Bipolar transistors with a common emitter, enhanced mode. What is a bipolar transistor and how to test it
Transistors are active components and are used throughout electronic circuits as amplifiers and switching devices (transistor switches). As amplification devices, they are used in high and low frequency devices, signal generators, modulators, detectors and many other circuits. IN digital circuits, in switching power supplies and controlled electric drives they serve as keys.
Bipolar transistors
This is the name of the most common type of transistor. They are divided into npn and pnp types. The material most often used for them is silicon or germanium. At first, transistors were made from germanium, but they were very sensitive to temperature. Silicon devices are much more resistant to vibrations and are cheaper to produce.
Various bipolar transistors are shown in the photo below.
Low-power devices are located in small plastic rectangular or metal cylindrical cases. They have three terminals: for the base (B), emitter (E) and collector (K). Each of them is connected to one of three layers of silicon with conductivity of either n-type (the current is generated by free electrons) or p-type (the current is generated by the so-called positively charged “holes”), which make up the structure of the transistor.
How does a bipolar transistor work?
The principles of operation of a transistor need to be studied, starting with its design. Consider the structure of an NPN transistor, which is shown in the figure below.
As you can see, it contains three layers: two with n-type conductivity and one with p-type conductivity. The type of conductivity of the layers is determined by the degree of doping of various parts of the silicon crystal with special impurities. The n-type emitter is very heavily doped to provide many free electrons as the majority current carriers. The very thin p-type base is lightly doped with impurities and has high resistance, and the n-type collector is very heavily doped to give it low resistance.
Transistor operating principles
The best way to get to know them is through experimentation. Below is a diagram of a simple circuit.
It uses a power transistor to control the light bulb. You will also need a battery, a small flashlight bulb of approximately 4.5 V/0.3 A, a variable resistor potentiometer (5K) and a 470 ohm resistor. These components must be connected as shown in the figure to the right of the diagram.
Turn the potentiometer slider to its lowest position. This will lower the base voltage (between base and ground) to zero volts (U BE = 0). The lamp does not light, which means there is no current flowing through the transistor.
If you now turn the handle from its lower position, then U BE gradually increases. When it reaches 0.6 V, current begins to flow into the base of the transistor and the lamp begins to glow. When the handle is moved further, the voltage U BE remains at 0.6 V, but the base current increases and this increases the current through the collector-emitter circuit. If the knob is moved to the up position, the voltage at the base will increase slightly to 0.75 V, but the current will increase significantly and the lamp will glow brightly.
What if you measure the transistor currents?
If we connect an ammeter between the collector (C) and the lamp (to measure I C), another ammeter between the base (B) and the potentiometer (to measure I B), and a voltmeter between common and base and repeat the whole experiment, we can get some interesting data. When the potentiometer knob is in its lowest position, U BE is 0 V, as are the currents IC and I B. When the handle is moved, these values increase until the light bulb begins to glow, when they are equal: U BE = 0.6 V, I B = 0.8 mA and I C = 36 mA.
As a result, we get from this experiment the following principles of transistor operation: in the absence of a positive (for npn-type) bias voltage at the base, the currents through its terminals are zero, and in the presence of base voltage and current, their changes affect the current in the collector-emitter circuit.
What happens when you turn on the power of a transistor
During normal operation, the voltage applied to the base-emitter junction is distributed such that the potential of the base (p-type) is approximately 0.6 V higher than that of the emitter (n-type). In this case, a forward voltage is applied to this junction, it is biased in the forward direction and is open to the flow of current from the base to the emitter.
Much more high voltage applied to the base-collector junction, and the potential of the collector (n-type) turns out to be higher than that of the base (p-type). So a reverse voltage is applied to the junction and it is reverse biased. This results in the formation of a fairly thick electron-depleted layer in the collector near the base when supply voltage is applied to the transistor. As a result, no current passes through the collector-emitter circuit. The distribution of charges in the junction zones of an npn transistor is shown in the figure below.
What is the role of base current?
How can we make our electronic device work? The principle of operation of the transistor is the influence of the base current on the state of the closed base-collector junction. When the base-emitter junction is forward biased, it does not high current will go to the database. Here its carriers are positively charged holes. These combine with electrons coming from the emitter to produce a current I BE. However, due to the fact that the emitter is very heavily doped, many more electrons flow from it into the base than can be combined with holes. This means that there is a large concentration of electrons in the base, and most of them cross it and enter the electron-depleted collector layer. Here they come under the influence of a strong electric field applied to the base-collector junction, pass through the electron-depleted layer and the main volume of the collector to its output.
Changes in the current flowing into the base affect the number of electrons attracted from the emitter. Thus, the operating principles of the transistor can be supplemented by the following statement: very small changes in the base current cause very large changes in the current flowing from the emitter to the collector, i.e. the current increases.
Types of field effect transistors
In English they are designated FETs - Field Effect Transistors, which can be translated as “field effect transistors”. Although there is a lot of confusion about the names for them, there are mainly two main types:
1. With control pn junction. In English literature they are designated JFET or Junction FET, which can be translated as “transitional field effect transistor" Otherwise they are called JUGFET or Junction Unipolar Gate FET.
2. With an insulated gate (otherwise MOS or MOS transistors). In English they are designated IGFET or Insulated Gate FET.
Outwardly, they are very similar to bipolar ones, as confirmed by the photo below.
Field effect transistor device
All field-effect transistors can be called UNIPOLAR devices, because the charge carriers that form the current through them are of a single type for a given transistor - either electrons or “holes,” but not both at the same time. This distinguishes the principle of operation of a field-effect transistor from a bipolar one, in which the current is generated simultaneously by both of these types of carriers.
Current carriers flow in junction field effect transistors through a layer of silicon without junctions, called a channel, with either n- or p-type conductivity between two terminals called "source" and "drain" - analogues of emitter and collector or , more precisely, the cathode and anode of a vacuum triode. The third terminal - the gate (analogue of the triode grid) - is connected to a layer of silicon with a different type of conductivity than that of the source-drain channel. The structure of such a device is shown in the figure below.
How does a field effect transistor work? Its operating principle is to control the channel cross-section by applying a voltage to the gate-channel junction. It is always reverse biased, so the transistor consumes virtually no current in the gate circuit, whereas a bipolar device requires a certain base current to operate. As the input voltage changes, the gate area can expand, blocking the source-drain channel until it is completely closed, thus controlling the drain current.
Electronics surround us everywhere. But almost no one thinks about how this whole thing works. It's actually quite simple. This is exactly what we will try to show today. Let's start with this important element, like a transistor. Let's tell you what it is, what it does, and how the transistor works.
What is a transistor?
Transistor – semiconductor device, designed to control electric current.
Where are transistors used? Yes everywhere! Almost no modern technology can do without transistors. electrical diagram. They are widely used in production computer technology, audio and video equipment.
Times when Soviet microcircuits were the largest in the world, have passed, and the size of modern transistors is very small. Thus, the smallest devices are on the order of a nanometer in size!
Prefix nano- denotes a value of the order of ten to the minus ninth power.
However, there are also giant specimens that are used primarily in the fields of energy and industry.
There are different types transistors: bipolar and polar, direct and reverse conduction. However, the operation of these devices is based on the same principle. A transistor is a semiconductor device. As is known, in a semiconductor the charge carriers are electrons or holes.
The region with excess electrons is indicated by the letter n(negative), and the region with hole conductivity is p(positive).
How does a transistor work?
To make everything very clear, let's look at the work bipolar transistor (the most popular type).
(hereinafter referred to simply as a transistor) is a semiconductor crystal (most often used silicon or germanium), divided into three zones with different electrical conductivities. The zones are named accordingly collector, base And emitter. The device of the transistor and its schematic representation are shown in the figure below
Separate forward and reverse conduction transistors. Pnp transistors are called forward conduction transistors, and npn transistors– from the reverse.
Now let's talk about the two operating modes of transistors. The operation of the transistor itself is similar to the operation of a water tap or valve. Only instead of water - electric current. There are two possible states of the transistor - operating (transistor open) and rest state (transistor closed).
What does it mean? When the transistor is turned off, no current flows through it. In the open state, when a small control current is applied to the base, the transistor opens and a large current begins to flow through the emitter-collector.
Physical processes in a transistor
And now more about why everything happens this way, that is, why the transistor opens and closes. Let's take a bipolar transistor. Let it be n-p-n transistor.
If you connect a power source between the collector and the emitter, the collector's electrons will begin to be attracted to the positive, but there will be no current between the collector and the emitter. This is hampered by the base layer and the emitter layer itself.
If you connect an additional source between the base and emitter, electrons from the n region of the emitter will begin to penetrate into the base region. As a result, the base area will be enriched with free electrons, some of which will recombine with holes, some will flow to the plus of the base, and some (most) will go to the collector.
Thus, the transistor turns out to be open, and the emitter-collector current flows in it. If the base voltage is increased, the collector-emitter current will also increase. Moreover, with a small change in the control voltage, a significant increase in the current through the collector-emitter is observed. It is on this effect that the operation of transistors in amplifiers is based.
That, in a nutshell, is the essence of how transistors work. You need to calculate a power amplifier based on bipolar transistors overnight, or perform laboratory work to study the operation of a transistor? This is not a problem even for a beginner if you use the help of our student service specialists.
Do not hesitate to seek professional help in such important issues like studying! And now that you already have an idea about transistors, we suggest you relax and watch the video by Korn “Twisted transistor”! For example, you decide to contact the Correspondence Student.
Bipolar transistors are made of alloyed materials and can be of two types - NPN and PNP. A transistor has three terminals known as emitter (E), base (B) and collector (K). The figure below shows an NPN transistor where, in the main operating modes (active, saturation, cutoff), the collector has a positive potential, the emitter is negative, and the base is used to control the state of the transistor.
The physics of semiconductors will not be discussed in this article, however, it is worth mentioning that a bipolar transistor consists of three separate parts, separated by two p-n junctions. A PNP transistor has one N region separated by two P regions:
An NPN transistor has one P region sandwiched between two N regions:
The connections between N and P regions are similar to transitions in, and they can also be with forward and reverse p-n offset transition. These devices can operate in different modes depending on the type of displacement:
- Cut-off: work in this mode also occurs when switching. No current flows between the emitter and collector, practically an “open circuit”, that is, “the contact is open”.
- Active mode: The transistor operates in amplifier circuits. IN this mode its characteristic is almost linear. A current flows between the emitter and collector, the magnitude of which depends on the value of the bias (control) voltage between the emitter and the base.
- Saturation: works when switching. Between the emitter and collector there is practically " short circuit", that is, "the contact is closed."
- Inverse active mode: As in active, the transistor current is proportional to the base current, but flows in the opposite direction. Very rarely used.
In an NPN transistor, a positive voltage is applied to the collector to create a current from the collector to the emitter. In a PNP transistor, a positive voltage is applied to the emitter to create a current from the emitter to the collector. In NPN, current flows from the collector (K) to the emitter (E):
And in PNP, the current flows from the emitter to the collector:
It is clear that the directions of current and voltage polarity in PNP and NPN are always opposite to each other. NPN transistors require power with positive polarity relative to the common terminals, and PNP transistors require negative power.
PNP and NPN work almost identically, but their modes are different due to the polarities. For example, to put NPN into saturation mode, U B must be higher than U K and U E. Below is brief description operating modes depending on their voltage:
The basic operating principle of any bipolar transistor is to control the base current to regulate the flow of current between the emitter and collector. Working principle of NPN and PNP transistors one and the same. The only difference is the polarity of the voltages applied to their N-P-N and P-N-P transitions, that is, to the emitter-base-collector.
TOPIC 4. BIPOLAR TRANSISTORS
4.1 Design and principle of operation
A bipolar transistor is a semiconductor device consisting of three regions with alternating types of electrical conductivity and is suitable for power amplification.
Currently produced bipolar transistors can be classified according to the following criteria:
By material: germanium and silicon;
According to the type of conductivity of the areas: p-n-p and n-p-n types;
By power: low (Pmax £ 0.3W), medium (Pmax £ 1.5W) and high power(Rmax > 1.5W);
By frequency: low-frequency, mid-frequency, high-frequency and microwave.
In bipolar transistors, the current is determined by the movement of charge carriers of two types: electrons and holes (or majority and minority). Hence their name - bipolar.
Currently, only transistors with planar p-n junctions are manufactured and used.
The structure of a planar bipolar transistor is shown schematically in Fig. 4.1.
It is a plate of germanium or silicon in which three regions with different electrical conductivities are created. For transistor type n-р-n average the region has hole, and the outer regions have electronic conductivity.
Transistors of the pnp type have a middle region with electronic conductivity, and outer regions with hole electrical conductivity.
The middle region of the transistor is called the base, one extreme region is the emitter, and the other is the collector. Thus, the transistor has two p-n junctions: the emitter - between the emitter and the base and the collector - between the base and the collector. The area of the emitter junction is smaller than the area of the collector junction.
The emitter is the region of the transistor whose purpose is to inject charge carriers into the base. A collector is a region whose purpose is to extract charge carriers from the base. The base is the region into which the emitter injects charge carriers that are non-majority for this region.
The concentration of the main charge carriers in the emitter is many times greater than the concentration of the main charge carriers in the base, and their concentration in the collector is somewhat less than the concentration in the emitter. Therefore, the emitter conductivity is several orders of magnitude higher than the base conductivity, and the collector conductivity is somewhat less than the emitter conductivity.
Conclusions are drawn from the base, emitter and collector. Depending on which of the terminals is common to the input and output circuits, there are three circuits for switching on the transistor: with a common base (CB), common emitter(OE), common collector (OK).
The input, or control, circuit serves to control the operation of the transistor. In the output, or controlled, circuit, amplified oscillations are obtained. The source of amplified oscillations is included in input circuit, and the load is turned on at the output.
Let's consider the principle of operation of a transistor using the example of a pnp type transistor connected according to a circuit with a common base (Fig. 4.2).
Figure 4.2 – Operating principle of a bipolar transistor (pnp type)
The external voltages of two power sources EE and Ek are connected to the transistor in such a way that the emitter junction P1 is biased in the forward direction (forward voltage), and the collector junction P2 is biased in the reverse direction (reverse voltage).
If a reverse voltage is applied to the collector junction and the emitter circuit is open, then a small reverse current Iko (units of microamps) flows in the collector circuit. This current occurs under the influence reverse voltage and is created by the directed movement of minority charge carriers, base holes and collector electrons through the collector junction. The reverse current flows through the circuit: +Ek, base-collector, -Ek. The magnitude of the reverse collector current does not depend on the collector voltage, but depends on the temperature of the semiconductor.
When a constant voltage EE is connected to the emitter circuit in the forward direction, the potential barrier of the emitter junction decreases. The injection of holes into the base begins.
The external voltage applied to the transistor turns out to be applied mainly to the transitions P1 and P2, because they have high resistance compared to the resistance of the base, emitter and collector regions. Therefore, holes injected into the base move through it through diffusion. In this case, the holes recombine with the electrons of the base. Since the carrier concentration in the base is much lower than in the emitter, very few holes recombine. With a small base thickness, almost all holes will reach the collector junction P2. In place of the recombined electrons, electrons from the power source Ek enter the base. Holes that recombine with electrons in the base create a base current IB.
Under the influence of reverse voltage Ek, the potential barrier of the collector junction increases, and the thickness of the junction P2 increases. But the potential barrier of the collector junction does not prevent holes from passing through it. The holes entering the region of the collector junction fall into a strong accelerating field created at the junction by the collector voltage, and are extracted (retracted) by the collector, creating a collector current Ik. The collector current flows through the circuit: +Ek, base-collector, -Ek.
Thus, three currents flow in the transistor: emitter, collector and base current.
In the wire, which is the base terminal, the emitter and collector currents are directed in opposite directions. Therefore, the base current is equal to the difference between the emitter and collector currents: IB = IE − IK.
Physical processes in a transistor n-p-n type proceed similarly to the processes in a pnp transistor.
The total emitter current IE is determined by the number of main charge carriers injected by the emitter. The main part of these charge carriers reaching the collector creates a collector current Ik. A small part of the charge carriers injected into the base recombine in the base, creating a base current IB. Consequently, the emitter current will be divided into base and collector currents, i.e. IE = IB + Ik.
The emitter current is the input current, the collector current is the output current. The output current is part of the input current, i.e.
where a is the current transfer coefficient for the OB circuit;
Since the output current is less than the input current, the coefficient a<1. Он показывает, какая часть инжектированных в базу носителей заряда достигает коллектора. Обычно величина a составляет 0,95¸0,995.
In a common emitter circuit, the output current is the collector current and the input current is the base current. Current gain for the OE circuit:
(4.3)
Consequently, the current gain for the OE circuit is tens of units.
The output current of the transistor depends on the input current. Therefore, a transistor is a current-controlled device.
Changes in emitter current caused by changes in emitter junction voltage are completely transmitted to the collector circuit, causing a change in collector current. And because The voltage of the collector power source Ek is significantly greater than the emitter Ee, then the power consumed in the collector circuit Pk will be significantly greater than the power in the emitter circuit Re. Thus, it is possible to control high power in the collector circuit of the transistor with low power spent in the emitter circuit, i.e. there is an increase in power.
4.2 Circuits for connecting bipolar transistors
The transistor is connected to the electrical circuit in such a way that one of its terminals (electrode) is the input, the second is the output, and the third is common to the input and output circuits. Depending on which electrode is common, there are three transistor switching circuits: OB, OE and OK. These circuits for a pnp transistor are shown in Fig. 4.3. For an n-p-n transistor in the switching circuits, only the polarity of the voltages and the direction of the currents change. For any transistor switching circuit (in active mode), the polarity of the power supplies must be selected so that the emitter junction is switched on in the forward direction, and the collector junction in the reverse direction.
Figure 4.3 – Connection circuits for bipolar transistors: a) OB; b) OE; c) OK
4.3 Static characteristics of bipolar transistors
The static mode of operation of the transistor is the mode when there is no load in the output circuit.
The static characteristics of transistors are the graphically expressed dependences of the voltage and current of the input circuit (input current-voltage characteristics) and the output circuit (output current-voltage characteristics). The type of characteristics depends on the method of switching on the transistor.
4.3.1 Characteristics of a transistor connected according to the OB circuit
IE = f(UEB) with UKB = const (Fig. 4.4, a).
IK = f(UKB) with IE = const (Fig. 4.4, b).
Figure 4.4 – Static characteristics of a bipolar transistor connected according to the OB circuit
The output current-voltage characteristics have three characteristic regions: 1 – strong dependence of Ik on UKB (nonlinear initial region); 2 – weak dependence of Ik on UKB (linear region); 3 – breakdown of the collector junction.
A feature of the characteristics in region 2 is their slight increase with increasing voltage UCB.
4.3.2 Characteristics of a transistor connected according to the OE circuit:
The input characteristic is the dependence:
IB = f(UBE) with UKE = const (Fig. 4.5, b).
The output characteristic is the dependence:
IK = f(UKE) with IB = const (Fig. 4.5, a).
Figure 4.5 – Static characteristics of a bipolar transistor connected according to the OE circuit
The transistor in the OE circuit provides current amplification. Current gain in the OE circuit: 
If coefficient a for transistors is a = 0.9¸0.99, then coefficient b = 9¸99. This is the most important advantage of switching the transistor according to the OE circuit, which, in particular, determines the wider practical application of this switching circuit compared to the OB circuit.
From the principle of operation of the transistor, it is known that two current components flow through the base terminal in the opposite direction (Fig. 4.6): the reverse current of the collector junction IKO and part of the emitter current (1 - a)IE. In this regard, the zero value of the base current (IB = 0) is determined by the equality of the specified current components, i.e. (1 − a)IE = IKO. Zero input current corresponds to the emitter current IE=IKO/(1−a)=(1+b)IKO and the collector current. In other words, at zero base current (IB = 0), a current flows through the transistor in the OE circuit, called the initial or through current IKO(E) and equal to (1+ b) IKO.
Figure 4.6 – Connection circuit for a transistor with a common emitter (OE circuit)
4.4 Basic parameters
To analyze and calculate circuits with bipolar transistors, the so-called h - parameters of the transistor connected according to the OE circuit are used.
The electrical state of a transistor connected according to the OE circuit is characterized by the values IB, IBE, IK, UKE.
The system of h − parameters includes the following quantities:
1. Input impedance
h11 = DU1/DI1 at U2 = const. (4.4)
represents the transistor’s resistance to alternating input current at which a short circuit occurs at the output, i.e. in the absence of AC output voltage.
2. Voltage feedback coefficient:
h12 = DU1/DU2at I1= const. (4.5)
shows what proportion of the input AC voltage is transferred to the input of the transistor due to feedback in it.
3. Current force coefficient (current transfer coefficient):
h21 = DI2/DI1at U2= const. (4.6)
shows the amplification of alternating current by the transistor in no-load mode.
4. Output conductivity:
h22 = DI2/DU2 at I1 = const. (4.7)
represents the conductance for alternating current between the output terminals of the transistor.
Output resistance Rout = 1/h22.
For a common emitter circuit, the following equations apply:
(4.8)
To prevent overheating of the collector junction, it is necessary that the power released in it during the passage of the collector current does not exceed a certain maximum value:
(4.9)
In addition, there are limitations on collector voltage:
and collector current:
4.5 Operating modes of bipolar transistors
A transistor can operate in three modes depending on the voltage at its junctions. When operating in active mode, the voltage at the emitter junction is direct, and at the collector junction it is reverse.
The cut-off, or blocking, mode is achieved by applying reverse voltage to both junctions (both p-n junctions are closed).
If the voltage at both junctions is direct (both p-n junctions are open), then the transistor operates in saturation mode.
In cutoff mode and saturation mode, there is almost no control of the transistor. In the active mode, such control is carried out most efficiently, and the transistor can perform the functions of an active element of an electrical circuit (amplification, generation, etc.).
4.6 Scope of application
Bipolar transistors are semiconductor devices for universal purposes and are widely used in various amplifiers, generators, pulse and switching devices.
4.7 The simplest amplifier stage using a bipolar transistor
The most widely used circuit is for switching on a transistor using a circuit with a common emitter (Fig. 4.7)
The main elements of the circuit are the power supply Ek, the controlled element - transistor VT and resistor Rk. These elements form the main (output) circuit of the amplifier stage, in which, due to the flow of controlled current, an amplified alternating voltage is created at the output of the circuit.
The remaining elements play a supporting role. Capacitor Cp is a separating capacitor. In the absence of this capacitor in the input signal source circuit, a direct current would be created from the power source Ek.
Figure 4.7 – Diagram of the simplest amplifier stage on a bipolar transistor according to a common-emitter circuit
Resistor RB, connected to the base circuit, ensures operation of the transistor in rest mode, i.e. in the absence of an input signal. The quiescent mode is ensured by the quiescent base current IB » Ek/RB.
With the help of resistor Rk, an output voltage is created, i.e. Rк performs the function of creating a varying voltage in the output circuit due to the flow of current in it, controlled through the base circuit.
For the collector circuit of the amplifier stage, we can write the following equation of electrical state:
Ek = Uke + IkRk, (4.10)
that is, the sum of the voltage drop across the resistor Rk and the collector-emitter voltage Uke of the transistor is always equal to a constant value - the emf of the power source Ek.
The amplification process is based on the conversion of the energy of a constant voltage source Ek into the energy of an alternating voltage in the output circuit by changing the resistance of the controlled element (transistor) according to the law specified by the input signal.
When an alternating voltage uin is applied to the input of the amplifier stage, an alternating component of the current IB~ is created in the base circuit of the transistor, which means the base current will change. A change in the base current leads to a change in the value of the collector current (IK = bIB), and therefore to a change in the voltage values across the resistance Rk and Uke. The amplifying abilities are due to the fact that the change in the collector current values is b times greater than the base current.
4.8 Calculation of electrical circuits with bipolar transistors
For the collector circuit of the amplifier stage (Fig. 4.7), in accordance with Kirchhoff’s second law, equation (4.10) is valid.
The volt-ampere characteristic of the collector resistor RK is linear, and the volt-ampere characteristics of the transistor are non-linear collector characteristics of the transistor (Fig. 4.5, a) connected according to the OE circuit.
The calculation of such a nonlinear circuit, that is, the determination of IK, URK and UKE for various values of base currents IB and resistor resistance RK, can be carried out graphically. To do this, on the family of collector characteristics (Fig. 4.5, a) it is necessary to draw from point EK on the abscissa axis the volt-ampere characteristic of the resistor RK, satisfying the equation:
Uke = Ek − RkIk. (4.11)
This characteristic is built at two points:
Uke = Ek with Ik = 0 on the abscissa and Ik = Ek/Rk with Uke = 0 on the ordinate. The I-V characteristic of the collector resistor Rk constructed in this way is called the load line. The points where it intersects with the collector characteristics provide a graphic solution to equation (4.11) for a given resistance Rк and various values of the base current IB. From these points you can determine the collector current Ik, which is the same for the transistor and resistor Rk, as well as the voltage UKE and URK.
The point of intersection of the load line with one of the static current-voltage characteristics is called the operating point of the transistor. By changing IB, you can move it along the load line. The initial position of this point in the absence of an input alternating signal is called the resting point - T0.
a) b)
Figure 4.8 – Graphic-analytical calculation of the operating mode of a transistor using output and input characteristics.
The rest point (operating point) T0 determines the current ICP and the voltage UCP in rest mode. Using these values, you can find the RKP power released in the transistor in rest mode, which should not exceed the maximum RK power max, which is one of the transistor parameters:
RKP = IKP ×UKEP £ RK max. (4.12)
Reference books usually do not provide a family of input characteristics, but only characteristics for UKE = 0 and for some UKE > 0.
The input characteristics for various UCEs exceeding 1V are located very close to each other. Therefore, the calculation of input currents and voltages can be approximately done using the input characteristic for UCE > 0, taken from the reference book.
Points A, To and B of the output operating characteristic are transferred to this curve, and points A1, T1 and B1 are obtained (Fig. 4.8, b). Operating point T1 determines constant voltage UBEP databases and constant current IBP bases.
The resistance of the resistor RB (ensures the operation of the transistor in rest mode), through which a constant voltage will be supplied from the source EK to the base:
(4.13)
In the active (amplifying) mode, the rest point of the transistor To is located approximately in the middle of the AB load line section, and the operating point does not extend beyond the AB section.
There is an obvious difference between a simple switching circuit and a linear transistor amplifier. In a normally operating linear amplifier, the collector current is always directly proportional to the base current. In a switching circuit such as in Fig. 1., the collector current is mainly determined by the supply voltage V CC and the load resistance R L. The saturation mode of the transistor is quite important and deserves detailed discussion.
Rice. 1. Illustration of saturation mode. The transistor acts as a switch to turn on the lamp.
Let's consider what happens to the collector current in the circuit in Fig. 1 if the base current is gradually increased starting from zero. When switch S 1 is open, no base current flows and the collector current is negligible. The short circuit of S 1 leads to the appearance of a base current I B = V CC /R B, where we have neglected the potential difference at the base-emitter junction. The collector current flowing through the load R L is equal to I C = h FE V CC / R B . For the specific circuit shown in the figure, with h FE = 100 and at the maximum value of R B (50 kOhm), we obtain:
I C =100x10/5000 A=20 mA
The voltage drop across R L is determined by the product R L I C and in our case is equal to 50 x 0.02 = 1 V. The transistor is in linear mode; a decrease in R B results in an increase in the base current, an increase in the collector current and, therefore, an increase in the voltage drop across R L . Under these conditions, the circuit could be used as a voltage amplifier.
Now consider the case when
and the base current is
I B =V CC /R B =V CC /(h FE R L)
Therefore, the collector current is equal to
I C =(h FE V CC)/(h FE R L)=V CC /R L
From a load point of view, the transistor behaves like a pair of switch contacts. It follows from Ohm's law that the load current in this situation cannot exceed the value V CC /R L. Therefore, a further increase in the base current cannot increase the collector current, which is now determined only by the load resistance and supply voltage. The transistor is in saturation. In practice, when a transistor is saturated, there is always a small voltage remaining between the collector and emitter, usually denoted V CE(sat). As a rule, it is less than 1 V and can reach up to 0.1 V for transistors specifically designed to work as switches. Typically, V CE(sat) decreases as more current flows through the base-emitter junction, that is, when the ratio of collector current IC to base current I B becomes significantly less than the transistor current gain h FE.
Roughly speaking, deep saturation (small value of V CE(sat)) occurs when
I C /I B< h FE /5
For a circuit like the one shown in Fig. 1, when the base current is given by simply connecting a resistor to the power supply, we select
R B /R L< h FE /5
Therefore, for the circuit in Fig. 1, taking the value of the current gain h FE = 150 typical for the 2N3053 transistor (analogue of KT630B - see analogues of domestic and foreign transistors), we have
R B /R L< 150/5 = 30.
Therefore, with R L = 50 Ohm we choose
R B< 30 х 50 Ом = 1,5 кОм.
So, if a lamp with a resistance of 50 Ohms is used as a load, then for it effective inclusion we should choose a base resistor value less than 1.5 kOhm. If this is not possible, when, for example, a photoresistor with a minimum resistance of 10 kOhm is used as RB, then a Darlington circuit should be used to increase the current gain.
If the bipolar transistor operates with a collector current close to the maximum, and it is necessary to maintain the voltage V CE (sat) at the level of fractions of a volt, then due to the decrease in h FE, a base current greater than I s / 10 may be required.
It may seem surprising that V CE(sat) can be much less than the voltage V BE, which for a silicon transistor is approximately 0.6 V. This happens because in saturation mode the collector-base junction is forward biased. Therefore we have two р-n junction, shifted in the forward direction, connected towards each other so that the voltage drops across them are mutually compensated. This ability of a bipolar transistor to have a very small voltage drop between collector and emitter in saturation mode makes it a very useful switching device. Many of the most important applications electronics, including the vast field of digital electronics, use switching circuits.
In switching mode, the transistor operates with either virtually zero collector current (transistor off) or virtually zero collector voltage (transistor on). In both cases, the power dissipated by the transistor is very small. Significant power is dissipated only when switching occurs: at this time, both the collector-emitter voltage and the collector current have finite values.
A low-power transistor such as the 2N3053, with a maximum power dissipation of less than one watt, can switch power into a load of several watts. It should be noted that maximum values collector voltage and current should not exceed permissible limits; In addition, it is desirable to switch as quickly as possible to avoid dissipating excessive power.