A transistor is a semiconductor device that can amplify, convert, and generate electrical signals. The first workable bipolar transistor was invented in 1947. The material for its manufacture was germanium. And already in 1956, a silicon transistor was born.
In a bipolar transistor, two types of charge carriers are used - electrons and holes, which is why such transistors are called bipolar. In addition to bipolar ones, there are unipolar (field-effect) transistors, which use only one type of carriers - electrons or holes. This article will cover.
Most silicon transistors have an n-p-n structure, which is also explained by the production technology, although there are silicon p-n-p transistors, but there are slightly fewer of them than n-p-n structures. Such transistors are used in complementary pairs (transistors of different conductivity with the same electrical parameters). For example, KT315 and KT361, KT815 and KT814, and in the output stages of the transistor UMZCH KT819 and KT818. In imported amplifiers, a powerful complementary pair of 2SA1943 and 2SC5200 is very often used.
Often transistors of the p-n-p structure are called forward conduction transistors, and the n-p-n structure is called reverse. For some reason, such a name is almost never found in the literature, but in the circle of radio engineers and radio amateurs it is used everywhere, everyone immediately understands what it is about. Figure 1 shows a schematic device of transistors and their conventional graphic designations.
Picture 1.
In addition to differences in conductivity type and material, bipolar transistors are classified according to power and operating frequency. If the power dissipation on the transistor does not exceed 0.3 W, such a transistor is considered low-power. With a power of 0.3 ... 3 W, the transistor is called an average power transistor, and with a power over 3 W, the power is considered large. Modern transistors are able to dissipate power of several tens or even hundreds of watts.
Transistors do not amplify electrical signals equally well: with increasing frequency, the gain of the transistor stage decreases, and at a certain frequency it stops altogether. Therefore, for operation in a wide frequency range, transistors are produced with different frequency properties.
By operating frequency, transistors are divided into low-frequency, - the operating frequency is not more than 3 MHz, medium-frequency - 3 ... 30 MHz, high-frequency - over 30 MHz. If the operating frequency exceeds 300 MHz, then these are microwave transistors.
In general, in serious thick reference books, over 100 different parameters of transistors are given, which also speaks of a huge number of models. And the number of modern transistors is such that it is no longer possible to place them in full in any directory. And the model range is constantly increasing, allowing you to solve almost all the tasks set by the developers.
There are many transistor circuits (just remember the number of household equipment) for amplifying and converting electrical signals, but, with all the diversity, these circuits consist of separate stages, which are based on transistors. To achieve the required signal amplification, it is necessary to use several amplification stages connected in series. To understand how the amplifier stages work, you need to get acquainted in more detail with the transistor switching circuits.
By itself, the transistor will not be able to amplify anything. Its amplifying properties are that small changes in the input signal (current or voltage) lead to significant changes in voltage or current at the output of the stage due to the consumption of energy from an external source. This property is widely used in analog circuits - amplifiers, television, radio, communications, etc.
To simplify the presentation, here we will consider circuits on transistors of the n-p-n structure. Everything that will be said about these transistors applies equally to p-n-p transistors. It is enough only to reverse the polarity of the power supplies, and, if any, to get a working circuit.
In total, three such schemes are used: a common emitter (OE) circuit, a common collector (OC) circuit and a common base (OB) circuit. All of these circuits are shown in Figure 2.
Figure 2.
But before moving on to considering these circuits, you should get acquainted with how the transistor works in the key mode. This familiarity should make it easier to understand in amplification mode. In a sense, the key scheme can be viewed as a kind of OE scheme.
Transistor operation in key mode
Before studying the operation of the transistor in the signal amplification mode, it is worth remembering that transistors are often used in the key mode.
This mode of operation of the transistor has been considered for a long time. In the August issue of the magazine "Radio" 1959 was published an article by G. Lavrov "Semiconductor triode in the key mode". The author of the article suggested changing the pulse duration in the control winding (OA). Now this method of regulation is called PWM and is used quite often. A diagram from a magazine of that time is shown in Figure 3.
Figure 3.
But the key mode is used not only in PWM systems. Often a transistor just turns something on and off.
In this case, a relay can be used as a load: an input signal is given - the relay is turned on, no - the relay is turned off. Instead of a relay in a key mode, light bulbs are often used. This is usually done for indication: the light is either on or off. The diagram of such a key stage is shown in Figure 4. Key stages are also used to work with LEDs or with optocouplers.
Figure 4.
In the figure, the cascade is controlled by a regular contact, although there may be a digital microcircuit or. Automobile light, this is used to illuminate the dashboard in "Zhiguli". Attention should be paid to the fact that the control voltage is 5V, and the commutated collector voltage is 12V.
There is nothing strange in this, since voltages in this circuit do not play any role, only currents matter. Therefore, the light bulb can be at least 220V, if the transistor is designed to work at such voltages. The collector voltage must also match the operating voltage of the load. Using such cascades, the load is connected to digital microcircuits or microcontrollers.
In this scheme, the base current controls the collector current, which, due to the energy of the power source, is several tens, or even hundreds of times greater (depending on the collector load) than the base current. It is easy to see that there is a current amplification. When the transistor is operating in the key mode, usually to calculate the cascade, they use the value called in the reference books "the current gain in the large signal mode" - in reference books it is denoted by the letter β. This is the ratio of the collector current, determined by the load, to the minimum possible base current. In the form of a mathematical formula, it looks like this: β = Ik / Ib.
For most modern transistors, the β coefficient is quite large, as a rule, from 50 and higher, therefore, when calculating the key stage, it can be taken equal to only 10. Even if the base current turns out to be greater than the calculated one, then the transistor will not open more from this, then it and key mode.
To light the lamp shown in Figure 3, Ib = Ik / β = 100mA / 10 = 10mA, this is at least. With a control voltage of 5V across the base resistor Rb, minus the voltage drop in the BE section, 5V will remain - 0.6V = 4.4V. The resistance of the base resistor will be: 4.4V / 10mA = 440 Ohm. A 430 ohm resistor is selected from the standard range. The voltage of 0.6V is the voltage at the B-E junction, and you should not forget about it in the calculations!
So that the base of the transistor does not remain "hanging in the air" when the control contact is opened, the B-E junction is usually shunted by the resistor Rbe, which reliably closes the transistor. One should not forget about this resistor, although for some reason it is not in some circuits, which can lead to false triggering of the cascade from interference. Actually, everyone knew about this resistor, but for some reason they forgot, and once again stepped on the "rake".
The value of this resistor should be such that when the contact is opened, the voltage at the base would not be less than 0.6V, otherwise the cascade will be uncontrollable, as if the section BE was simply short-circuited. In practice, the resistor Rbe is set with a nominal value of about ten times more than Rb. But even if the Rb rating is 10KΩ, the circuit will work reliably enough: the base and emitter potentials will be equal, which will lead to the transistor closing.
Such a key cascade, if it is working properly, can turn on the light bulb at full incandescence, or turn it off completely. In this case, the transistor can be fully on (saturation state) or completely off (cutoff state). Immediately, of course, the conclusion suggests itself that between these "boundary" states there is such a thing when the light bulb is full of light. Is the transistor half on or half off in this case? It is like the problem of filling a glass: an optimist sees a glass half filled, while a pessimist sees it as half empty. This mode of operation of the transistor is called amplifying or linear.
Transistor operation in signal amplification mode
Almost all modern electronic equipment consists of microcircuits in which transistors are "hidden". It is enough to simply select the operating mode of the operational amplifier to obtain the required gain or bandwidth. But, despite this, cascades on discrete ("spread") transistors are often used, and therefore an understanding of the operation of an amplifying stage is simply necessary.
The most common inclusion of a transistor in comparison with OK and OB is a common emitter (OE) circuit. The reason for this prevalence is primarily the high voltage and current gain. The highest amplification factor of the OE stage is provided when half of the power supply voltage Epit / 2 drops across the collector load. Accordingly, the second half falls in the K-E section of the transistor. This is achieved by tuning the cascade, which will be discussed below. This amplification mode is called class A.
When the transistor with OE is turned on, the output signal on the collector is in antiphase with the input one. As disadvantages, it can be noted that the input resistance of the OE is small (no more than several hundred Ohms), and the output resistance is within tens of KOhms.
If in the key mode the transistor is characterized by the current gain in the large signal mode β, then in the amplification mode the "current gain in the small signal mode" is used, denoted in the reference books h21e. This designation came from the representation of the transistor in the form of a four-pole. The letter "e" indicates that the measurements were made when the transistor with a common emitter was turned on.
The coefficient h21e, as a rule, is somewhat greater than β, although it can also be used in calculations in the first approximation. All the same, the scatter of the parameters β and h21e is so large even for one type of transistor that the calculations are only approximate. After such calculations, as a rule, configuration of the scheme is required.
The gain of a transistor depends on the thickness of the base, so it cannot be changed. Hence, there is a large spread in the gain of transistors taken even from the same box (read one batch). For low-power transistors, this coefficient ranges from 100 ... 1000, and for high-power transistors 5 ... 200. The thinner the base, the higher the ratio.
The simplest circuit for switching on the OE transistor is shown in Figure 5. This is just a small piece from Figure 2, shown in the second part of the article. This is called a fixed base current circuit.
Figure 5.
The scheme is extremely simple. The input signal is fed to the base of the transistor through the blocking capacitor C1, and, being amplified, is removed from the collector of the transistor through the capacitor C2. The purpose of the capacitors is to protect the input circuits from the DC component of the input signal (just remember a carbon or electret microphone) and to provide the necessary bandwidth of the stage.
Resistor R2 is the collector load of the stage, and R1 provides DC bias to the base. With the help of this resistor, they try to make the collector voltage Epit / 2. This state is called the operating point of the transistor, in this case the gain of the stage is maximum.
The approximate resistance of the resistor R1 can be determined by the simple formula R1 ≈ R2 * h21e / 1.5 ... 1.8. The coefficient 1.5 ... 1.8 is substituted depending on the supply voltage: at low voltage (no more than 9V) the value of the coefficient is no more than 1.5, and starting from 50V, it approaches 1.8 ... 2.0. But, in fact, the formula is so approximate that the resistor R1 most often has to be selected, otherwise the required value of Epit / 2 on the collector will not be obtained.
The collector resistor R2 is set as a condition of the problem, since the collector current and the gain of the stage as a whole depend on its value: the greater the resistance of the resistor R2, the higher the gain. But you must be careful with this resistor, the collector current must be less than the maximum allowable for this type of transistor.
The scheme is very simple, but this simplicity also gives it negative properties, and this simplicity comes at a price. Firstly, the amplification of the cascade depends on the specific instance of the transistor: if you replaced the transistor during repair, select the offset again, bring it to the operating point.
Secondly, from the ambient temperature, - as the temperature rises, the collector reverse current Ico increases, which leads to an increase in the collector current. And where, then, is half of the supply voltage at the collector Epit / 2, the same operating point? As a result, the transistor heats up even more, after which it breaks down. To get rid of this dependence, or at least to minimize it, additional elements of negative feedback - OOS - are introduced into the transistor stage.
Figure 6 shows a fixed bias circuit.
Figure 6.
It would seem that the voltage divider Rb-k, Rb-e will provide the required initial displacement of the stage, but in fact, all the disadvantages of a fixed-current circuit are inherent in such a stage. Thus, the circuit shown is just a variation on the fixed current circuit shown in Figure 5.
Thermal stabilization circuits
The situation is somewhat better in the case of using the schemes shown in Figure 7.
Figure 7.
In a collector-stabilized circuit, the bias resistor R1 is not connected to the power supply, but to the collector of the transistor. In this case, if the reverse current increases with increasing temperature, the transistor opens more strongly, and the collector voltage decreases. This decrease leads to a decrease in the bias voltage applied to the base through R1. The transistor begins to close, the collector current decreases to an acceptable value, and the operating point is restored.
It is quite obvious that such a stabilization measure leads to a slight decrease in the gain of the stage, but this is not a problem. The missing gain is usually added by increasing the number of amplifier stages. On the other hand, such an OOS makes it possible to significantly expand the range of operating temperatures of the cascade.
The circuitry of a cascade with emitter stabilization is somewhat more complicated. The amplifying properties of such stages remain unchanged over an even wider temperature range than that of a collector-stabilized circuit. And one more indisputable advantage - when replacing a transistor, you do not have to re-select the operating modes of the cascade.
The emitter resistor R4, providing temperature stabilization, also reduces the gain of the stage. This is for DC. In order to exclude the influence of the resistor R4 on the amplification of the alternating current, the resistor R4 is shunted by the capacitor Ce, which represents a negligible resistance for alternating current. Its value is determined by the frequency range of the amplifier. If these frequencies are in the audio range, then the capacitance of the capacitor can be from units to tens or even hundreds of microfarads. For radio frequencies, this is already hundredths or thousandths, but in some cases the circuit works fine without this capacitor.
In order to better understand how emitter stabilization works, it is necessary to consider the switching circuit of a transistor with a common OK collector.
Common collector circuit (CC) Shown in Figure 8. This circuit is a snippet of Figure 2, from the second part of the article, where all three transistor switching circuits are shown.
Figure 8.
The load of the stage is the emitter resistor R2, the input signal is fed through the capacitor C1, and the output signal is removed through the capacitor C2. Here you can ask why this scheme is called OK? Indeed, if we recall the OE circuit, then it is clearly visible that the emitter is connected to the common wire of the circuit, relative to which the input signal is supplied and the output signal is removed.
In the OK circuit, the collector is simply connected to the power source, and at first glance it seems that it has nothing to do with the input and output signals. But in fact, the EMF source (power battery) has a very small internal resistance, for a signal it is practically one point, one and the same contact.
In more detail, the operation of the OK circuit can be viewed in Figure 9.
Figure 9.
It is known that for silicon transistors the junction voltage b-e is in the range of 0.5 ... 0.7 V, so you can take it on average 0.6 V, if you do not set the goal to carry out calculations with an accuracy of tenths of a percent. Therefore, as can be seen in Figure 9, the output voltage will always be less than the input voltage by the value of Ub-e, namely by the same 0.6V. Unlike the OE circuit, this circuit does not invert the input signal, it simply repeats it, and even reduces it by 0.6V. This circuit is also called an emitter follower. Why is such a scheme needed, what is its use?
The OK circuit amplifies the current signal by a factor of h21e, which indicates that the input resistance of the circuit is h21e times greater than the resistance in the emitter circuit. In other words, you can apply voltage directly to the base (without a limiting resistor) without fear of burning the transistor. Just take the base pin and connect it to the + U power rail.
The high input impedance allows you to connect a high impedance (complex impedance) input source such as a piezo pickup. If such a pickup is connected to the cascade according to the OE scheme, then the low input impedance of this cascade will simply "sink" the pickup signal - "the radio will not play."
A distinctive feature of the OK circuit is that its collector current Ik depends only on the load resistance and voltage of the input signal source. In this case, the parameters of the transistor do not play any role here. Such circuits are said to be covered by 100% voltage feedback.
As shown in Figure 9, the current in the emitter load (aka emitter current) Iн = Iк + Ib. Taking into account that the base current Ib is negligible compared to the collector current Ic, it can be assumed that the load current is equal to the collector current Iн = Iк. The load current will be (Uin - Ube) / Rn. In this case, we will assume that Ube is known and is always equal to 0.6V.
It follows that the collector current Ik = (Uin - Ube) / Rn depends only on the input voltage and load resistance. The load resistance can be changed over a wide range, however, you do not need to be especially zealous. After all, if instead of Rn put a nail - weaving, then no transistor will withstand!
The OK circuit makes it easy to measure the static current transfer coefficient h21e. How to do this is shown in Figure 10.
Figure 10.
First, measure the load current as shown in Figure 10a. In this case, the base of the transistor does not need to be connected anywhere, as shown in the figure. After that, the base current is measured in accordance with Figure 10b. In both cases, measurements should be made in the same quantities: either in amperes or in milliamperes. Power supply voltage and load must remain constant in both measurements. To find out the static current transfer coefficient, it is enough to divide the load current by the base current: h21e ≈ In / Ib.
It should be noted that with an increase in the load current h21e decreases slightly, and with an increase in the supply voltage it increases. Emitter followers are often built in a push-pull configuration using complementary pairs of transistors to increase the output power of the device. Such an emitter follower is shown in Figure 11.
Figure 11.
Figure 12.
The inclusion of transistors according to the scheme with a common base OB
Such a circuit gives only voltage gain, but has better frequency properties compared to the OE circuit: the same transistors can operate at higher frequencies. The main application of the OB circuit is antenna amplifiers of the UHF ranges. Antenna amplifier circuit is shown in Figure 12.
Consider the circuit for switching on a transistor with a common emitter.
- the very term of the name of this inclusion already speaks of the specifics of this scheme. The common emitter, and in kration it is an OE, implies the fact that the input of this circuit and the output have a common emitter.
Consider the circuit:
In this circuit, we see two power supplies, the first 1.5 volts, used as an input signal for the transistor and the entire circuit. The second power supply is 4.5 volts, its role is to power the transistor, and the entire circuit. An element of the Rn circuit is the load of the transistor or, more simply, the consumer.
Now let's trace the operation of this circuit itself: a 1.5 volt power supply serves as an input signal for the transistor, entering the base of the transistor, it opens it. If we consider the full cycle of the base current, then it will be like this: the current passes from plus to minus, that is, based on a 1.5 volt power source, namely from the + terminal, the current passes through the common emitter passing through the base and closes its circuit at the battery terminal 1.5 volts. At the moment the current passes through the base, the transistor is open, thereby allowing the second 4.5 volt power supply to power Rn. let's see the passage of current from the second power supply 4.5 volts. When the transistor is opened by the input current of the base, from the 4.5 volt power supply, the current flows through the emitter of the transistor and leaves the collector directly to the load Rн.
The gain is equal to the ratio of the collector current to the base current and can usually reach from tens to several hundred. A transistor connected according to a common emitter circuit can theoretically give the maximum signal power amplification, relative to other options for turning on the transistor.
Now consider the circuit for switching on a transistor with a common collector: 
In this diagram, we see that there is a common collector at the input and output of the transistor. Therefore, this circuit is called OK with a common collector.
Consider its operation: as in the previous circuit, an input signal arrives at the base, (in our case, this is the base current) opens the transistor. When the transistor is opened, the current from the 4.5 volt battery passes from the battery terminal + through the load Rн enters the emitter of the transistor, passes through the collector and ends its circle. The input of the cascade with such a switching on of the OK has a high resistance, usually from tenths of a megaohm to several megohms due to the fact that the collector junction of the transistor is locked. And the output impedance of the stage is, on the contrary, small, which makes it possible to use such stages to match the previous stage with the load. A cascade with a transistor connected according to a common collector circuit does not amplify the voltage, but amplifies the current (usually by a factor of 10 ... 100). We will return to these details in the following articles, since it is not possible to cover everything and everyone at once.
Consider the circuit for switching on a transistor with a common base. 
The name ABOUT this already tells us a lot - it means, by turning on the transistor, a common base relative to the input and output of the transistor.
In this circuit, the input signal is fed between the base and the emitter - which is what a 1.5 V battery serves for us, the current passing its cycle from plus through the emitter of the transistor along its base, thereby opening the transistor for the voltage to pass from the collector to the load Rн. The input impedance of the stage is small and usually lies in the range from units to hundreds of ohms, which is attributed to the disadvantage of the described switching on of the transistor. In addition, for the operation of a stage with a transistor connected according to a circuit with a common base, two separate power supplies are required, and the current gain of the stage is less than one. The voltage gain of the stage often reaches from tens to several hundred times.
Here we examined three circuits for switching on a transistor, to expand knowledge, I can add the following:
The higher the frequency of the signal entering the input of the transistor stage, the lower the current gain.
The collector junction of the transistor has a high resistance. An increase in frequency leads to a decrease in the reactive capacitance of the collector junction, which leads to its significant shunting and deterioration of the amplifying properties of the cascade.
Good day, dear radio amateurs!
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In this lesson Beginner radio amateur schools we will continue to study semiconductors... In the last lesson, we considered diodes, and in this lesson we will consider a more complex semiconductor element - transistors.
Transistor is a more complex semiconductor structure than diode... It consists of three layers of silicon (there are also germanium transistors) with different conductivity. These can be structures of the n-p-n or p-n-p type. The functioning of transistors, as well as diodes, is based on the properties of pn junctions.
The central, or middle layer, is called base(B), and the other two, respectively - emitter(E) and collector(TO). It should be noted that there is no significant difference between the two types of transistors, and many circuits can be assembled with one type or another, while observing the appropriate polarity of the power supply. The figure below shows a schematic diagram of the transistors, the p-n-p transistor differs from the n-p-n transistor in the direction of the emitter arrow:
There are two main types of transistors: bipolar and unipolar, which differ in design features. Within each type, there are many varieties. The main difference between these two types of transistors is that the control of the processes occurring during the operation of the device in a bipolar transistor is carried out by the input current, and in a unipolar transistor - by the input voltage.
Bipolar transistors, as mentioned above, are a three-layer layered cake. In a simplified form, a transistor can be represented as two oppositely connected diodes:
(in this case, it should be noted that the base-emitter junction is an ordinary zener diode, the stabilization voltage of which is 7 ... 10 volts). The health of the transistor can be checked in the same way as the health of the diode, with an ordinary ohmmeter, by measuring the resistance between its terminals. Transitions similar to those found in a diode exist in a transistor between base and collector, and between base and emitter. In practice, this method is used very often to test transistors. If an ohmmeter is connected between the collector and emitter terminals, the device will show an open circuit (with a working transistor), which is natural since the diodes are turned on in the opposite direction. This means that for any polarity of the applied voltage, one of the diodes is switched on in the forward direction, and the second in the opposite direction, so the current will not pass.
Combining two pairs of transitions leads to the manifestation of an extremely interesting property called transistor effect... If a voltage is applied to the transistor between the collector and the emitter, there will be practically no current (as discussed above). If you make the connection in accordance with the diagram (as in the figure below), where a voltage is applied to the base through the limiting resistance (so as not to damage the transistor), then a current stronger than the base current will pass through the collector. As the base current increases, the collector current will also increase.
Using a measuring device, you can determine the ratio of base, collector and emitter currents. This can be checked in a simple way. If you keep the supply voltage, for example at 4.5 V, by changing the resistance value in the base circuit from R to R / 2, the base current will double, and the collector current will proportionally increase, for example:
Therefore, for any voltage across the resistance R, the collector current will be 99 times the base current, that is, the transistor has a current gain equal to 99. In other words, the transistor amplifies the base current by 99 times. This coefficient is denoted by the letter ? . The gain is equal to the ratio of the collector current to the base current:
? = Ik / Ib
An alternating voltage can also be applied to the base of the transistor. But, it is necessary that the transistor works in a linear mode. For normal operation in linear mode, the transistor should be supplied to the base with a constant bias voltage and an alternating voltage, which it will amplify. In this way, transistors amplify weak voltages, for example from a microphone, to a level that can drive a loudspeaker. If the gain is not sufficient, you can use multiple transistors or their series in stages. In order not to violate the operating modes of each of them in direct current (at which linearity is ensured) when connecting the cascades, decoupling capacitors are used. Bipolar transistors have electrical characteristics that give them distinct advantages over other amplifier components.
As we already know, there are also (except for bipolar) and unipolar transistors... Let's take a quick look at two of them - field and unijunction transistors. Like bipolar, they are of two types and have three outputs:
Field-effect transistor electrodes are: gate- Z, runoff- C corresponding to the manifold and source- And, identified with the emitter. The n- and p-channel FETs differ in the direction of the gate arrow. Single junction transistors, sometimes referred to as double base diodes, are mainly used in pulse periodic signal generator circuits.
There are three fundamental circuits for switching on transistors in an amplifier stage:
? common emitter(but)
? with common collector(b)
? with a common base(in)
Common-emitter bipolar transistor, depending on the output resistance of the power supply R1 and the load resistance Rн, amplifies the input signal in both voltage and current. The gain of a bipolar transistor is denoted as h21e(read: ash-two-one-e, where e is a circuit with a common emitter), and it is different for each transistor. The value of the coefficient h21e (its full name is static current transfer coefficient of the base h21e) depends only on the thickness of the base of the transistor (it cannot be changed) and on the voltage between the collector and the emitter, therefore, at a low voltage (less than 20 V), its current transfer coefficient at any collector current is practically unchanged and slightly increases with increasing voltage on the collector.
Current gain – Kus.i and voltage gain – Kus.u of a bipolar transistor connected in a common-emitter circuit depends on the ratio of the load resistance (designated as Rn in the diagram) and the signal source (designated as R1 in the diagram). If the resistance of the signal source is h21e times less than the load resistance, then the voltage gain is slightly less than unity (0.95 ... 0.99), and the current gain is h21e. When the resistance of the signal source is more than h21e times less than the load resistance, then the current gain remains unchanged (equal to h21e), and the voltage gain decreases. If, on the contrary, the input resistance is reduced, then the voltage gain becomes greater than unity, and the current gain, when the current flowing through the base-emitter junction of the transistor is limited, does not change. The common emitter circuit is the only bipolar transistor switching circuit that requires limiting the input (control) current. Several conclusions can be drawn:- the base current of the transistor must be limited, otherwise either the transistor or the circuit that controls it will burn out; - with the help of a transistor connected according to the OE scheme, it is very easy to control a high-voltage load with a low-voltage signal source. A significant current flows through the base, and therefore the collector junctions at a base-emitter voltage of only 0.8 ... 1.5 V. If the amplitude (voltage) is greater than this value, a current-limiting resistor (R1) must be placed between the base of the transistor and the output of the control circuit. You can calculate its resistance using the formulas:
Ir1 = Irн / h21э R1 = Ucontr / Ir1 where:
Irн- current through the load, A; Ucont- signal source voltage, V; R1- resistor resistance, Ohm.
Another feature of the OE circuit is that the voltage drop at the collector-emitter junction of the transistor can be practically reduced to zero. But for this it is necessary to significantly increase the base current, which is not very profitable. Therefore, this mode of operation of transistors is used only in pulse, digital circuits.
Transistor, analog amplifier circuit, should provide approximately the same amplification of signals with different amplitudes relative to a certain “average” voltage. To do this, you need to open it up a little, trying not to overdo it. As seen in the picture below (left):
the collector current and the voltage drop across the transistor with a smooth increase in the base current initially change almost linearly, and only then, with the onset saturation transistor, are pressed against the axes of the graph. We are only interested in the straight parts of the lines (to saturation) - it is obvious that they symbolize the linear amplification of the signal, that is, when the control current changes several times, the collector current (voltage in the load) will change by the same amount.
The analog waveform is shown in the figure above (right)... As can be seen from the graph, the signal amplitude constantly pulsates relative to a certain average voltage Uav, and it can both increase and decrease. But the bipolar transistor reacts only to an increase in the input voltage (or rather, current). Conclusion: you need to make sure that the transistor is slightly open even with the minimum amplitude of the input signal. With an average amplitude Uav, it will open a little stronger, and with a maximum Umax, it will open as much as possible. But at the same time, it should not enter the saturation mode (see the figure above) - in this mode, the output current ceases to linearly depend on the input current, as a result of which a strong signal distortion occurs.
Let's look at the analog waveform again. Since both the maximum and minimum amplitudes of the input signal relative to the average are approximately the same in magnitude (and opposite in sign), we need to supply such a constant current (bias current - Icm) to the base of the transistor so that at the “average” voltage at the input the transistor is open exactly half. Then, with a decrease in the input current, the transistor will close and the collector current will decrease, and with an increase in the input current, it will open even more.
We learned how the transistor works, in general terms we examined the manufacturing technologies germanium and silicon transistors and figured out how they are marked.
Today we will conduct several experiments and make sure that the bipolar transistor really consists of two diodes, included in the opposite direction, and that the transistor is signal amplifier.
We need a low-power germanium transistor of the p-n-p structure from the MP39 - MP42 series, an incandescent lamp designed for a voltage of 2.5 Volts and a power source for 4 - 5 Volts. In general, for novice radio amateurs, I recommend assembling a small adjustable one, with which you will power your designs.
1. The transistor consists of two diodes.
To make sure of this, let's put together a small circuit: the base of the transistor VT1 connect to the minus of the power supply, and the collector terminal to one of the terminals of the incandescent lamp EL... Now if the second terminal of the lamp is connected to the plus of the power source, the lamp will light up.
The light came on because we applied to the collector junction of the transistor direct- the throughput voltage that opened the collector junction and flowed through it direct current collector Iк... The magnitude of this current depends on the resistance filaments lamps and internal resistance power supply.
And now we will consider the same circuit, but we will depict the transistor in the form of a semiconductor plate.
Main charge carriers in the base electrons, overcoming the p-n junction, fall into the hole region collector and become minor. Having become minority, the base electrons are absorbed by the majority carriers in the hole region of the collector holes... In the same way, holes from the collector region, falling into the electronic region of the base, become minority and are absorbed by the majority charge carriers in the base. electrons.
The base pin connected to the negative pole of the power supply will to act practically unlimited number electrons, replenishing the decay of electrons from the base region. And the collector contact, connected to the positive pole of the power source through the lamp filament, is capable of to accept the same number of electrons, due to which the concentration of holes in the region base.
Thus, the conductivity of the p-n junction will become large and the resistance to current will be small, which means that the collector current will flow through the collector junction. Iк... And what more there will be this current, the brighter the lamp will be on.
The light will also be on if it is included in the emitter junction circuit. The figure below shows exactly this variant of the scheme.
Now let's change the circuit and base of the transistor a little. VT1 connect to plus power supply. In this case, the lamp will not burn, since we turned on the p-n junction of the transistor in reverse direction. This means that the resistance of the p-n junction has become great and only very little flows through it reverse current collector Ikbo incapable of incandescent lamp filament EL... In most cases, this current does not exceed a few microamperes.
And in order to finally make sure of this, again consider a circuit with a transistor depicted in the form of a semiconductor plate.
Electrons in the area base will move to plus power source, moving away from the pn junction. Holes in the area collector, will also move away from the p-n junction, moving to negative pole of the power supply. As a result, the border of the regions, as it were will expand, which is why a zone depleted in holes and electrons is formed, which will provide high resistance to the current.
But, since in each of the areas of the base and the collector there are non-core charge carriers, then small exchange electrons and holes between the regions will still occur. Therefore, a current that is many times smaller than the direct current will flow through the collector junction, and this current will not be enough to ignite the lamp filament.
2. Operation of the transistor in switching mode.
Let's make another experiment showing one of the modes of operation of the transistor.
Between the collector and the emitter of the transistor, we connect a series-connected power supply and the same incandescent lamp. We connect the plus of the power supply to the emitter, and the minus through the filament of the lamp with the collector. The lamp is off. Why?
Everything is very simple: if you apply a supply voltage between the emitter and the collector, then for any polarity one of the transitions will be in the forward direction, and the other in the opposite direction and will interfere with the passage of current. This is not difficult to see if you look at the following figure.
The figure shows that the base-emitter emitter junction is included in direct direction and is open and ready to accept an unlimited number of electrons. Collector junction base-collector, on the contrary, is included in reverse direction and prevents the passage of electrons to the base.
Hence it follows that the majority charge carriers in the emitter region holes, repelled by the plus of the power source, rush to the base region and there they mutually absorb (recombine) with the main charge carriers in the base electrons... At the moment of saturation, when there are no free charge carriers on either side, their movement will stop, which means that the current stops flowing. Why? Because from the side of the collector there will be no recharge electrons.
It turns out that the main charge carriers in the collector holes attracted by the negative pole of the power source, and some of them mutually absorbed electrons coming from the negative side of the power supply. And at the moment of saturation, when on both sides there will be no free charge carriers, holes, due to their predominance in the collector region, will block further passage of electrons to the base.
Thus, a zone depleted in holes and electrons is formed between the collector and the base, which will provide high resistance to the current.
Of course, thanks to the magnetic field and thermal effect, a scanty current will still flow, but the strength of this current is so small that it is not able to incandesce the filament of the lamp.
Now add to the circuit jumper wire and close the base with the emitter to it. The light included in the collector circuit of the transistor will not light again. Why?
Because when the base and emitter are closed with a jumper, the collector junction becomes just a diode to which the opposite voltage. The transistor is in the closed state and only a slight reverse collector current flows through it. Ikbo.
Now let's change the circuit a little more and add a resistor Rb resistance 200 - 300 Ohm, and one more voltage source GB in the form of a finger-type battery.
Connect the minus of the battery through a resistor Rb with the base of the transistor, and plus batteries with an emitter. The lamp came on.
The lamp lit up because we connected a battery between the base and the emitter, and thereby applied to the emitter junction direct unlocking voltage. The emitter junction opened and went through it straight current that opened collector junction of the transistor. The transistor opened and along the circuit emitter-base-collector collector current drip Iк, many times greater than the circuit current emitter-base... And thanks to this current, the light came on.
If we change the polarity of the battery and apply a plus to the base, then the emitter junction will close, and the collector junction will also close with it. Reverse collector current will flow through the transistor Ikbo and the light goes out.
Resistor Rb limits the current in the base circuit. If the current is not limited and all 1.5 volts are applied to the base, then too much current will flow through the emitter junction, as a result of which thermal breakdown transition and the transistor will fail. Typically for germanium transistors, the unlocking voltage is not more than 0,2 volts, and for silicon no more 0,7 volts.
And again we will analyze the same circuit, but represent the transistor in the form of a semiconductor plate.
When the unlocking voltage is applied to the base of the transistor, it opens emitter transition and free holes from the emitter begin to interabsorb with electrons base creating a small forward base current Ib.
But not all holes introduced from the emitter into the base recombine with its electrons. Typically, the base area is done thin, and in the manufacture of transistors of the p-n-p structure, the concentration of holes in emitter and manifold make many times greater than the concentration of electrons in base, therefore, only a small part of the holes is absorbed by the electrons of the base.
The main mass of the emitter holes passes the base and falls under the influence of a higher negative voltage acting in the collector, and already together with the collector holes moves to its negative contact, where it is mutually absorbed by the introduced electrons by the negative pole of the power source GB.
As a result, the resistance of the collector circuit emitter-base-collector will decrease and direct collector current flows in it Iк many times the base current Ib chains emitter-base.
How more more holes is introduced from the emitter to the base, so more significant collector current. And vice versa than less the unlocking voltage at the base, the less collector current.
If at the time of operation of the transistor, a milliammeter is included in the base and collector circuits, then with the transistor closed, there would be practically no currents in these circuits.
When the transistor is open, the base current Ib would be 2-3 mA, and the collector current Iк would be about 60 - 80 mA. All this suggests that the transistor can be current amplifier.
In these experiments, the transistor was in one of two states: open or closed. The switching of the transistor from one state to another occurred under the influence of the unlocking voltage at the base Uб... This transistor mode is called switching mode or key... This mode of operation of the transistor is used in devices and automation devices.
This concludes, and in the next part we will analyze the operation of the transistor in the example of a simple audio frequency amplifier assembled on a single transistor.
Good luck!
Literature:
1. Borisov VG - Young radio amateur. 1985
2. E. Iceberg - Transistor? .. It's very simple! 1964
There are several diagrams of simple devices and assemblies that can be made by novice radio amateurs.
Single-stage AF amplifier
This is the simplest design that allows you to demonstrate the amplifying capabilities of the transistor. True, the voltage gain is small - it does not exceed 6, so the scope of such a device is limited.
Nevertheless, it can be connected, say, to a detector radio (it must be loaded with a 10 kΩ resistor) and use the BF1 headset to listen to the transmissions of the local radio station.
The amplified signal is fed to the input jacks X1, X2, and the supply voltage (as in all other designs of this author, it is 6 V - four galvanic cells with a voltage of 1.5 V, connected in series) is fed to the jacks X3, X4.
Divider R1R2 sets the bias voltage at the base of the transistor, and resistor R3 provides current feedback, which contributes to temperature stabilization of the amplifier.
Rice. 1. Scheme of a single-stage AF amplifier on a transistor.
How does stabilization take place? Suppose that the transistor collector current has increased under the influence of temperature. Accordingly, the voltage drop across the resistor R3 will increase. As a result, the emitter current will decrease, and hence the collector current - it will reach its initial value.
The load of the amplifier stage is a headset with a resistance of 60 .. 100 Ohm. It is not difficult to check the operation of the amplifier, you need to touch the input socket X1, for example, with tweezers a weak buzz should be heard in the phone, as a result of the induction of an alternating current. The collector current of the transistor is about 3 mA.
Two-stage ultrasonic frequency converter on transistors of different structures
It is designed with direct coupling between stages and deep negative DC feedback, which makes it independent of the ambient temperature. The basis of temperature stabilization is the resistor R4, which works similarly to the resistor R3 in the previous design.
The amplifier is more "sensitive" in comparison with a single-stage amplifier - the voltage gain reaches 20. AC voltage with an amplitude of no more than 30 mV can be applied to the input jacks, otherwise there will be distortions heard in the headphone.
They check the amplifier by touching the input socket X1 with tweezers (or just your finger) - a loud sound will be heard in the phone. The amplifier draws a current of about 8 mA.
Rice. 2. Scheme of a two-stage AF amplifier on transistors of different structures.
This design can be used to amplify weak signals from a microphone, for example. And of course it will significantly enhance the signal 34 taken from the load of the detector receiver.
Two-stage ultrasonic frequency converter on transistors of the same structure
Here, a direct connection between the cascades is also used, but the stabilization of the operating mode is somewhat different from the previous designs.
Suppose that the collector current of the transistor VT1 has decreased.The voltage drop across this transistor will increase, which will lead to an increase in the voltage across the resistor R3 connected to the emitter circuit of the transistor VT2.
Due to the connection of the transistors through the resistor R2, the base current of the input transistor will increase, which will lead to an increase in its collector current. As a result, the initial change in the collector current of this transistor will be compensated.
Rice. 3. Scheme of a two-stage AF amplifier on transistors of the same structure.
The sensitivity of the amplifier is very high - the gain reaches 100. The gain depends to a large extent on the capacitance of the capacitor C2 - if you turn it off, the gain will decrease. The input voltage should be no more than 2 mV.
The amplifier works well with a detector receiver, electret microphone, and other weak signal sources. The current consumed by the amplifier is about 2 mA.
It is made on transistors of different structures and has a voltage gain of about 10. The highest input voltage can be 0.1 V.
The first two-stage amplifier is assembled on the VT1 transistor, the second - on VT2 and VTZ of different structures. The first stage amplifies the voltage signal 34, with both half-waves being the same. The second one amplifies the current signal, but the cascade on the VT2 transistor “works” with positive half-waves, and on the VTZ transistor - with negative ones.
Rice. 4. Push-pull power amplifier AF on transistors.
The DC mode is chosen such that the voltage at the junction point of the emitters of the second stage transistors is approximately half the voltage of the power supply.
This is achieved by switching on the feedback resistor R2. The collector current of the input transistor, flowing through the diode VD1, leads to a voltage drop across it. which is the bias voltage at the bases of the output transistors (relative to their emitters) - it allows you to reduce the distortion of the amplified signal.
The load (several headphones connected in parallel or a dynamic head) is connected to the amplifier through an oxide capacitor C2.
If the amplifier will operate on a dynamic head (with a resistance of 8-10 ohms), the capacitance of this capacitor should be at least twice as large. Pay attention to the connection of the load of the first stage - resistor R4. , and with the lower output of the load.
This is the so-called voltage boost circuit, in which a small voltage of the AF positive feedback enters the base circuit of the output transistors, which equalizes the operating conditions of the transistors.
Two-level voltage indicator
Such a device can be used. for example, to indicate the "exhaustion" of the battery or indicate the level of the reproduced signal in a household tape recorder. The indicator layout will demonstrate how it works.
Rice. 5. Scheme of a two-level voltage indicator.
In the lower position of the engine of the variable resistor R1, both transistors are closed, the LEDs HL1, HL2 are off. When moving the resistor slider up, the voltage across it increases. When it reaches the opening voltage of the VT1 transistor, the HL1 LED will flash
If you continue to move the engine. there will come a moment when, after the diode VD1, the transistor VT2 opens. The HL2 LED will also flash. In other words, a low voltage at the indicator input causes only the HL1 LED to light up and more than both LEDs.
Smoothly decreasing the input voltage with a variable resistor, we note that the HL2 LED goes out first, and then HL1. The brightness of the LEDs depends on the limiting resistors R3 and R6 as their resistances increase, the brightness decreases.
To connect the indicator to a real device, you need to disconnect the upper terminal of the variable resistor from the positive wire of the power source and apply a controlled voltage to the extreme terminals of this resistor. By moving its slider, the indicator response threshold is selected.
When monitoring only the voltage of the power source, it is permissible to install a green LED AL307G in place of HL2.
It emits light signals on the principle of less than normal - normal - more than normal. For this, the indicator uses two red and one green LEDs.
Rice. 6. Three-level voltage indicator.
At a certain voltage on the engine of the variable resistor R1 (voltage is normal), both transistors are closed and only the green LED HL3 is working. Moving the resistor slider up the circuit leads to an increase in voltage (more than normal), the VT1 transistor opens on it.
The HL3 LED goes out and HL1 comes on. If the slider is moved down and thus the voltage across it (‘less than normal”), the VT1 transistor will close and VT2 will open. The following picture will be observed: first, the HL1 LED will go out, then HL3 will light up and soon go out, and finally HL2 will flash.
Due to the low sensitivity of the indicator, a smooth transition from the extinguishing of one LED to the ignition of another is obtained has not yet completely extinguished, for example, HL1, but HL3 is already on.
Schmitt trigger
As you know, this device is usually used to convert a slowly varying voltage into a square wave signal. When the slider of the variable resistor R1 is in the lower position according to the circuit, the transistor VT1 is closed.
The voltage at its collector is high, as a result, the VT2 transistor turns out to be open, which means that the HL1 LED is on. A voltage drop forms on the resistor R3.
Rice. 7. Simple Schmitt trigger on two transistors.
By slowly moving the slider of the variable resistor up the circuit, it will be possible to reach the moment when the abrupt opening of the transistor VT1 and the closing of VT2 occur.This will happen when the voltage at the base of VT1 exceeds the voltage drop across the resistor R3.
The LED will turn off. If you then move the slider down, the trigger will return to its original position - the LED will flash. This will happen when the voltage on the slider is less than the LED off voltage.
Waiting multivibrator
Such a device has one stable state and goes into another only when the input signal is applied. In this case, the multivibrator generates a pulse of its own duration, regardless of the duration of the input signal. We will verify this by conducting an experiment with the layout of the proposed device.
Rice. 8. Schematic diagram of the waiting multivibrator.
In the initial state, the VT2 transistor is open, the HL1 LED is on. It is now enough to short-circuit the slots X1 and X2 for a current pulse through the capacitor C1 to open the VT1 transistor. The voltage on its collector will decrease and the capacitor C2 will be connected to the base of the VT2 transistor in such a polarity that it will close. The LED will turn off.
The capacitor will start to discharge, the discharge current will flow through the resistor R5, keeping the VT2 transistor closed. As soon as the capacitor is discharged, the VT2 transistor will open again and the multivibrator will go back to standby mode.
The duration of the pulse generated by the multivibrator (the duration of being in an unstable state) does not depend on the duration of the trigger, but is determined by the resistance of the resistor R5 and the capacitance of the capacitor C2.
If you connect a capacitor of the same capacity in parallel to C2, the LED will remain off for twice as long.
I. Bokomchev. P-06-2000.
