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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Transistor radios. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur The first practical steps of a radio amateur begin, as a rule, with the construction of detector receivers. But their volume is low, and it is not possible to significantly increase it even with a good outdoor antenna. How to be! This is discussed in the article. The sound volume of the detector receiver will increase if you add a small radio component to it - a transistor. It allows you to amplify the signal by tens and hundreds of times, despite its tiny size. The transistor consumes very little energy and is able to work even at a supply voltage of less than a volt! You already know that a transistor has three terminals - base, emitter and collector. In most cases, the input signal is fed to the base, and the amplified signal is removed from the collector. But sometimes it is necessary to remove the signal from the emitter. An example of this is our first construction. DETECTOR-TRANSISTOR RECEIVER Its diagram is shown in Figure 1. The input oscillatory circuit consists of an inductor L1 and a variable capacitor C1. This is followed by a cascade on the transistor V1, connected in parallel to the oscillatory circuit. The base of the transistor is connected through a resistor R1 to the collector - through this resistor, the bias voltage necessary for the operation of the transistor is applied to the base. Capacitor C2 and headphones B1 are included in the emitter circuit of the transistor. Power is supplied to the transistor stage through the switch S1 from the galvanic cell G1 with a voltage of 1,5 V.
Why is the receiver called detector-transistor? Because when the power is off, the base-emitter section of the transistor works like a regular diode and the whole structure turns into a detector receiver. When power is applied to the transistor, it begins not only to detect, but also to amplify sound vibrations, and the transmission volume increases. The receiver is tuned to the radio station with a variable capacitor C1. The antenna is connected to socket X1, and the ground is connected to socket X2. About the details of the receiver. Take a transistor of the type P416B (you can P401-P403, P422) with a current transfer coefficient (formerly called the gain) from 60 to 100. Capacitor C1 - KP-180 or another small-sized variable capacitor with a maximum capacitance of at least 180 pF. If, for example, a capacitor from a Selga radio receiver is used, the range of the receiver will expand towards longer waves, since the maximum capacitance of this capacitor is 270 pF. Capacitor C2 - BMT-2 or another type with a capacity of 3300 to 9100 pF. Resistor MLT-0,5. (you can MLT-0,25 or MLT-0,125). Headphones B1 type TON-1, TON-2 or any other high-resistance (with a resistance of at least 3 kOhm). The power source can be any element with a voltage of 1,5 V (316, 332, 343, 373) or a small-sized disk battery type D-0,1 or D-0,2. Switch S1 - toggle switch TV2-1. Wind the inductor with a PEL or PEV wire with a diameter of 0,15-0,2 mm on a ferrite rod with a diameter of 8 mm and a length of 40-50 mm. You will not find such a rod on sale, so you will have to break it off from a longer rod for a pocket receiver. Do it like this. Wrap the rod with a cloth and clamp it in a vise so that a part of the desired length protrudes over the jaws. A sharp blow with a hammer on the protruding end is now enough, and it will break off. Sharpen the sharp edges of the rod in the place of the chip with a file. After that, wind 80 turns of the indicated wire on the rod, winding turn to turn. With such a coil and the variable capacitor shown in the diagram, the receiver will operate in the medium wave range (approximately from 250 to 600 m). Mount the inductor, variable capacitor and several other parts on the board (Fig. 2) of insulating material. At the soldering points of the leads of the parts, drill holes in the board and insert mounting studs made of thick tinned copper wire into the board. To keep the studs from falling out, flatten them slightly with pliers on both sides of the board.
Strengthen the ferrite rod in racks made of wire, passed from the bottom side of the board through the holes and bent from above in a semicircle. Attach the variable capacitor to the board with screws or, in extreme cases, glue it. First, solder all the parts to the studs. except for the transistor. Before soldering the transistor, first determine its conclusions. To do this, just look at the transistor from above - on its side you will see a colored mark in the form of a dot. The pin next to the label is the emitter pin. Next to it is the output of the collector, and the remaining extreme is the output of the base. Now turn the transistor upside down, bend its leads with tweezers, shorten them with wire cutters so that they protrude 10-15 mm above the transistor, and bend the ends of the leads into a ring. These rings and solder to the mounting studs. But here, as in the future, when soldering the transistor leads, it is desirable to follow a certain sequence: the base lead is soldered first, then the emitter, and lastly the collector. Before installing the board in the case, make sure that the receiver's ability is working. To do this, connect the remaining parts (power supply, switch and headphones) to the board-studs and connect the wire from the external antenna and ground. Without turning on the power, tune the receiver with a variable capacitor to some radio station. If you now turn on the power, the sound volume in the phones will increase several times. If the volume does not increase, check the correct connection of the galvanic cell - if the polarity is reversed, compared to that indicated in the diagram, the transistor will not work. After correcting the error, check how many stations are heard when the variable capacitor tuning knob is fully turned. If some station is heard in one of the extreme positions of the knob, change the number of turns of the coil, turn on a constant capacitor in series with the antenna or connect a constant capacitor in parallel (select its capacitance so that the station can be heard at some distance from the extreme position of the variable capacitor knob ). Which of the following measures is better to apply? Let's figure it out. When the station is heard in the extreme clockwise position of the variable capacitor knob, it means that the capacitance of the capacitor is minimal. In this case, you need to unwind several turns of wire from the coil or connect a constant capacitor in series with the antenna. If, on the contrary, the station is heard in the opposite extreme position of the capacitor knob, then its capacity is maximum. In this case, you need to add a few turns to the coil or connect a constant in parallel with the variable capacitor. After such a check and adjustment, you can strengthen the board in a case of suitable dimensions (Fig. 3). On the side walls of the Case, install sockets for connecting the antenna and grounding, as well as a connector for the headphone plug. Attach the galvanic cell with a metal bracket to the side wall of the housing from the inside.
The switch can be installed both on the top panel and on the side wall. The bottom cover of the housing must, of course, be removable. RADIO ON A SINGLE TRANSISTOR This receiver is more sensitive than the previous one, although it also contains only one transistor (Fig. 4). The thing is that the transistor is turned on a little differently - the headphones are in the collector circuit. In this mode, the cascade has a higher gain than when the phones are connected to the emitter circuit.
The input part of the receiver is made somewhat differently. Two inductors are now placed on a common ferrite rod - a loop L1 (with a variable capacitor C1 it makes up an oscillatory circuit) and a coupling coil L2. Moreover, the number of turns of the coupling coil is less than that of the loop coil, and only a part of the received signal is fed to the transistor. This is done so that the transistor does not affect the oscillatory circuit and does not change its settings. So, from the coupling coil, the signal enters the base of the transistor through capacitor C2. Here it is detected, that is, an audio frequency signal is extracted from it, which is then amplified by a transistor. Radio transmission is heard from the headphones. As in the previous receiver, the bias to the base of the transistor is fed through the resistor R1. The letter designation of the resistor has an asterisk. It shows that this resistor may have to be selected (that is, to specify the resistance of the resistor) when setting up the receiver. This will be discussed later. The power source of the receiver is a battery of galvanic cells, therefore it is designated GB1. In this case, a Krona battery with a voltage of 9 V is used. Capacitors, resistor, transistor, switch and headphones are the same as in the previous receiver. The coils are wound on a ferrite rod with a diameter of 8 mm and a length of 40-50 mm. Coil L1 contains 80 turns, and L2 - 20 turns of PEL or PEV wire with a diameter of 0,15-0,2 mm. The distance between the coils is about 5 mm, winding coil by coil. Mount part of the receiver parts on a board (Fig. 5) made of insulating material, resembling the board of the previous receiver. After installation, check the correctness of all connections and only then connect the power supply, headphones, antenna and ground to the mounting studs. Turn on the power switch (there will be a click in the headphones) and immediately measure the voltage between the emitter and collector of the transistor. The voltmeter needle should show a voltage of about 4,5 V. If it differs significantly (more than 20%) from the specified one, select resistor R1 - install another one with less or more resistance instead.
It is not difficult to determine which resistor is needed. With a lower measured voltage, you need to install a resistor with a resistance greater than that indicated in the diagram (390 kOhm, 430 kOhm, 470 kOhm, etc.). On the contrary, if the measured voltage exceeds the specified one, the resistance of the resistor should be reduced (install a resistor with a resistance of 300 kOhm, 270 kOhm, 240 kOhm). You can do otherwise: instead of the resistor R1, turn on two series-connected resistors - a constant resistance of 100 kOhm, and a variable resistance of 1 mOhm. By moving the variable resistor slider, achieve the desired voltage, measure the total resistance (solder the circuit from the board at the same time) and install a constant resistor on the board with approximately the same resistance. In practice, such an adjustment rarely has to be done, since the required current transfer coefficient of the transistor (60-100) is specified, and when using a transistor with this parameter, the resistor indicated in the diagram ensures its operation mode. This is true, of course, only with a "fresh" battery. Therefore, measure its voltage with the connected receiver - it must be at least 8 V. Otherwise, the battery will have to be replaced. After checking and setting the voltage on the collector, touch the base terminal of the transistor with tweezers. A weak sound should be heard in the phones - an alternating current background. Now you can check how many radio stations and with what volume your homemade product is receiving. If you notice sound distortion in phones, unwind one or two turns from the L2 communication coil. If the sound volume is excessive (especially when receiving nearby powerful radio stations), connect a small constant capacitor (10-15 pF) between the outdoor antenna and the receiver. In any case, you can change the operating range of the receiver using the same means as in the previous design. Mount the board and the remaining parts (sockets, connector, switch and battery) in a housing that can be structurally the same as that of the first receiver. You can solder the power conductors directly to the battery terminals or use a connector - a block from a worthless "Krona" to connect the battery to the receiver. Author: B.Ivanov
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