of an ohm.

The turn-on voltage is several hundred volts. The dinistor opens and significant currents flow through it. The voltage drop across the dinistor in the open state is 1-2 volts and depends little 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 the power source E. The voltage across the dinistor corresponding to the maximum permissible point Iopen.max is called the open state voltage Uokr. The maximum permissible current ranges 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 limit values ​​of dinistor currents and voltages. The recovery time of transition 2 resistance after removing the supply voltage is about 10-30 μs.

By their principle, dinistors are key action devices. In the on state (BV section) it is similar to a closed key, and in the off state (EG section) it is like an open key.

The design and principle of operation of a thyristor (thyristor)

The thyristor is a controlled device. It contains a control electrode (CE) connected to a p-type semiconductor or an n-type semiconductor of the middle junction 2.

The structure, UGO and current-voltage characteristics of a trinistor (usually called a thyristor) are shown in the figure:



The voltage Uoff, at which an avalanche-like increase in current begins, can be reduced by introducing minority charge carriers into any of the layers adjacent to junction 2. The extent to which Uon decreases is shown on the current-voltage characteristic. An important parameter is the unlocking control current Iу.оt, which ensures that the thyristor switches to the open state at voltages lower than the voltage Uon. The figure shows three values ​​of switching voltage UI on< Un вкл < Um вкл соответствует трем значениям управляющего тока UI у.от >Un u.ot > Um u.ot .

Let's consider the simplest circuit with a thyristor loaded onto a resistor load Rн






• Ia – anode current (power current in the anode-cathode circuit of the thyristor);
• Uak – voltage between anode and cathode;
• Iу – control electrode current (in real circuits current pulses are used);
• Uuk is the voltage between the control electrode and the cathode;
• Upit – supply voltage.

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

A characteristic feature of the non-lockable thyristor in question, 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, reverse voltage Uac 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 design and principle of operation of a triac

So-called symmetrical thyristors (triacs, triacs) are widely used. Each triac is similar to a pair of the considered thyristors, connected back-to-back. Symmetrical thyristors are a controlled device with a symmetrical current-voltage characteristic. To obtain a symmetrical characteristic, double-sided p-n-p-n-p semiconductor structures are used.

The structure of the triac, its UGO and current-voltage characteristics are shown in the figure:



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

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

When a control voltage is applied to the UE, depending on its polarity and magnitude, the switch voltage Uon changes

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

Electronic keys

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

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

UGO ideal keys are shown in the figure:

Keys that operate to open and close, 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 across an ideal closed switch is 0. With an open switch, the current is 0.

Real keys are also characterized by two extreme resistance values ​​Rmax and Rmin. The transition from one resistance value to another in real switches occurs in a finite time. The voltage drop across a real closed switch is not zero.

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

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

1. Key resistances in open and closed states;
2. Performance – the time it takes for a key to transition from one state to another;
3. Voltage drop on a closed switch and leakage current on an open switch;
4. Noise immunity – the ability of a key to remain in one of the states when exposed to interference;
5. The sensitivity of the key is the magnitude of the control signal that transfers the key from one state to another;
6. Threshold voltage - the value of the control voltage, in the vicinity of which there is a sharp change in the resistance of the electronic key.

Diode electronic keys

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

The principle of operation of a diode electronic switch is based on changing the value of the differential resistance of a semiconductor diode in the vicinity of the threshold voltage value on the diode Uthr. Figure "c" shows the current-voltage characteristic of a semiconductor diode, which shows the value of Uthr. This value is located at the intersection of the voltage axis with the tangent drawn to the ascending member of the current-voltage characteristic.

Figure "d" shows the dependence of the differential resistance on the voltage across the diode. It follows from the figure that in the vicinity of the threshold voltage of 0.3 V there is a sharp change in the differential resistance of the diode with extreme values ​​of 900 and 35 Ohms (Rmin = 35 Ohms, Rmax = 900 Ohms).

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

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

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

Diode switches do not allow electrical separation of the control and controlled circuits, which is often required in practical circuits.

Transistor switches

The majority of circuits used in computers, remote control devices, automatic control systems, etc. are based on transistor switches.

The switch circuits on the bipolar transistor and the current-voltage characteristics are shown in the figure:

The first state “off” (transistor closed) is determined by point A1 on the output characteristics of the transistor; it is called cutoff mode. In the cutoff mode, the base current Ib = 0, the collector current Ik1 is equal to the initial collector current, and the collector voltage Uk = Uk1 ≈ Ek. The cutoff mode is implemented at Uin = 0 or at negative base potentials. In this state, the switch resistance reaches its maximum value: Rmax = , where RT is the resistance of the transistor in the closed state, more than 1 MOhm.

The second state “on” (the transistor is open) is determined by point A2 on the current-voltage characteristic and is called the saturation mode. From the cutoff mode (A1) to the saturation mode (A2), the transistor is switched by a positive input voltage Uin. In this case, the voltage Uout takes a minimum value Uk2 = Uk.e.us of the order of 0.2-1.0 V, the collector current Ik2 = Ik.us ≈ Ek / Rk. The base current in saturation mode is determined from the condition: Ib > Ib.us = Ik.us / h21.

The input voltage required to switch the transistor to the open state is determined from the condition: U in > Ib.us · Rb + Uk.e.us

Good noise immunity and low power dissipation in the transistor are explained by the fact that most of the time the transistor 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 switches on bipolar transistors is determined by the barrier capacitances of the p-n junctions and the processes of accumulation and resorption of minority charge carriers in the base.

To increase the speed and input resistance, field-effect transistor switches are used.

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

For any switch on a field-effect transistor Rн > 10-100 kOhm.

The control signal Uin at the gate is about 10-15 V. The resistance of the field-effect transistor in the closed state is high, about 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 along the control circuit can be 108 -109 Ohms. (circuits “a” and “b”) and 1012 -1014 Ohms (circuits “c” and “d”).

Power (power) semiconductor devices

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

Power semiconductor devices include thyristors (dinistors, thyristors, triacs), transistors (bipolar and field-effect) and statically induced bipolar transistors (IGBT). They are used as electronic keys that switch electronic circuits. They try to bring their characteristics closer to the characteristics of ideal keys.

According to the operating principle, characteristics and parameters, high-power transistors are similar to low-power transistors, but there are certain features.

Power field effect transistors

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

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

DMOS transistor

This MOS transistor, manufactured by the double diffusion method, has a horizontal channel. The figure shows a structure element containing a channel.

VMOS transistor

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

It is easy to see that the structures of a VMOS transistor and a DMIS transistor are similar.

IGBT transistor

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

Typically, IGBTs use an n-type induced channel MOS transistor structure. The structure of this transistor differs from the structure of a DMIS transistor by an additional layer of p-type semiconductor.

Please note that the terms “emitter”, “collector” and “gate” are commonly used to refer to IGBT electrodes.

Adding a p-type layer results in the formation of a second bipolar transistor structure (pnp type). Thus, IGBT has two bipolar structures - n-p-n type and p-n-p type.

The UGO and the IGBT switch-off circuit are shown in the figure:

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

SIT transistor

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

The regions of a p-type semiconductor have the shape of cylinders, the diameter of which is a few micrometers or more. This cylinder system acts as a shutter. Each cylinder is connected to a gate electrode (in figure “a” the gate electrode is not shown).

The dotted line indicates the areas of p-n junctions. The actual number of channels can be thousands. Typically SIT is used in common source circuits.

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

The main area of ​​application of thyristors are low-frequency devices with high switching power up to several megawatts, which do not impose serious requirements on speed.

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 a few W to several kW. The optimal range of switching voltages is 200-2000 V.

Field-effect transistors (MOSFETs) are used as electronic switches for switching low-voltage, high-frequency devices. The optimal values ​​of switching voltages do not exceed 200 V (maximum value up to 1000 V), while the switching frequency can range from a few kHz to 105 kHz. The range of switched currents is 1.5-100 A. The positive properties of this device are controllability by voltage rather than current, and less dependence on temperature compared to other devices.

Insulated gate bipolar transistors (IGBTs) are used at frequencies below 20 kHz (some types of devices are used at frequencies above 100 kHz) with switching powers above 1 kW. Switched voltages are not lower than 300-400 V. Optimal values ​​of switched voltages are above 2000 V. IGBT and MOSFET require a voltage of no higher than 12-15 V for full switching on; negative voltage is not required to close the devices. They are characterized by high switching speeds.

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

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

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

Bipolar transistor

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

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

Transistor connection circuits

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The procedure and example of calculating a transistor cascade with OE

Initial data:

Supply voltage U i.p.=12 V.

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

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

We accept R k =10*R e

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

Solution:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Then the total resistance of the resistors

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

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

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

Where I k0— quiescent current of the transistor.

8. Determining the voltage at the base

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

From here, through the voltage divider formula we find:

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

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

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

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

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