Which awl is better. How to make a homemade awl with interchangeable nozzles? A shoemaker needs a hook and an awl in two sizes: for the upper and for the sole
How does a transistor work?
Take a good look at Fig. 93. On the left in this figure you see a simplified circuit of an amplifier on a p-n-p transistor and illustrations that explain the essence of the operation of this amplifier. Here, as in the previous figures, the holes of the p-type regions are conventionally depicted by circles, and the electrons of the n-type region by black balls of the same size. Remember the names p-n junction ov: between the collector and the base - the collector, between the emitter and the base - the emitter.
Rice. 93. A simplified diagram of a p-n-p transistor amplifier and graphs illustrating its operation.
Between the collector and the emitter, a battery Bk (collector) is connected, which creates a negative voltage of the order of several volts on the collector with respect to the emitter. In the same circuit, called the collector, the load R n is included, which can be a telephone or other device - depending on the purpose of the amplifier.
If the base is not connected to anything, a very weak current (tenths of a milliamp) will appear in the collector circuit, since with such a polarity of the battery B being switched on, the resistance of the collector p-n junction will be very large; for the collector junction, this will be the reverse current. The current of the collector circuit Ik sharply increases if a bias element Bc is included between the base and the emitter, applying a small, at least a tenth of a volt, negative voltage to the base with respect to the emitter. Here is what will happen. With this inclusion of element B c (meaning that the clamps for connecting the source of the amplified signal, indicated in the diagram by the sign "~" - a sinusoid, are short-circuited) in this new circuit, called the base circuit, some forward current I b will flow; as in a diode, holes in the emitter and electrons in the base will move in opposite directions and neutralize, causing current through the emitter junction.
But the fate of most of the holes introduced from the emitter into the base is different than disappearing when they meet electrons. The fact is that in the manufacture of transistors of the p-n-p structure, the saturation of holes in the emitter (and collector) is always made greater than the saturation of electrons in the base. Due to this, only a small part of the holes (less than 10%), having met with electrons, disappears. The main mass of holes freely passes into the base, falls under a higher negative voltage on the collector, enters the collector and, in the general flow with its holes, moves to its negative contact. Here they are neutralized by counter electrons introduced into the collector by the negative pole of the battery Bk. As a result, the resistance of the entire collector circuit decreases and a current flows in it that is many times greater than the reverse current of the collector junction. The greater the negative voltage on the base, the more holes are introduced from the emitter into the base, the greater the current of the collector circuit. And, conversely, the lower the negative voltage on the base, the lower the current of the collector circuit of the transistor.
And if an alternating electrical signal is introduced into the base circuit in series with a constant voltage source supplying this circuit? The transistor will amplify it.
The amplification process in general terms is as follows. In the absence of signal voltage in the base and collector circuits, currents of a certain magnitude flow (section O a in the graphs in Fig. 93), determined by the voltages of the batteries and the properties of the transistor. As soon as a signal appears in the base circuit, the currents in the transistor circuits begin to change accordingly: during negative half-cycles, when the total negative voltage at the base increases, the circuit currents increase, and during positive half-cycles, when the voltages of the signal and element B c are opposite and , therefore, the negative voltage on the base decreases, the currents in both circuits also decrease. Amplification occurs in voltage and current.
If an electrical signal of an audio frequency is applied to the input circuit, i.e., to the base circuit, and the telephone is the load of the output - collector - circuit, it converts the amplified signal into sound. If the load is a resistor, then the voltage of the variable component of the amplified signal created on it can be applied to the input circuit of the second transistor for additional amplification. One transistor can amplify the signal by 30 to 50 times.
Transistors work the same way. n-p-n structures, only in them the main current carriers are not holes, but electrons. In this regard, the polarity of switching on the elements and batteries that feed the base and collector circuits of n-p-n transistors should not be the same as for p-n-p transistors, but reverse.
Remember a very important circumstance: a constant voltage, called the bias voltage, must be applied to the base of the transistor (relative to the emitter), along with the voltage of the amplified signal, which opens the transistor.
In the amplifier according to the circuit in Fig. 93 the role of the bias voltage source is performed by element B s. For a germanium transistor of the p-n-p structure, it should be negative and be 0.1-0.2 V, and for a transistor of the n-p-n structure, it should be positive. For silicon transistors, the bias voltage is 0.5 -0.7 V. Without an initial bias voltage, the emitter p-n junction will “cut off”, like a diode, positive (p-n-p transistor) or negative (n-p-n transistor) signal half-waves, which will cause amplification to be accompanied by distortion. The only time the base is not biased is when the emitter junction of the transistor is used to detect a high frequency modulated signal.
Is it necessary to apply a special cell or battery to supply the initial bias voltage to the base? Of course not. For this purpose, the voltage of the collector battery is usually used, connecting the base to this power source through a resistor. The resistance of such a resistor is often selected empirically, since it depends on the properties of a given transistor.
At the beginning of this part of the conversation, I said that a bipolar transistor can be imagined as two back-to-back diodes connected in one semiconductor plate and having one common cathode, the role of which is played by the base of the transistor. It is easy to verify this in experiments, for which you will need any used, but not damaged germanium low-frequency transistor of the p-n-p structure, for example MP39 or similar transistors MP40 - MP42. Between the collector and the base of the transistor, turn on a 3336L battery connected in series and a light bulb from a pocket flashlight, rated for a voltage of 2.5 V and a current of 0.075 or 0.15 A. If the plus of the battery is connected (through a light bulb) to the collector, and the minus to the base ( Fig. 94, a), then the light will be on. With a different polarity of switching on the battery (Fig. 94, b), the light should not light.
Rice. 94. Experiments with a transistor.
How to explain these phenomena? First, you applied a direct, i.e. forward voltage, to the collector p-n junction. In this case, the collector junction is open, its resistance is low, and direct collector current Ik flows through it. The value of this current in this case is determined mainly by the resistance of the light bulb filament and the internal resistance of the battery. When the battery was switched on for the second time, its voltage was applied to the collector junction in the opposite, impermeable direction. In this case, the junction is closed, its resistance is high, and only a small reverse collector current flows through it. For a serviceable low-power low-frequency transistor, the reverse collector current I of the OBE does not exceed 30 μA. Such a current, of course, could not glow the filament of the light bulb, so it did not burn.
Carry out a similar experiment with an emitter junction. The result will be the same: with reverse voltage, the transition will be closed - the light is off, and with direct voltage it will be open - the light is on.
The following experiment, illustrating one of the modes of operation of the transistor, is carried out according to the circuit shown in Fig. 95 a. Between the emitter and collector of the same transistor, turn on a 3336L battery and an incandescent bulb connected in series. The positive pole of the battery must be connected to the emitter, and the negative pole to the collector (through the filament of the light bulb). Is the light bulb on? No, it doesn't burn. Connect a jumper wire between the base and the emitter, as shown in the diagram with a dashed line. A light bulb included in the collector circuit of the transistor will also not burn. Remove the jumper and instead connect to these electrodes a series-connected resistor with a resistance of 200 - 300 Ohms and one galvanic element Eb, for example, type 332, but so that the minus of the element is on the base, and the plus is on the emitter. The light should now be on. Reverse the polarity of connecting the element to these transistor electrodes. In this case, the lamp will not light up. Repeat this experiment several times and you will be convinced that the light bulb in the collector circuit will burn only when a negative voltage acts on the base of the transistor relative to the emitter.
Rice. 95. Experiments illustrating the operation of a transistor in switching mode (a) and in amplifying mode (b).
Let's take a look at these experiences. In the first of these, when you short-circuited the emitter junction by connecting the base to the emitter with a jumper, the transistor became just a diode, which was supplied with a reverse voltage that closed the transistor. Only a slight reverse current of the collector junction went through the transistor, which could not glow the light bulb filament. At this time, the transistor was in the closed state. Then, by removing the jumper, you restored the emitter junction. By first turning on the element between the base and the emitter, you applied a direct voltage to the emitter junction. The emitter junction opened, a direct current went through it, which opened the second junction of the transistor - the collector. The transistor turned out to be open and the current of the transistor went through the emitter - base - collector circuit, which is many times greater than the current of the emitter - base circuit. It was he who lit the filament of the light bulb. When you changed the polarity of the inclusion of the element to the reverse, then its voltage closed the emitter junction, and at the same time the collector junction also closed. At the same time, the transistor current almost stopped (only the collector reverse current was flowing) and the light bulb did not burn.
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 action of voltage at the base of UB. This mode of operation of the transistor, illustrated by the graphs in Fig. 95, a, is called the switching mode or, what is the same, the key mode. This mode of operation of transistors is used mainly in electronic automation equipment.
What is the role of the resistor R b in these experiments? In principle, this resistor may not exist. I recommended turning it on solely in order to limit the current in the base circuit. Otherwise, too much forward current will flow through the emitter junction, as a result of which a thermal breakdown of the junction may occur and the transistor will fail.
If, during these experiments, measuring instruments were included in the base and collector circuits, then with the transistor closed, there would be almost no currents in its circuits. With the transistor open, the base current I B would be no more than 2 - 3 mA, and the collector current I K was 60 - 75 mA. This means that the transistor can be a current amplifier.
In receivers and audio frequency amplifiers, transistors operate in amplifying mode. This mode differs from the switching mode in that by using small currents in the base circuit, we can control much larger currents in the collector circuit of the transistor.
The operation of a transistor in amplification mode can be illustrated by such an experiment (Fig. 95, b). In the collector circuit of the transistor T, turn on the electromagnetic telephone Tf 2 between the base and the minus of the power source B - resistor R b with a resistance of 200 - 250 kOhm. Turn on the second phone Tf 1 between the base and the emitter through a coupling capacitor C sv with a capacity of 0.1 - 0.5 microfarads. You will get a simple amplifier that can, for example, play the role of a one-way telephone. If your friend speaks softly in front of the phone connected to the input of the amplifier, you will hear his conversation in the phones connected to the output of the amplifier.
What is the role of the resistor R b in this amplifier? Through it, a small initial bias voltage is supplied to the base of the transistor from battery B, which opens the transistor and thereby ensures its operation in amplification mode. At the input of the amplifier, instead of the phone TF 1, you can turn on the pickup and play the record. Then the sounds of the melody or the voice of the singer, recorded on a gramophone record, will be clearly audible in Tf2 phones.
In this experiment, an alternating voltage of audio frequency was applied to the input of the amplifier, the source of which was a telephone, which, like a microphone, converts sound vibrations into electrical vibrations, or a sound pickup, which converts mechanical vibrations its needles into electrical vibrations. This voltage created a weak alternating current in the emitter-base circuit, which controlled a much larger current in the collector circuit: with negative half-cycles on the base, the collector current increased, and with positive ones, it decreased (see graphs in Fig. 95, b). The signal was amplified, and the signal amplified by the transistor was converted by the telephone included in the collector circuit into sound vibrations. The transistor was in amplifying mode.
You can carry out similar experiments with an n-p-n structure transistor, for example, MP35 type. In this case, it is only necessary to change the polarity of switching on the power supply of the transistor: a minus should be connected to the emitter, and a plus of the batteries should be connected to the collector (via the phone).
Briefly about the electrical parameters of bipolar transistors. The quality and amplifying properties of bipolar transistors are evaluated by several parameters, which are measured using special instruments. You, from a practical point of view, should be primarily interested in three main parameters: the reverse current of the collector I KBO, the static current transfer coefficient h 21E (read as follows: ash two one e) and the cutoff frequency of the current transfer coefficient gr.
The collector reverse current I KBO is an uncontrolled current through the collector p-n junction, which is created by the minor current carriers of the transistor. Parameter I KBO characterizes the quality of the transistor: the smaller it is, the higher the quality of the transistor. For low-power low-frequency transistors, for example, types MP39 - MP42, I KBO should not exceed 30 μA, and for low-power high-frequency transistors - no more than 5 μA. Transistors with large values I OBEs are unstable in operation.
The static current transfer coefficient h 21E characterizes the amplifying properties of the transistor. It is called static because this parameter is measured at constant voltages on its electrodes and constant currents in its circuits. The large (capital) letter "E" in this expression indicates that when measuring, the transistor is turned on according to a circuit with a common emitter (I will talk about transistor switching circuits in the next conversation). The coefficient h 21E is characterized by the ratio of the constant collector current to the constant base current for a given constant collector-emitter reverse voltage and emitter current. The greater the numerical value of the coefficient h 21E, the greater the signal amplification this transistor can provide.
The cutoff frequency of the current transfer coefficient gr, expressed in kilohertz or megahertz, makes it possible to judge the possibility of using a transistor to amplify oscillations of certain frequencies. The cutoff frequency of the MP39 transistors, for example, is 500 kHz, and the P401 - P403 transistors are more than 30 MHz. In practice, transistors are used to amplify frequencies much less than the boundary ones, since with increasing frequency, the current transfer coefficient h 21E of the transistor decreases.
AT practical work it is necessary to take into account such parameters as the maximum allowable collector-emitter voltage, the maximum allowable collector current, as well as the maximum allowable dissipated power of the transistor collector - the power that turns into heat inside the transistor.
You will find basic information about low-power transistors for mass use in App. 4.
Consider the switching circuit of a transistor with a common emitter.
- the very term of the name of this inclusion already speaks of the specifics of this scheme. A 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 schema:
in this circuit we see two power supplies, the first 1.5 volts is 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. The circuit element Rn is the load of the transistor or, more simply, the consumer.
Now let's trace the very operation of this circuit: 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 passage of the base current, 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 the transistor allows the second power source of 4.5 volts to power Rn. let's see the current flow from the second 4.5 volt power supply. When the transistor is opened by the base input current, a current flows through the emitter of the transistor from the 4.5 volt power source and exits 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 hundreds. A transistor connected according to a common emitter circuit can theoretically give the maximum signal amplification in terms of power, 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 with a common collector OK.
Let's consider its work: as in the previous circuit, the 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 V battery passes from the battery terminal + through the load Rn, enters the emitter of the transistor, passes through the collector and ends its circle. The input of the cascade with this inclusion of OK has a high resistance, usually from tenths of a megaohm to several megaohms due to the fact that the collector junction of the transistor is locked. And the output impedance of the cascade, on the contrary, is small, which makes it possible to use such cascades to match the previous cascade 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 10 ... 100 times). We will return to these details in the following articles, since it is not possible to cover everything and everyone at once.
Let's consider the switching circuit of a transistor with a common base. 
The name of the OB already tells us a lot now - it means that by turning on the transistor, the common base regarding the input and output of the transistor.
In this circuit, the input signal is applied between the base and the emitter - what a battery with a nominal value of 1.5 V serves us, the current passing its cycle from the plus through the emitter of the transistor along its base, thereby opening the transistor for the passage of voltage from the collector to the load Rн. The input impedance of the cascade is small and usually ranges 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 the cascade with a common-base transistor, two separate power supplies are required, and the cascade current gain is less than unity. The voltage gain of the cascade often reaches from tens to several hundred times.
Here we considered three transistor switching circuits, to expand knowledge I can add the following:
The higher the frequency of the signal at 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.
Several schemes are given simple devices and nodes that can be made by novice radio amateurs.
Single stage AF amplifier
This is simplest design, which 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 to, say, a detector radio (it must be loaded with a 10 kΩ resistor) and, using the BF1 headphone, listen to the transmission of a local radio station.
The amplified signal is fed to the input sockets 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 X3, X4 sockets.
Divider R1R2 sets the bias voltage at the base of the transistor, and resistor R3 provides current feedback, which contributes to the temperature stabilization of the amplifier.
Rice. 1. Scheme of a single-stage AF amplifier on a transistor.
How does stabilization take place? Suppose that under the influence of temperature, the collector current of the transistor has increased. 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 original value.
The load of the amplifying stage is a headphone with a resistance of 60 .. 100 Ohms. It is not difficult to check the operation of the amplifier, you need to touch the X1 input jack, for example, a weak buzz should be heard with tweezers in the phone, as a result of alternating current pickup. 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 connection between the stages and deep negative DC feedback, which makes its mode 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" compared to a single-stage one - the voltage gain reaches 20. An alternating voltage with an amplitude of no more than 30 mV can be applied to the input jacks, otherwise there will be distortion heard in the headphone.
They check the amplifier by touching the X1 input jack with tweezers (or just a finger) - a loud sound will be heard in the phone. The amplifier consumes 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 such as from a microphone. And of course, it will significantly amplify 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 previous designs.
Assume 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 included in 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 is highly dependent 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, an 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 a VT1 transistor, the second - on VT2 and VTZ of different structures. The first stage amplifies signal 34 in terms of voltage, and both half-waves are the same. The second one amplifies the current signal, but the cascade on the VT2 transistor “works” with positive half-waves, and on the VТЗ transistor - with negative ones.
Rice. 4. Push-pull AF power amplifier on transistors.
The DC mode is chosen so that the voltage at the junction point of the emitters of the transistors of the second stage is approximately half the voltage of the power source.
This is achieved by turning 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 parallel-connected headphones or a dynamic head) is connected to the amplifier through an oxide capacitor C2.
If the amplifier will work on a dynamic head (with a resistance of 8 -.10 ohms), the capacitance of this capacitor should be at least twice as large , but with a lower load output.
This is the so-called voltage boost circuit, in which a small positive feedback voltage is supplied to 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 “depletion” of the battery or to indicate the level of the reproduced signal in a household tape recorder. The layout of the indicator will allow you to demonstrate the principle of its operation.
Rice. 5. Scheme of a two-level voltage indicator.
In the lower position of the variable resistor R1 engine according to the diagram, both transistors are closed, the LEDs HL1, HL2 are off. When moving the slider of the resistor up, the voltage across it increases. When it reaches the opening voltage of the transistor VT1, 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 input of the indicator causes only the HL1 LED to glow, and more than both LEDs.
By smoothly reducing 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 top 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 engine, the threshold of the indicator is selected.
When monitoring only the voltage of the power source, it is permissible to install the AL307G green LED in place of HL2.
It emits light signals according to the principle less than normal- the norm is more than the norm. To do this, the indicator uses two red LEDs and one green LED.
Rice. 6. Three-level voltage indicator.
At a certain voltage on the engine of the variable resistor R1 (the voltage is normal), both transistors are closed and only the green LED HL3 (works). Moving the resistor slider up the circuit leads to an increase in voltage (more than normal), the transistor VT1 opens on it.
LED HL3 goes out, and HL1 lights up. If the engine is moved down and thus the voltage on it is reduced ('less than normal'), the transistor VT1 will close, and VT2 will open. The following picture will be observed: first, the HL1 LED will go out, then it will light up and soon HL3 will go out, and finally HL2 will flash.
Due to the low sensitivity of the indicator, a smooth transition is obtained from the extinction of one LED to the ignition of the other, for example, HL1 has not yet completely gone out, but HL3 is already on.
Schmitt trigger
As you know, this device is usually used to convert a slowly changing voltage into a rectangular signal. When the engine of the variable resistor R1 is in the lower position according to the circuit, the transistor VT1 is closed.
The voltage on its collector is high, as a result, the transistor VT2 is open, which means that the HL1 LED is lit. A voltage drop forms on the resistor R3.
Rice. 7. Simple Schmitt trigger on two transistors.
By slowly moving the variable resistor slider up the circuit, it will be possible to reach the moment when the transistor VT1 suddenly opens and VT2 closes. This will happen when the voltage on the base of VT1 exceeds the voltage drop across the resistor R3.
The LED will turn off. If after that you 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 switches to another only when an input signal is applied. In this case, the multivibrator generates an impulse of its duration, regardless of the duration of the input. We will verify this by conducting an experiment with the layout of the proposed device.
Rice. eight. circuit diagram waiting multivibrator.
In the initial state, the transistor VT2 is open, the LED HL1 is lit. Now it is enough to briefly close the sockets X1 and X2 so that the current pulse through the capacitor C1 opens the transistor VT1. The voltage on its collector will decrease and the capacitor C2 will be connected to the base of the transistor VT2 in such polarity that it will close. The LED will turn off.
The capacitor starts to discharge, the discharge current will flow through the resistor R5, keeping the transistor VT2 in the closed state. As soon as the capacitor is discharged, the transistor VT2 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 with C2, the LED will remain off for twice as long.
I. Bokomchev. R-06-2000.
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In this lesson Beginner radio schools we will continue to study semiconductors. In the last lesson, we looked at 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 n-p-n or p-n-p structures. The functioning of transistors, as well as diodes, is based on the properties of p-n 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, as long as the correct polarity of the power supply is observed. The figure below shows a schematic diagram of transistors, the p-n-p transistor is different from the transistor n-p-n direction emitter arrows:
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 the bipolar transistor is carried out by the input current, and in the unipolar transistor - by the input voltage.
Bipolar transistors, as mentioned above, are a layer cake of three layers. In a simplified form, a transistor can be represented as two back-to-back diodes:
(at the same time, it should be noted that the base-emitter junction is a conventional 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 a conventional ohmmeter, 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 for testing transistors is used very often. 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 connected in opposite directions. And this means that for any polarity of the applied voltage, one of the diodes is turned on in the forward direction, and the second in the opposite direction, so no current will flow.
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 (which was discussed a little higher). If, however, the connection is made in accordance with the diagram (as in the figure below), where 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 flow 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 the level of 4.5 V, changing the resistance value in the base circuit from R to R / 2, the base current will double, and the collector current will increase proportionally, for example:
Therefore, for any voltage across 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 work in linear mode. For normal operation in linear mode, the transistor must apply a DC bias voltage to the base and supply an AC voltage, which it will amplify. Thus, transistors amplify weak voltages coming from a microphone, for example, to a level that can drive a loudspeaker. If the gain is not sufficient, several transistors or their series stages can be used. In order not to violate the operating modes of each of them in direct current (at which linearity is ensured), when connecting cascades, isolation capacitors are used. Bipolar transistors have electrical characteristics that provide them with certain advantages over other amplifying components.
As we already know, there are also (except bipolar) and unipolar transistors. Let's briefly look at two of them - field and unijunction transistors. Like bipolar, they are of two types and have three outputs:
The electrodes of field-effect transistors are: gate– Z, stock- C, corresponding to the collector and source– And, identified with the emitter. FETs with n- and p-channel differ in the direction of the gate arrow. Unijunction transistors, sometimes referred to as double-base diodes, are mainly used in pulsed periodic signal generator circuits.
There are three fundamental circuits for switching on transistors in an amplifier stage:
? common emitter(a)
? with common manifold(b)
? with common base(in)
Common emitter bipolar transistor, depending on the output impedance of the power supply R1 and the load resistance Rl amplifies the input signal both in voltage and current. The gain of a bipolar transistor is denoted as h21e(it reads: 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 base current transfer coefficient h21e) depends only on the thickness of the base of the transistor (it cannot be changed) and on the voltage between the collector and 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 collector voltage.
current gain – Kus.i and voltage gain – Kus.u of a bipolar transistor connected according to a common emitter circuit depends on the ratio of the load resistance (indicated as Rn in the diagram) and the signal source (indicated as R1 in the diagram). If the resistance of the signal source in 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 signal source impedance 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, while limiting the current flowing through the base-emitter junction of the transistor, does not change. The common emitter circuit is the only bipolar transistor circuit that requires input (drive) current limiting. Several conclusions can be drawn:- the base current of the transistor must be limited, otherwise either the transistor or the circuit controlling it will burn out; - with the help of a transistor connected according to the OE circuit, it is very easy to control a high-voltage load with a low-voltage signal source. A significant current flows through the base, and hence the collector junctions, at a base-emitter voltage of only 0.8 ... 1.5 V. If the amplitude (voltage) is greater than this value, you need to put a current-limiting resistor (R1) between the base of the transistor and the output of the control circuit. You can calculate its resistance using the formulas:
Ir1=Irn/h21e R1=Ucontrol/Ir1 where:
Irn– current through the load, A; Upr– signal source voltage, V; R1 is the resistance of the resistor, Ohm.
Another feature of the OE circuit is that the voltage drop across 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 pulsed, digital circuits.
Transistor, operating in an analog signal amplifier circuit, should provide approximately the same amplification of signals with different amplitudes relative to some “average” voltage. To do this, you need to “open” it a little, trying not to “overdo it”. As you can see from 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 (before saturation) - it is obvious that they symbolize the linear amplification of the signal, that is, if the control current changes several times, the collector current (load voltage) 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 either increase or decrease. But the bipolar transistor reacts only to an increase in input voltage (or rather current). Conclusion: you need to make sure that the transistor, even with the minimum amplitude of the input signal, is slightly ajar. 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 saturation mode (see figure above) - in this mode, the output current ceases to depend linearly on the input, as a result of which a strong signal distortion occurs.
Let's turn again to the form of an analog signal. 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 apply such a direct current (bias current - Icm) to the base of the transistor so that the transistor is open at the “average” input voltage 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.
A transistor is a semiconductor device that can amplify, convert, and generate electrical signals. The first workable bipolar transistor was invented in 1947. Germanium served as a material for its manufacture. And already in 1956, the 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, there are unipolar (field) transistors, which use only one type of carrier - electrons or holes. This article will cover.
Most silicon transistors have an npn structure, which is also due to the production technology, although silicon transistors also exist. pnp type, but there are somewhat fewer of them than n-p-n structures. Such transistors are used as part of 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 2SA1943 and 2SC5200 is very often used.
Often, p-n-p structure transistors are called direct conduction transistors, and structures n-p-n reverse. For some reason, this 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 symbols.
Picture 1.
In addition to differences in type of conductivity and material, bipolar transistors are classified by power and operating frequency. If the power dissipation on the transistor does not exceed 0.3 W, such a transistor is considered low-power. At a power of 0.3 ... 3 W, the transistor is called a medium power transistor, and at a power of more than 3 W, the power is considered high. Modern transistors are able to dissipate power of several tens and even hundreds of watts.
Transistors amplify electrical signals not equally well: with increasing frequency, the amplification of the transistor stage drops, and at a certain frequency it stops altogether. Therefore, to operate in a wide frequency range, transistors are produced with different frequency properties.
According to the operating frequency, transistors are divided into low-frequency ones - the operating frequency is not more than 3 MHz, mid-frequency - 3 ... 30 MHz, high-frequency - over 30 MHz. If the operating frequency exceeds 300 MHz, then these are already microwave transistors.
In general, over 100 different transistor parameters are given in serious thick reference books, which also indicates a huge number of models. And the number of modern transistors is such that it is no longer possible to put them in full in any reference book. And the lineup is constantly increasing, allowing to solve almost all the tasks set by the developers.
There are many transistor circuits (just remember the number of at least household equipment) for amplifying and converting electrical signals, but, for all their diversity, these circuits consist of separate cascades, 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 amplifying stages work, you need to become more familiar with transistor switching circuits.
By itself, the transistor will not be able to amplify anything. Its amplifying properties lie in the fact that small changes in the input signal (current or voltage) lead to significant changes in the voltage or current at the output of the stage due to the expenditure of energy from an external source. It is this property that is widely used in analog circuits - amplifiers, television, radio, communications, etc.
To simplify the presentation, circuits based on transistors of the n-p-n structure will be considered here. Everything that will be said about these transistors applies equally to p-n-p transistors. It is enough just to reverse the polarity of the power supplies, and, if any, to get a working circuit.
In total, there are three such circuits: a circuit with a common emitter (CE), a circuit with a common collector (OC) and a circuit with a common base (OB). All these schemes are shown in Figure 2.
Figure 2.
But before proceeding to the consideration of these circuits, you should get acquainted with how the transistor works in the key mode. This introduction should make it easier to understand in boost mode. In a certain sense, the key circuit can be considered as a kind of circuit with OE.
Transistor operation in key mode
Before studying the operation of a transistor in signal amplification mode, it is worth remembering that transistors are often used in a key mode.
This mode of operation of the transistor has been considered for a long time. In the August issue of the magazine "Radio" in 1959, an article by G. Lavrov "Semiconductor triode in key mode" was published. The author of the article suggested changing the duration of the pulses in the control winding (OC). Now this method of regulation is called PWM and is used quite often. The diagram from the 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 simply turns something on and off.
In this case, a relay can be used as a load: an input signal is applied - the relay is turned on, no - the relay signal is turned off. Light bulbs are often used instead of relays in key mode. Usually this is done to indicate: the light bulb is either on or off. A 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 conventional contact, although it may be a digital microcircuit or instead. A car bulb, this is used to illuminate the dashboard in the Zhiguli. Attention should be paid to the fact that 5V is used for control, and the switched 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 operate at such voltages. The collector source voltage must also match the operating voltage of the load. With the help of 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 more (depending on the collector load) than the base current. It is easy to see that there is an increase in current. When the transistor is operating in the key mode, it is usually used to calculate the cascade by the value called in the reference books "current gain in the large signal mode" - in the 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 coefficient β is large enough, as a rule, from 50 and higher, therefore, when calculating the key stage, it can be taken equal to only 10. and key mode.
To light the bulb shown in Figure 3, Ib \u003d Ik / β \u003d 100mA / 10 \u003d 10mA, this is at least. With a control voltage of 5V on the base resistor Rb, minus the voltage drop in the B-E section, 5V - 0.6V = 4.4V will remain. The resistance of the base resistor will be: 4.4V / 10mA = 440 ohms. A resistor with a resistance of 430 ohms 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 when calculating!
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 with a resistor Rbe, which reliably closes the transistor. This resistor should not be forgotten, although for some reason it is not in some circuits, which can lead to false operation of the noise stage. Actually, everyone knew about this resistor, but for some reason they forgot, and once again stepped on the "rake".
The value of this resistor must be such that when the contact opens, the voltage at the base would not be less than 0.6V, otherwise the cascade will be uncontrollable, as if the B-E section 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 value of Rb is 10Kom, the circuit will work quite reliably: the potentials of the base and emitter will be equal, which will lead to the closing of the transistor.
Such a key cascade, if it is in good condition, 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 fully closed (cutoff state). Immediately, by itself, the conclusion suggests itself that between these "boundary" states there is such a thing when the light bulb shines half-heartedly. Is the transistor half open or half closed in this case? It's like filling a glass: an optimist sees the glass as half full, 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 just to choose the operating mode of the operational amplifier in order to obtain the required gain or bandwidth. But, despite this, cascades on discrete ("loose") transistors are often used, and therefore an understanding of the operation of the amplifying cascade is simply necessary.
The most common transistor connection compared to OK and OB is the common emitter (CE) circuit. The reason for this prevalence, first of all, is the high voltage and current gain. The highest gain of the OE stage is provided when half of the voltage of the power supply Epit/2 drops across the collector load. Accordingly, the second half falls on section K-E transistor. This is achieved by setting the cascade, which will be discussed below. This mode of amplification is called class A.
When the transistor with OE is turned on, the output signal at the collector is in antiphase with the input signal. As disadvantages, it can be noted that the input resistance of the OE is small (no more than a few hundred ohms), and the output resistance is within tens of kΩ.
If in the key mode the transistor is characterized by a 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 quadripole. 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 larger than β, although it can also be used in calculations in the first approximation. All the same, the spread of 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, it is required to adjust the scheme.
The gain of the transistor depends on the thickness of the base, so it cannot be changed. Hence the large variation in the gain of transistors taken even from one box (read one batch). For low-power transistors, this coefficient ranges from 100 ... 1000, and for powerful ones it is 5 ... 200. The thinner the base, the higher the coefficient.
The simplest circuit for switching on an OE transistor is shown in Figure 5. This is just a small piece from Figure 2, shown in the second part of the article. Such a circuit is called a fixed base current circuit.
Figure 5
The scheme is extremely simple. The input signal is applied to the base of the transistor through the decoupling capacitor C1, and, being amplified, is taken from the collector of the transistor through the capacitor C2. The purpose of capacitors is to protect the input circuits from the constant component of the input signal (just remember a carbon or electret microphone) and provide the necessary bandwidth of the cascade.
Resistor R2 is the collector load of the stage, and R1 supplies a DC bias to the base. With the help of this resistor, they try to make the voltage across the collector be Epit / 2. This state is called the operating point of the transistor, in this case the gain of the cascade is maximum.
Approximately the resistance of the resistor R1 can be determined by a 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 not more than 1.5, and starting from 50V, it approaches 1.8…2.0. But, indeed, 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 cascade as a whole depend on its value: the greater the resistance of the resistor R2, the higher the gain. But with this resistor you need to be careful, the collector current must be less than the maximum allowable for this type of transistor.
The circuit is very simple, but this simplicity gives it negative properties, and there is a price to be paid for this simplicity. Firstly, the amplification of the cascade depends on the specific instance of the transistor: I replaced the transistor during repair, - select the offset again, bring it to the operating point.
Secondly, from the ambient temperature, with increasing temperature, the reverse collector current Ico increases, which leads to an increase in the collector current. And where, then, is half the supply voltage on the Epit / 2 collector, that same operating point? As a result, the transistor heats up even more, after which it fails. To get rid of this dependence, or at least reduce it to a minimum, additional negative feedback elements are introduced into the transistor cascade - OOS.
Figure 6 shows a circuit with a fixed bias voltage.
Figure 6
It would seem that the voltage divider Rb-k, Rb-e will provide the required initial bias of the cascade, but in fact, such a cascade has all the disadvantages of a fixed current circuit. Thus, the circuit shown is just a variation of the fixed current circuit shown in Figure 5.
Circuits with thermal stabilization
The situation is somewhat better in the case of applying 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, the collector voltage decreases. This decrease leads to a decrease in the bias voltage applied to the base through R1. The transistor starts to close, the collector current decreases to an acceptable value, the position of the operating point is restored.
It is quite obvious that such a measure of stabilization leads to some reduction in the gain of the cascade, but this does not matter. The missing amplification, as a rule, is added by increasing the number of amplifying stages. But such an environmental protection allows you to significantly expand the operating temperature range of the cascade.
The circuitry of the cascade with emitter stabilization is somewhat more complicated. The amplifying properties of such cascades remain unchanged over an even wider temperature range than that of a collector-stabilized circuit. And one more indisputable advantage - when replacing the 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 cascade. This is for direct current. In order to eliminate the influence of the resistor R4 on the amplification of the alternating current, the resistor R4 is shunted by the capacitor Ce, which presents little resistance to the alternating current. Its value is determined by the frequency range of the amplifier. If these frequencies lie in the audio range, then the capacitance of the capacitor can be from units to tens and even hundreds of microfarads. For radio frequencies, this is already hundredths or thousandths, but in some cases the circuit works fine even without this capacitor.
In order to better understand how emitter stabilization works, it is necessary to consider the circuit for switching on a transistor with a common collector OK.
The Common Collector Circuit (CC) is shown in Figure 8. This circuit is a piece of Figure 2, from the second part of the article, which shows all three transistor switching circuits.
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 taken through the capacitor C2. Here you can ask why this scheme is called OK? After all, if we recall the OE circuit, then it is clearly seen that the emitter is connected to the common wire of the circuit, relative to which the input signal is applied 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 signal. But in fact, the EMF source (power battery) has a very small internal resistance; for a signal, this is practically one point, the same contact.
In more detail, the operation of the OK circuit can be seen in Figure 9.
Figure 9
It is known that for silicon transistors the voltage b-e transition is in the range of 0.5 ... 0.7V, so you can take it on average 0.6V, if you do not set out 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 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 h21e times, which means that the input impedance of the circuit is h21e times greater than the resistance in the emitter circuit. In other words, without fear of burning the transistor, apply voltage directly to the base (without a limiting resistor). Simply 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 piezoelectric 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 “land” 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 the voltage of the input signal source. In this case, the parameters of the transistor do not play any role here at all. Such circuits are said to be covered by 100% voltage feedback.
As shown in Figure 9, the current in the emitter load (aka the emitter current) In = Ik + Ib. Taking into account that the base current Ib is negligible compared to the collector current Ik, it can be assumed that the load current is equal to the collector current In = Ik. The current in the load 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, it is not necessary to be particularly zealous. After all, if instead of Rn you put a nail - a hundredth, then no transistor will survive!
The OK circuit makes it quite 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. Measurements should in both cases be made in the same quantities: either in amperes or in milliamps. The power supply voltage and load must remain the same for 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 somewhat decreases, and with an increase in the supply voltage, it increases. Emitter followers are often built in a push-pull circuit using complementary pairs of transistors, which allows you 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 ABOUT
Such a circuit provides 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. The antenna amplifier circuit is shown in Figure 12.
