Just about the transistor and transistor circuits. What is a transistor - types of semiconductor devices and methods of testing. Real circuit operation
The original name of the radio component is a triode, according to the number of contacts. This radioelement is able to control the current in the electrical circuit, under the influence external signal... Unique properties are used in amplifiers, generators and other similar circuit solutions.
Designation of transistors in the diagram
For a long time, tube triodes reigned in radio electronics. Three main components of the triode were placed inside a sealed flask, in a special gas or vacuum environment:
- Cathode
- Net
When a low power control signal was applied to the grid, incomparably large values could be passed between the cathode and anode. The magnitude of the operating current of the triode is many times higher than that of the control one. It is this property that allows the radio element to act as an amplifier.
Triodes based on radio tubes work quite efficiently, especially when high power... However, their dimensions do not allow them to be used in modern compact devices.
Imagine mobile phone or a pocket player made on such elements.
The second problem is catering. For normal operation, the cathode must be very hot for electron emission to begin. Heating the coil requires a lot of electricity. Therefore, scientists around the world have always strived to create a more compact device with the same properties.
The first samples appeared in 1928, and in the middle of the last century, a working semiconductor triode was presented, made using bipolar technology. The name "transistor" was assigned to it.
What is a transistor?
A transistor is a semiconductor electrical device with or without a housing, which has three contacts for operation and control. The main property is the same as that of a triode - changing the parameters of the current between the working electrodes using a control signal.
Due to the absence of the need to warm up, transistors spend a meager amount of energy to ensure their own performance. And the compact dimensions of the working semiconductor crystal make it possible to use the radio component in small-sized structures.
Due to independence from the working environment, semiconductor crystals can be used both in a separate package and in microcircuits. Complete with the rest of the radioelements, transistors are grown directly on a single crystal.
The outstanding mechanical properties of the semiconductor have found application in mobile and portable devices. Transistors are insensitive to vibration, sharp shocks. They have good temperature resistance (cooling radiators are used under heavy load).
What does the name "transistor" mean?
The transistor did not immediately receive such a familiar name. Initially, by analogy with lamp technology, it was called semiconductor triode... The modern name consists of two words. The first word - "transfer" (here "transformer" is immediately remembered) means a transmitter, a transformer, a carrier. And the second half of the word resembles the word "resistor" - a detail of electrical circuits, the main property of which is electrical resistance.
It is this resistance that is found in Ohm's law and many other formulas in electrical engineering. Therefore, the word "transistor" can be interpreted as a resistance converter. In much the same way as in hydraulics, the change in fluid flow is controlled by a valve. In a transistor, such a "latch" changes the amount of electric charges that create an electric current. This change is nothing but change internal resistance semiconductor device.
Amplification of electrical signals
The most common operation performed transistors, is an amplification of electrical signals... But this is not quite the correct expression, because a weak signal from the microphone remains so.
Amplification is also required in radio and television: a weak signal from a billionths of a watt antenna must be amplified to such an extent that it can produce sound or images on a screen. And this is already a capacity of several tens, and in some cases even hundreds of watts. Therefore, the amplification process is reduced to obtaining a powerful copy of a weak input signal with the help of additional energy sources received from the power supply unit. In other words, a low-power input stimulus controls powerful energy flows.
Amplification in other fields of technology and nature
Such examples can be found not only in electrical diagrams... For example, pressing the accelerator pedal increases the speed of the vehicle. At the same time, you do not have to press the gas pedal very hard - compared to the engine power, the power of pressing the pedal is negligible. To reduce the speed, the pedal will have to be released a little, to weaken the input effect. In this situation, gasoline is a powerful source of energy.
The same effect can be observed in hydraulics: very little energy is used to open an electromagnetic valve, for example, in a machine tool. And the oil pressure on the piston of the mechanism is capable of creating a force of several tons. This force can be adjusted if an adjustable gate valve is provided in the oil line, as in a conventional kitchen faucet. I covered it a little - the pressure dropped, the effort decreased. If I opened more, then the pressure increased.
Turning the valve also does not require special efforts. V this case an external source of energy is the pumping station of the machine. And you can see a great variety of similar influences in nature and technology. But still, we are more interested in the transistor, so further we will have to consider ...
Amplifiers of electrical signals
In all experiments, transistors KT315B, diodes D9B, miniature incandescent lamps 2.5V x 0.068A are used. Headphones - high impedance, type TON-2. Variable capacitor - any, with a capacity of 15 ... 180 pF. The power supply battery consists of two series-connected 4.5V batteries of standard size 3R12. The lamps can be replaced with a series connected LED of the AL307A type and a 1 kOhm resistor.
EXPERIMENT 1
ELECTRICAL DIAGRAM (conductors, semiconductors and insulators)
Electric current is the directed movement of electrons from one pole to the other under the influence of voltage (9 V battery).
All electrons have the same negative charge. Atoms of different substances have different numbers of electrons. Most of the electrons are firmly bound to atoms, but there are also so-called "free" or valence electrons. If a voltage is applied to the ends of the conductor, then free electrons will begin to move towards the positive pole of the battery.
In some materials, the movement of electrons is relatively free, they are called conductors; in others, movement is difficult, they are called semiconductors; thirdly, it is generally impossible, such materials are called insulators, or dielectrics.
Metals are good current conductors. Substances such as mica, porcelain, glass, silk, paper, cotton are classified as insulators.
Semiconductors include germanium, silicon, etc. These substances become conductors under certain conditions. This property is used in the production of semiconductor devices - diodes, transistors.
Rice. 1. Determination of water conductivity
This experiment demonstrates the operation of a simple electrical circuit and the difference in conductivity of conductors, semiconductors and dielectrics.
Assemble the circuit as shown in fig. 1, and lead the bare ends of the wires to the front of the board. Connect the bare ends together, the light will be on. This indicates that an electric current is passing through the circuit.
Using two wires, you can check the conductivity of different materials. To accurately determine the conductivity of certain materials, special instruments are needed. (By the brightness of the light bulb, one can only determine whether the material under study is a good or bad conductor.)
Attach the bare ends of the two conductors to a piece of dry wood a short distance apart. The light will not be on. This means that dry wood is a dielectric. If the bare ends of two conductors are connected to aluminum, copper or steel, the light will be on. This suggests that metals are good conductors. electric current.
Dip the bare ends of the conductors into a glass of tap water (Fig. 1, a). The light is off. This means that water is a poor current conductor. If you add a little salt to the water and repeat the experiment (Fig. 1, b), the light will be on, which indicates the flow of current in the circuit.
The 56 Ohm resistor in this circuit and in all subsequent experiments serves to limit the current in the circuit.
EXPERIMENT 2
DIODE ACTION
The purpose of this experiment is to visually demonstrate that the diode conducts current well in one direction and does not conduct in the opposite direction.
Assemble the circuit as shown in fig. 2, a. The lamp will be on. Turn the diode 180 ° (Fig. 2, b). The light will not be on.
Now let's try to understand the physical essence of the experiment.
Rice. 2. The action of a semiconductor diode in an electronic circuit.
The semiconducting substances germanium and silicon each have four free, or valence, electrons. Semiconductor atoms are bound into dense crystals (crystal lattice) (Fig. 3, a).
Rice. 3. Semiconductor crystal lattice.
If an impurity is introduced into a semiconductor with four valence electrons, for example, arsenic, which has five valence electrons (Fig. 3b), then the fifth electron in the crystal will be free. Such impurities provide electronic conductivity, or n-type conductivity.
Impurities with a lower valence than semiconductor atoms have the ability to attach electrons to themselves; such impurities provide hole conductivity, or p-type conductivity (Fig. 3, c).
Rice. 4.p-n-junctions in semiconductor diode.
A semiconductor diode consists of a junction of p- and n-type materials (p-n-junction) (Fig. 4, a). Depending on the polarity of the applied voltage, the p-n-junction can either facilitate (Fig. 4, d) or prevent (Fig. 4, c) the passage of electric current. On the border of two semiconductors, even before the external voltage is applied, a binary electric layer with a local electric field of intensity E 0 is created (Fig. 4, b).
If you pass through the diode alternating current, then the diode will transmit only the positive half-wave (Fig. 4d), and the negative one will not pass (see Fig. 4, c). The diode thus converts, or “rectifies,” the alternating current to direct current.
EXPERIMENT 3
HOW THE TRANSISTOR WORKS
This experiment clearly demonstrates the main function of a transistor, which is a current amplifier. A small drive current in the base circuit can cause a large current in the emitter-collector circuit. By changing the resistance of the base resistor, you can change the collector current.
Assemble the circuit (fig. 5). Put resistors in the circuit one by one: 1 MΩ, 470 kΩ, 100 kΩ, 22 kΩ, 10 kΩ. You can see that with 1 MΩ and 470 kΩ resistors the light is off; 100 kOhm - the light is barely lit; 22 kOhm - the lamp is brighter; full brightness is observed when a 10 kΩ base resistor is connected.
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Rice. 6. Transistor with an n-p-n structure. |
Rice. 7. Transistor with p-n-p structure. |
A transistor is essentially two semiconductor diodes with one common area - the base. If at the same time the area with p-conductivity turns out to be common, then you get a transistor with an n-p-n structure (Fig. 6); if general area will be with n-conductivity, then the transistor will be with a p-n-p structure (Fig. 7).
The region of the transistor that emits (emigrates) current carriers is called the emitter; the area that collects current carriers is called a collector. The zone enclosed between these areas is called the base. The transition between emitter and base is called emitter, and between base and collector is called collector.
In fig. 5 shows the inclusion of an n-p-n type transistor in electrical circuit.
When included in the circuit of the transistor type p-n-p the polarity of battery B is reversed.
For the currents flowing through the transistor, there is a dependence
I e = I b + I to
Transistors are characterized by a current gain, denoted by the letter β, is the ratio of the increase in collector current to the change in base current.
The β value ranges from several tens to several hundred units, depending on the type of transistor.
EXPERIMENT 4
CONDENSER PROPERTIES
Having studied the principle of operation of a transistor, you can demonstrate the properties of a capacitor. Assemble the circuit (Fig. 8), but do not connect a 100 μF electrolytic capacitor. Then connect it for a while to position A (Fig. 8, a). The light will come on and off. This indicates that the capacitor charge current was flowing in the circuit. Now place the capacitor in position B (Fig. 8, b), do not touch the terminals with your hands, otherwise the capacitor may be discharged. The light will turn on and off, the capacitor has been discharged. Now put the capacitor back in position A. It has been charged. Place the capacitor on the insulating material for a while (10 s), then place it in position B. The light will turn on and off. It can be seen from this experiment that a capacitor is capable of accumulating and storing an electric charge long time... The accumulated charge depends on the capacitance of the capacitor.
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Rice. 8. Diagram explaining the principle of the capacitor. |
Rice. 9. The change in voltage and current on the capacitor over time. |
Charge the capacitor by setting it to position A, then discharge it by connecting conductors with bare ends to the terminals of the capacitor (hold the conductor by the insulated part!), And place it in position B. The light will not light up. As you can see from this experiment, the charged capacitor acts as a power source (battery) in the base circuit, but after use electric charge the light goes out. In fig. 9 shows the dependences on time: voltage of the capacitor charge; charge current flowing in the circuit.
EXPERIMENT 5
TRANSISTOR AS A SWITCH
Assemble the circuit according to fig. 10, but do not install resistor R1 and transistor T1 in the circuit yet. Key B must be connected to the circuit at points A and E so that the connection point of the resistors R3, R1 can be closed to a common wire (negative bus of the printed circuit board).
Rice. 10. The transistor in the circuit works like a switch.
Connect the battery, the lamp in the collector circuit T2 will be on. Now close the circuit with switch B. The light will go out, since the switch connects point A with the negative bus, thereby reducing the potential of point A, and therefore the potential of the base T2. If the switch is returned to its original position, the lamp will light up. Now disconnect the battery and connect T1, do not connect the resistor R1. Connect the battery, the light will turn on again. As in the first case, the transistor T1 is open and an electric current passes through it. Now put resistor R1 (470 kOhm) at points C and D. The light goes out. Remove the resistor and the light will turn on again.
When the voltage on the T1 collector drops to zero (when installing a 470 kΩ resistor), the transistor turns on. The base of transistor T2 is connected through T1 to the negative bus, and T2 is closed. The light goes out. Thus, the transistor T1 acts as a switch.
In previous experiments, the transistor was used as an amplifier, now it is used as a switch.
The possibilities of using a transistor as a key (switch) are shown in experiments 6, 7.
EXPERIMENT 6
ALARM
A feature of this circuit is that the transistor T1, used as a switch, is controlled by the photoresistor R2.
The photoresistor available in this kit changes its resistance from 2 kOhm in strong light to several hundred kOhm in the dark.
Assemble the circuit according to fig. 11. Depending on the lighting of the room where you are conducting the experiment, select the resistor R1 so that the light bulb burns normally without darkening the photoresistor.
Rice. 11. The circuit of the photoresistor-based alarm signaling.
The state of the transistor T1 is determined by a voltage divider consisting of a resistor R1 and a photoresistor R2.
If the photoresistor is illuminated, its resistance is low, the transistor T1 is closed, there is no current in its collector circuit. The state of the transistor T2 is determined by applying a positive potential by the resistors R3 and R4 to the base of T2. Therefore, the transistor T2 opens, the collector current flows, the light is on.
When the photoresistor is darkened, its resistance increases greatly and reaches a value when the divider applies a voltage to the base of T1, sufficient to open it. The voltage on the collector T1 drops to almost zero, through the resistor R4 it locks the transistor T2, the light goes out.
In practice, in such circuits, other actuators (bell, relay, etc.) can be installed in the collector circuit of transistor T2.
In this and in the following circuits, a photoresistor of the SF2-9 type or similar can be used.
EXPERIMENT 7
AUTOMATIC LIGHT SWITCHING DEVICE
Unlike experiment 6, in this experiment when the photoresistor R1 is dimmed, the light is on (Fig. 12).
Rice. 12. A circuit that turns on the light automatically.
When light hits the photoresistor, its resistance is greatly reduced, which leads to the opening of the transistor T1, and, consequently, to the closure of T2. The light is off.
In the dark, the light turns on automatically.
This property can be used to turn lamps on and off depending on the lighting conditions.
EXPERIMENT 8
SIGNAL DEVICE
A distinctive feature of this circuit is its high sensitivity. In this and a number of subsequent experiments, a combined connection of transistors (composite transistor) is used (Fig. 13).
Rice. 13. Optoelectronic signaling device.
The principle of operation of this circuit does not differ from the circuit. At a certain meaning the resistance of the resistors R1 + R2 and the resistance of the photoresistor R3 in the base circuit of the transistor T1 current flows. In the collector circuit T1, a current also flows, but (3 times higher than the base current T1. Suppose that (β = 100. All the current passing through the emitter T1 must pass through the junction emitter - base T2. Then the collector current T2 is β times more than T1 collector current, T1 collector current is β times higher than T1 base current, T2 collector current is approximately 10,000 times higher than T1 base current. Thus, a composite transistor can be considered as a single transistor with a very high gain and high sensitivity. a composite transistor is that the transistor T2 must be powerful enough, while the transistor T1 that controls it may be low-power, since the current passing through it is 100 times less than the current passing through T2.
The performance of the circuit shown in Fig. 13, is determined by the illumination of the room where the experiment is being carried out, therefore it is important to select the resistance R1 of the divider of the upper arm so that the lamp does not burn in the illuminated room, but burns when the photoresistor is darkened by hand, the room is darkened with curtains or when the light is turned off, if the experiment is carried out in the evening.
EXPERIMENT 9
HUMIDITY SENSOR
This circuit (Fig. 14) also uses a composite transistor with high sensitivity to determine the moisture content of the material. The base bias of T1 is provided by resistor R1 and two bare-ended conductors.
Test the electrical circuit by lightly squeezing the bare ends of the two conductors with the fingers of both hands, without connecting them to each other. The resistance of the fingers is sufficient to trigger the circuit, and the light comes on.
Rice. 14. Moisture sensor circuit. The bare ends of the conductors pierce the blotting paper.
Now pass the bare ends through blotting paper at a distance of about 1.5-2 cm, attach the other ends to the diagram according to Fig. 14. Then dampen the absorbent paper between the wires with water. The light comes on (In this case, the decrease in resistance was due to the dissolution of the salts in the paper with water.).
If the blotting paper is soaked with brine and then dried and repeated, the experiment is more efficient and the ends of the conductors can be separated a greater distance.
EXPERIMENT 10
SIGNAL DEVICE
This circuit is similar to the previous one, the only difference is that the lamp burns when the photoresistor is illuminated and goes out when it is dimmed (Fig. 15).
Rice. 15. Signal device on the photoresistor.
The circuit works as follows: under normal illumination of the photoresistor R1, the light will be on, since the resistance of R1 is small, the transistor T1 is open. When the light is turned off, the lamp will go out. The light from a pocket torch or lighted matches will make the light bulb turn on again. The sensitivity of the circuit is regulated by increasing or decreasing the resistance of the resistor R2.
EXPERIMENT 11
PRODUCT COUNTER
This experiment should be carried out in a semi-darkened room. All the time when the light falls on the photoresistor, the L2 indicator lamp is on. If you place a piece of cardboard between the light source (lamp L1 and the photoresistor, lamp L2 goes out. If you remove the cardboard, lamp L2 lights up again (Fig. 16).
Rice. 16. Product counter.
For the experiment to be successful, it is necessary to adjust the circuit, that is, select the resistance of the resistor R3 (the most suitable in this case is 470 Ohm).
This scheme can practically be used for counting a batch of products on a conveyor. If the light source and the photoresistor are placed in such a way that a batch of products passes between them, the circuit turns on and off, since the flow of light is interrupted by passing products. Instead of the indicator lamp L2, a special counter is used.
EXPERIMENT 12
SIGNAL TRANSMISSION BY LIGHT
Rice. 23. Frequency divider on transistors.
Transistors T1 and T2 open alternately. The control signal is sent to the trigger. When the transistor T2 is open, the lamp L1 does not light up. Lamp L2 lights up when the transistor T3 is open. But transistors T3 and T4 open and close alternately, therefore, the L2 lamp lights up at every second control signal sent by the multivibrator. Thus, the frequency of the lamp L2 burning is 2 times less than the burning frequency of the lamp L1.
This property can be used in an electric organ: the frequencies of all notes of the upper octave of the organ are halved and a tone one octave lower is created. The process can be repeated.
EXPERIMENT 18
SCHEME "I" BY UNITS
In this experiment, a transistor is used as a switch and a light bulb is an output indicator (Fig. 24).
This scheme is logical. The light will be on if there is a high potential at the base of the transistor (point C).
Suppose points A and B are not connected to the negative bus, they have a high potential, therefore, at point C there is also a high potential, the transistor is open, the light is on.
Rice. 24. Logic element 2I on the transistor.
Let's take conditionally: high potential - logical "1" - the light is on; low potential - logical "0" - the lamp is off.
Thus, if there are logical "1" at points A and B, at point C there will also be "1".
Now let's connect point A to the negative bus. Its potential will become low (drop to "0" V). Point B has a high potential. On the R3 - D1 circuit, the battery will flow current. Therefore, at point C there will be a low potential or "0". The transistor is closed, the light is off.
Let's connect point B to the ground. The current now flows through the R3 - D2 - battery circuit. The potential at point C is low, the transistor is closed, the light is off.
If both points are connected to ground, there will also be a low potential at point C.
Similar schemes can be used in the electronic examiner and others logic circuits ah, where the output signal will be only in the presence of simultaneous signals in two or more input channels.
Possible states of the circuit are shown in the table.
Truth table of circuit AND
EXPERIMENT 19
ORDER SCHEME BY UNITS
This scheme is the opposite of the previous one. For point C to be "0", it is necessary that points A and B also have "0", that is, points A and B must be connected to the negative bus. In this case, the transistor will close, the light will go out (Fig. 25).
If now only one of the points, A or B, is connected to the negative bus, then at point C it will still be high level, ie "1", the transistor is open, the light is on.
Rice. 25. Logic element 2 OR on the transistor.
When point B is connected to the negative bus, current will flow through R2, D1 and R3. The current will not flow through the diode D2, since it is turned on in the opposite direction for conduction. At point C there will be about 9 V. The transistor is open, the light is on.
Now we connect point A to the negative bus. The current will go through R1, D2, R3. The voltage at point C will be about 9 V, the transistor is open, the light is on.
Truth table of OR circuit
EXPERIMENT 20
SCHEME "NOT" (INVERTER)
This experiment demonstrates the operation of a transistor as an inverter - a device that can reverse the polarity of the output signal relative to the input signal. In the experiments, the transistor was not part of the operating logic circuits, it only served to turn on the light bulb. If point A is connected to the negative bus, then its potential will drop to, "0", the transistor will close, the light will go out, at point B there is a high potential. This means a logical "1" (Fig. 26).
Rice. 26. The transistor works like an inverter.
If point A is not connected to the negative bus, that is, at point A - "1", then the transistor is open, the light is on, the voltage at point B is close to "0" or it is a logical "0".
In this experiment, the transistor is part of logic circuit and can be used to convert an OR circuit to OR-NOT and an AND circuit to AND-NOT.
Truth table of NOT circuit
EXPERIMENT 21
SCHEME "AND-NOT"
This experiment combines two experiments: 18 - circuit I and 20 - circuit NOT (Fig. 27).
This circuit functions similarly to the circuit, forming a "1" or "0" based on the transistor.
Rice. 27. Logic element 2I-NOT on the transistor.
The transistor is used as an inverter. If a "1" appears at the base of the transistor, then the point at the output is "0" and vice versa.
If the potentials at point D are compared with the potentials at point C, it can be seen that they are inverted.
Truth table of the NAND circuit
EXPERIMENT 22
SCHEME "OR NOT"
This experiment combines two experiments: - OR circuit and - NOT circuit (Fig. 28).
Rice. 28. Logic element 2 OR NOT on the transistor.
The circuit functions in exactly the same way as in Experiment 20 (a "0" or "1" is generated on the basis of the transistor). The only difference is that the transistor is used as an inverter: if "1" is at the input of the transistor, then "0" is at its output and vice versa.
Truth table of the OR-NOT circuit
EXPERIMENT 23
SCHEME "AND-NOT" ASSEMBLED ON TRANSISTORS
This circuit consists of two logic circuits NOT, the collectors of the transistors of which are connected at point C (Fig. 29).
If both points, A and B, are connected to the negative bus, then their potentials will become equal to "0". The transistors will close, there will be a high potential at point C, the light will not burn.
Rice. 29. Logic element 2I-NOT.
If only point A is connected to the negative bus, at point B there is a logical "1", T1 is closed, and T2 is open, the collector current flows, the light is on, at point C there is a logical "0".
If point B is connected to the negative bus, then the output will also be "0", the light will be on, in this case T1 is open, T2 is closed.
And finally, if points A and B are logic "1" (not connected to the negative rail), both transistors are on. On their collectors "0", the current flows through both transistors, the light is on.
Truth table of the NAND circuit
EXPERIMENT 24
PHONE SENSOR AND AMPLIFIER
In the experimental circuit, both transistors are used as an amplifier. sound signals(fig. 30).
Rice. 30. Inductive phone sensor.
The signals are picked up and fed to the base of transistor T1 using inductive coil L, then they are amplified and fed to the telephone. When you have finished assembling the circuit on the board, position the ferrite rod near the phone, perpendicular to the incoming wires. Speech will be heard.
In this scheme and in the future, a ferrite rod with a diameter of 8 mm and a length of 100-160 mm, brand 600NN, is used as an inductive coil L. The winding contains about 110 turns of insulated copper wire with a diameter of 0.15..0.3 mm, type PEL or PEV.
EXPERIMENT 25
MICROPHONE AMPLIFIER
If available extra phone(fig. 31), it can be used instead of the inductor in the previous experiment. As a result, we will have a sensitive microphone amplifier.
Rice. 31. Microphone amplifier.
Within assembled circuit you can get a semblance of a two-way communication device. Telephone 1 can be used as a receiving device (connection at point A), and telephone 2 as an output device (connection at point B). In this case, the second ends of both phones must be connected to the negative bus.
EXPERIMENT 26
AMPLIFIER FOR PLAYER
With the help of a gramophone amplifier (Fig. 32), you can listen to the recordings without disturbing the peace of others.
The circuit consists of two audio amplification stages. The input signal is the signal coming from the pickup.
Rice. 32. Amplifier for the player.
In the diagram, the letter A denotes the sensor. This transducer and capacitor C2 act as a capacitive voltage divider to reduce the original loudness. Trimmer capacitor C3 and capacitor C4 are the secondary voltage divider. With the help of C3 the volume is adjusted.
EXPERIMENT 27
"ELECTRONIC VIOLIN"
Here the multivibrator circuit is for making electronic music. The scheme is similar. The main difference is that the base bias resistor of transistor T1 is variable. A 22 kΩ resistor (R2) in series with the variable resistor provides a minimum base bias resistance, T1 (Figure 33).
Rice. 33. Multivibrator for creating music.
EXPERIMENT 28
FLASHING MORSE BUZZER
In this circuit, the multivibrator is designed to generate pulses with a tonal frequency. The light comes on when the circuit is powered on (fig. 34).
The phone in this circuit is connected to the circuit between the collector of the transistor T2 through the capacitor C4 and the negative bus of the board.
Rice. 34. Generator for learning Morse code.
With this diagram you can practice learning Morse code.
If you are not satisfied with the tone of the sound, swap capacitors C2 and C1.
EXPERIMENT 29
METRONOME
A metronome is a device for setting a rhythm (tempo), for example, in music. For these purposes, a pendulum metronome was previously used, which gave both a visual and audible indication of the tempo.
In this circuit, these functions are performed by a multivibrator. The tempo frequency is approximately 0.5 s (Fig. 35).
Rice. 35. Metronome.
Thanks to the phone and the indicator light, it is possible to hear and visually feel a given rhythm.
EXPERIMENT 30
SOUND SIGNAL DEVICE WITH AUTOMATIC RETURN TO START POSITION
This circuit (Fig. 36) demonstrates the use of a one-shot, the operation of which is described in experiment 14. In the initial state, the transistor T1 is open, and T2 is closed. The telephone is used here as a microphone. Whistling into the microphone (you can just blow it) or light tapping excites an alternating current in the microphone circuit. Negative signals, arriving at the base of the transistor T1, close it, and therefore, open the transistor T2, a current appears in the collector circuit T2, and the lamp lights up. At this time, the capacitor C1 is charged through the resistor R1. The voltage of the charged capacitor C2 is sufficient to open the transistor T1, that is, the circuit returns to its original state spontaneously, while the light goes out. The lamp burns time is about 4 s. If the capacitors C2 and C1 are interchanged, then the lamp burning time will increase to 30 s. If the resistor R4 (1 kOhm) is replaced with 470 kOhm, then the time will increase from 4 to 12 s.
Rice. 36. Acoustic signaling device.
This experiment can be thought of as a trick that can be shown in a circle of friends. To do this, you need to remove one of the phone microphones and put it under the board near the light bulb so that the hole in the board coincides with the center of the microphone. Now, if you blow on the hole in the board, it will seem that you are blowing on the light bulb and therefore it lights up.
EXPERIMENT 31
SOUND SIGNAL DEVICE WITH MANUAL RETURN TO START POSITION
This circuit (Fig. 37) is similar in principle to the previous one, with the only difference that when switching the circuit does not automatically return to its original state, but is done using switch B.
Rice. 37. Acoustic signaling device with manual reset.
The circuit ready state or initial state will be when the transistor T1 is open, T2 is closed, the lamp is off.
A light whistle into the microphone gives a signal that turns off the transistor T1, while opening the transistor T2. The signal light comes on. It will stay on until the transistor T2 closes. To do this, it is necessary to short-circuit the base of the transistor T2 to the negative bus ("ground") using the key B. Other actuators, such as relays, can be connected to similar circuits.
EXPERIMENT 32
THE SIMPLEST DETECTOR RECEIVER
A novice radio amateur should start designing radio receivers with the simplest structures, for example, with a detector receiver, the diagram of which is shown in Fig. 38.
The detector receiver works as follows: electromagnetic waves sent over the air by radio stations, crossing the receiver antenna, induce a voltage in it with a frequency corresponding to the frequency of the radio station signal. The induced voltage enters the input circuit L, C1. In other words, this circuit is called resonant, since it is pre-tuned to the frequency of the desired radio station. In the resonant circuit, the input signal is amplified tens of times and then enters the detector.
Rice. 38. Detector receiver.
The detector is assembled on a semiconductor diode, which serves to rectify the modulated signal. The low-frequency (sound) component will pass through the headphones, and you will hear speech or music depending on the transmission of this radio station. The high-frequency component of the detected signal, bypassing the headphones, will pass through the capacitor C2 to the ground. The capacitance of the capacitor C2 determines the degree of filtration of the high-frequency component of the detected signal. Usually, the capacitance of the capacitor C2 is chosen in such a way that for sound frequencies it represents a large resistance, and for the high-frequency component its resistance is small.
As a capacitor C1, you can use any small-sized variable capacitor with a measurement range of 10 ... 200 pF. In this constructor, a ceramic trimmer capacitor of the KPK-2 type with a capacity of 25 to 150 pF is used to tune the circuit.
The inductor L has the following parameters: the number of turns - 110 ± 10, wire diameter - 0.15 mm, type - PEV-2, diameter of the frame made of insulating material - 8.5 mm.
ANTENNA
A properly assembled receiver starts working immediately when an external antenna is connected to it, which is a piece of copper wire 0.35 mm in diameter, 15-20 m long, suspended on insulators at a certain height above the ground. The higher the antenna is above the ground, the better the reception of radio signals will be.
GROUNDING
The volume of the reception increases if the ground is connected to the receiver. The ground wire should be short and have little resistance. Its end is connected to a copper pipe going deep into the ground.
EXPERIMENT 33
DETECTOR RECEIVER WITH LOW FREQUENCY AMPLIFIER
This circuit (Fig. 39) is similar to the previous circuit of the detector receiver with the only difference that the simplest low-frequency amplifier, assembled on the transistor T, is added here. The low-frequency amplifier serves to increase the power of the signals detected by the diode. Tuning scheme oscillatory circuit connected to the diode through the capacitor C2 (0.1 μF), and the resistor R1 (100 kΩ) provides the diode with a constant bias.
Rice. 39. Detector receiver with single-stage ULF.
For normal operation of the transistor, a 9 V power supply is used. Resistor R2 is necessary in order to supply voltage to the base of the transistor to create the necessary mode of its operation.
This circuit, as in the previous experiment, requires an external antenna and ground.
EXPERIMENT 34
SIMPLE TRANSISTOR RECEIVER
The receiver (Fig. 40) differs from the previous one in that instead of diode D, a transistor is installed, which simultaneously works both as a detector of high-frequency oscillations and as a low-frequency amplifier.
Rice. 40. Single transistor receiver.
The detection of a high-frequency signal in this receiver is carried out at the base - emitter section, therefore, such a receiver does not require a special detector (diode). A transistor with an oscillatory circuit is connected, as in the previous circuit, through a capacitor with a capacity of 0.1 μF and is decoupling. Capacitor C3 serves to filter the high-frequency component of the signal, which is also amplified by the transistor.
EXPERIMENT 35
REGENERATIVE RECEIVER
In this receiver (Fig. 41), regeneration is used to improve the sensitivity and selectivity of the loop. This role is played by the L2 coil. The transistor in this circuit is connected a little differently than in the previous one. The signal voltage from the input circuit goes to the base of the transistor. The transistor detects and amplifies the signal. The high-frequency component of the signal does not immediately go to the filter capacitor C3, but first passes through the winding feedback L2, which is on the same core as the loop coil L1. Due to the fact that the coils are placed on the same core, there is an inductive coupling between them, and part of the amplified voltage of the high-frequency signal from the collector circuit of the transistor again enters the input circuit of the receiver. At correct inclusion the ends of the communication coil L2, the feedback voltage entering the L1 circuit due to the inductive coupling coincides in phase with the signal coming from the antenna, and there is, as it were, an increase in the signal. This increases the sensitivity of the receiver. However, with a large inductive coupling, such a receiver can turn into a continuous oscillation generator, and a sharp whistle is heard in the phones. To eliminate excessive excitation, it is necessary to reduce the degree of coupling between the coils L1 and L2. This is achieved either by moving the coils away from each other, or by reducing the number of turns of the L2 coil.
Rice. 41. Regenerative receiver.
It may happen that the feedback does not give the desired effect and the reception of stations that were well heard earlier, when the feedback is introduced, stops altogether. This suggests that instead of a positive feedback, a negative feedback has formed and the ends of the L2 coil need to be swapped.
On short distances from the radio station, the described receiver works well without external antenna, for one magnetic antenna.
If the audibility of the radio station is low, an external antenna must still be connected to the receiver.
The receiver with one ferrite antenna must be installed so that the electromagnetic waves coming from the radio station create the largest signal in the coil of the oscillating circuit. Thus, when you have tuned in to the signal of the radio station with the help of a variable capacitor, if the audibility is poor, turn the circuit to receive the signals in the phones at the volume you need.
EXPERIMENT 36
TWO-TRANSISTOR REGENERATIVE RECEIVER
This circuit (Fig. 42) differs from the previous one in that it uses a low-frequency amplifier assembled on T2 transistors.
With the help of a two-transistor regenerative receiver, you can receive a large number of radio stations.
Rice. 42. Regenerative receiver with low frequency amplifier.
Although this kit (kit # 2) only has a long wave coil, the circuit can operate on both medium and short wavelengths using the appropriate trim coils. You can make them yourself.
EXPERIMENT 37
"PELENGATOR"
The scheme of this experiment is similar to the scheme of experiment 36 without antenna and "ground".
Tune into a powerful radio station. Take the board in your hands (it should be horizontal) and rotate until the sound (signal) disappears, or at least decreases to a minimum. In this position, the axis of the ferrite points precisely to the transmitter. If you now rotate the board 90 °, the signals will be clearly audible. But more accurately the location of the radio station can be determined by the graph-mathematical method, using a compass to determine the angle in azimuth.
To do this, you need to know the direction of the transmitter location from different positions - A and B (Fig. 43, a).
Let's say we are at point A, we have determined the direction of the transmitter, it is 60 °. Let's move now to point B, while measuring the distance AB. Let's determine the second direction of the transmitter location, it is 30 °. The intersection of the two directions is the location of the transmitting station.
Rice. 43. Diagram of the direction finding of the radio station.
If you have a map with the location of broadcasting stations on it, then it is possible to pinpoint your location.
Tune in station A, keep it at a 45 ° angle, and then tune in station B; its azimuth, let's say, is 90 °. Considering these angles, draw lines on the map through points A and B, their intersection will give your location (Fig. 43, b).
In the same way, ships and planes are oriented in the process of movement.
CIRCUIT CONTROL
In order for the circuits to work reliably during experiments, you need to make sure that the battery is charged, all connections are clean, and all nuts are securely tightened. The battery leads must be properly connected; when connecting, it is necessary to strictly observe the polarity of electrolytic capacitors and diodes.
INSPECTION OF COMPONENTS
Diodes can be tested at; transistors - in; electrolytic capacitors (10 and 100 μF) - c. You can also test the headset by connecting it to the battery - you will hear a "crackle" in the headphone.
At one time, transistors replaced electronic tubes. This is due to the fact that they are smaller, more reliable and less expensive to manufacture. Now, bipolar transistorsare essential elements in all amplifying circuits.
It is a semiconductor element with a three-layer structure that forms two electron-hole transitions. Therefore, the transistor can be represented as two oppositely connected diodes. Depending on what will be the main charge carriers, distinguish p-n-p and n-p-n transistors.
Base- the semiconductor layer, which is the basis of the transistor design.
Emitter called the semiconductor layer, the function of which is the injection of charge carriers into the base layer.
Collector is called a semiconductor layer, the function of which is to collect charge carriers that have passed through the base layer.
As a rule, the emitter contains much large quantity main charges than the base. This is the main condition for the operation of the transistor, because in this case, with the forward bias of the emitter junction, the current will be determined by the main carriers of the emitter. The emitter will be able to carry out its main function- injection of carriers into the base layer. Emitter reverse current is usually tried to be kept as small as possible. An increase in the majority of emitter carriers is achieved using a high impurity concentration.
The base is made as thin as possible... This is due to the lifetime of the charges. The charge carriers must cross the base and recombine as little as possible with the base carriers in order to reach the collector.
In order for the collector to be able to more fully collect the carriers that have passed through the base, they are trying to make it wider.
The principle of operation of the transistor
Consider at example p-n-p transistor.
In the absence of external stresses, a potential difference is established between the layers. Potential barriers are being erected at crossings. Moreover, if the number of holes in the emitter and collector is the same, then the potential barriers will be the same width.
In order for the transistor to work correctly, the emitter junction must be forward biased, and the collector junction in the opposite direction.... This will correspond to the active mode of the transistor. In order to make such a connection, two sources are required. A source with voltage Ue is connected with the positive pole to the emitter, and the negative pole to the base. A source with voltage Uk is connected with the negative pole to the collector, and the positive pole to the base. And Ue< Uк.
Under the influence of voltage Ue, the emitter junction is shifted in the forward direction. As is known, with a forward bias of the electron-hole transition, the external field is directed opposite to the transition field and therefore decreases it. Most carriers begin to pass through the transition, in the emitter these are holes 1-5, and in the base electrons are 7-8. And since the number of holes in the emitter is greater than the number of electrons in the base, the emitter current is mainly due to them.
The emitter current is the sum of the hole component of the emitter current and the electronic component of the base.
Since only the hole component is useful, they try to make the electronic component as small as possible. The qualitative characteristic of the emitter junction is injection ratio.
They try to bring the injection coefficient closer to 1.
Holes 1-5 that have passed into the base accumulate at the border of the emitter junction. Thus, a high concentration of holes near the emitter and a low concentration near the collector junction is created, as a result of which the diffusion movement of holes from the emitter to the collector junction begins. But near the collector junction, the hole concentration remains zero, because as soon as the holes reach the junction, they are accelerated by its internal field and are extracted (drawn in) into the collector. Electrons are repelled by this field.
While the holes cross the base layer, they recombine with the electrons located there, for example, like hole 5 and electron 6. And since holes come in constantly, they create an excess positive charge, therefore, electrons must also enter, which are drawn in through the base terminal and form the base current Ibr. This is an important condition for the operation of the transistor. - the concentration of holes in the base should be approximately equal to the concentration of electrons. In other words the base must be electrically neutral.
The number of holes reaching the collector is less than the number of holes leaving the emitter by the amount of the recombined holes in the base. That is, the collector current differs from the emitter current by the base current.
From here comes carryover ratio carriers, which are also trying to bring it closer to 1.
The collector current of the transistor consists of the hole component Icr and the reverse collector current.
The reverse collector current arises as a result of reverse bias of the collector junction, therefore it consists of minority carriers of a hole 9 and an electron 10. Precisely because the reverse current is formed by minority carriers, it depends only on the thermogeneration process, that is, on temperature. Therefore, it is often called thermal current.
The quality of the transistor depends on the magnitude of the thermal current; the smaller it is, the better the transistor is.
The collector current is associated with the emitter current transfer ratio.
If we consider mechanical analogs, then the operation of transistors resembles the principle of operation of a hydraulic power steering in a car. But, the similarity is true only at a first approximation, since there are no valves in transistors. In this article, we will take a separate look at the operation of a bipolar transistor.
Bipolar transistor device
The basis of a bipolar transistor device is a semiconductor material. The first semiconductor crystals for transistors were made from germanium; today, silicon and gallium arsenide are more commonly used. First, a pure semiconductor material with a well-ordered crystal lattice is produced. Then they give the required shape to the crystal and introduce into its composition a special impurity (alloy the material), which gives it certain properties of electrical conductivity. If conductivity is due to the movement of excess electrons, it is defined as donor (electron) n-type. If the conductivity of a semiconductor is due to the successive substitution of electrons for vacant places, the so-called holes, then such conductivity is called acceptor (hole) and is denoted by p-type conductivity.
Picture 1.
The crystal of the transistor consists of three parts (layers) with sequential alternation of the conductivity type (n-p-n or p-n-p). Transitions from one layer to another form potential barriers. The transition from base to emitter is called emitter(EP), to the collector - collector(KP). In Figure 1, the structure of the transistor is shown symmetrical, idealized. In practice, during manufacturing, the dimensions of the regions are significantly asymmetric, approximately as shown in Figure 2. The collector junction area is much larger than the emitter junction. The base layer is very thin, on the order of a few microns.
Figure 2.
The principle of operation of a bipolar transistor
Any pn junction of the transistor works in the same way. When a potential difference is applied to its poles, it is "displaced". If the applied potential difference is conditionally positive, while the p-n junction opens, they say that the junction is displaced in the forward direction. When a conditionally negative potential difference is applied, a reverse displacement of the transition occurs, at which it is locked. A feature of the operation of the transistor is that with a positive bias of at least one transition, the common region, called the base, is saturated with electrons, or electronic vacancies (depending on the type of conductivity of the base material), which causes a significant decrease in the potential barrier of the second transition and, as a consequence, its reverse bias conductivity.
Modes of operation
All transistor switching circuits can be divided into two types: normal and inverse.
Figure 3.
Normal transistor switching circuit involves changing the electrical conductivity of the collector junction by controlling the offset of the emitter junction.
Inverse circuit, as opposed to normal, allows you to control the conductance of the emitter junction by controlling the offset of the collector. The inverse circuit is a symmetrical analogue of the normal one, but in view of the constructive asymmetry of the bipolar transistor, it is ineffective for use, has more stringent restrictions on the maximum permissible parameters and is practically not used.
With any switching scheme, the transistor can operate in three modes: Cutoff mode, active mode and saturation mode.
To describe the work, the direction of the electric current in this article is conventionally taken as the direction of the electrons, i.e. from the negative pole of the power supply to the positive pole. Let's use the scheme in Figure 4 for this.
Figure 4.
Cutoff mode
For p-n junction there is a minimum forward bias voltage value at which electrons are able to overcome the potential barrier of this transition. That is, with forward bias voltages up to this threshold, no current can flow through the junction. For silicon transistors, the value of such a threshold is approximately 0.6 V. Thus, with a normal switching circuit, when the forward bias of the emitter junction does not exceed 0.6 V (for silicon transistors), the current does not flow through the base, it is not saturated with electrons, and as a consequence, there is no emission of base electrons into the collector region, i.e. there is no collector current (zero).
Thus, for the cutoff mode necessary condition are the identities:
U BE<0,6 В
IB = 0
Active mode
In the active mode, the emitter junction shifts in the forward direction until the moment of unlocking (the beginning of the current flow) with a voltage greater than 0.6 V (for silicon transistors), and the collector junction - in the opposite direction. If the base has p-type conductivity, there is a transfer (injection) of electrons from the emitter to the base, which are instantly distributed in a thin layer of the base and almost all reach the collector boundary. The saturation of the base with electrons leads to a significant decrease in the size of the collector junction, through which electrons under the action of a negative potential from the emitter and base are displaced into the collector region, flowing down through the collector terminal, thereby causing the collector current. A very thin layer of the base limits its maximum current passing through a very small cross-section in the direction of the base pin. But this small thickness of the base causes its rapid saturation with electrons. The area of the junctions is significant, which creates conditions for the flow of a significant emitter-collector current, tens and hundreds of times higher than the base current. Thus, by passing insignificant currents through the base, we can create conditions for the passage of currents of much greater magnitude through the collector. The higher the base current, the greater its saturation, and the higher the collector current. This mode allows you to smoothly control (regulate) the conductivity of the collector junction by corresponding change (regulation) of the base current. This property of the active mode of the transistor is used in the circuits of various amplifiers.
In active mode, the emitter current of the transistor is the sum of the base and collector currents:
I E = I K + I B
Collector current can be expressed as:
I K = α I e
where α is the emitter current transfer coefficient
From the given equalities, you can get the following:
where β is the base current amplification factor.
Saturation mode
The limit of the increase in the base current until the moment when the collector current remains unchanged determines the point of maximum saturation of the base with electrons. A further increase in the base current will not change the degree of its saturation, and in no way will it affect the collector current, it can lead to overheating of the material in the base contact area and the failure of the transistor. The reference data for transistors can indicate the values of the saturation current and the maximum allowable base current, or the emitter-base saturation voltage and the maximum allowable emitter-base voltage. These limits determine the saturation mode of the transistor under normal operating conditions.
Cutoff mode and saturation mode are effective when transistors operate as electronic switches for switching signal and power circuits.
The difference in the principle of operation of transistors with different structures
Above, the case of operation of an n-p-n transistor was considered. Transistors p-n-p structures work in a similar way, but there are fundamental differences that you should be aware of. A semiconductor material with p-type acceptor conductivity has a relatively low electron transmission capacity, since it is based on the principle of electron transfer from one vacant place (hole) to another. When all vacancies are replaced by electrons, their motion is possible only as vacancies appear from the direction of motion. With a considerable length of a section of such a material, it will have significant electrical resistance, which leads to greater problems when using it as the most massive collector and emitter of bipolar p-n-p transistors than when using n-p-n transistors in a very thin base layer. The n-type semiconductor material has the electrical properties of conductive metals, making it more advantageous for use as an emitter and collector, as in n-p-n type transistors.
This distinctive feature of different structures of bipolar transistors leads to great difficulties in the production of pairs of components with different structures and similar electrical characteristics to each other. If you pay attention to the reference data of the characteristics of pairs of transistors, you can see that when the same characteristics of two transistors of different types are achieved, for example, KT315A and KT361A, despite their identical collector power (150 mW) and approximately the same current gain (20-90) , they have different maximum permissible collector currents, emitter-base voltages, etc.
P.S. This description of the principle of operation of the transistor was interpreted from the standpoint of the Russian Theory, therefore, there is no description of the action of electric fields on fictitious positive and negative charges. Russian Physics makes it possible to use simpler, understandable mechanical models that are closest to reality than abstractions in the form of electric and magnetic fields, positive and electric charges, which the traditional school treacherously slips at us. For this reason, I do not recommend using the stated theory without preliminary analysis and reflection when preparing for the delivery of tests, coursework and other types of work, your teachers may simply not accept dissent, even competitive and quite consistent from the point of view of common sense and logic. In addition, on my part, this is the first attempt to describe the operation of a semiconductor device from the standpoint of Russian Physics, it can be refined and supplemented in the future.