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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Designs by I. Bakomchev. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Single-stage AF amplifier (Fig. 1)
This is the simplest design that allows you to demonstrate the amplifying capabilities of a 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 receiver (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 sockets X3, X4. The divider R1 R2 sets the bias voltage at the base of the transistor, and the resistor R3 provides current feedback, which contributes to the temperature stabilization of the amplifier. 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, with tweezers - a weak buzz should be heard in the phone as a result of AC interference. The collector current of the transistor is about 3 mA. Two-stage AF amplifier on transistors of different structures (Fig. 2)
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. This design can be used to amplify weak signals, such as those from a microphone. And of course, it will significantly amplify the AF signal taken from the load of the detector receiver. Two-stage AF amplifier on transistors of the same structure (Fig. 3)
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 transistor VT1 has decreased. The voltage drop across this transistor will increase, which will increase 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. 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. Push-pull power amplifier AF (Fig. 4)
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 amplifier is two-stage: the first is assembled on a transistor VT1, the second - on VT2 and VT3 of different structures. The first stage amplifies the AF voltage signal, 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 VT3 transistor - with negative ones. 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 including a feedback resistor R2. The collector current of the input transistor, flowing through the VD1 diode, leads to a voltage drop on it, which is the bias voltage at the bases of the output transistors (relative to their emitters), which reduces 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. Pay attention to the connection of the load of the first stage - the resistor R4. Its top output according to the diagram is not connected to the power plus, as is usually done, but to the 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 (Fig. 5)
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. 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 you move the resistor slider 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, following the diode VD1, the transistor VT2 opens. The HL2 LED will also flash. In other words, a low voltage at the indicator input causes only the HL1 LED to glow, and a larger one causes both LEDs to glow. 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 "operation" of the indicator is selected. When monitoring only the voltage of the power source, it is permissible to install a green LED (AL2G) in place of HL307. Three-level voltage indicator (Fig. 6)
It gives out light signals according to the principle less than the norm - the norm - more than the norm. To do this, the indicator uses two red LEDs and one green LED. At a certain voltage on the engine of the variable resistor R1 ("voltage is normal"), both transistors are closed and only the green LED HL3 "works". Moving the resistor slider up the circuit leads to an increase in voltage ("more than normal") on it. The transistor VT1 opens. 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 another: it has not yet completely gone out, for example, HL1, but HL3 is already on. Schmitt trigger (Fig. 7)
As you know, this device is usually used to convert a slowly changing voltage into a square wave signal. When the engine of the variable resistor R1 is in the lower position according to the diagram, the transistor VT1 is closed. The voltage on its collector is high. As a result, the transistor VT2 is open, which means that the LED HL1 is lit. A voltage drop is formed across resistor R3. 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 at the base of VT1 exceeds the voltage drop across the resistor R3. The LED will turn off. If you then move the slider down, the trigger will return to its original position - the LED will flash. This will happen when the voltage on the engine is less than the LED off voltage. Standby multivibrator (Fig. 8)
Such a device has one stable state and changes to another only when an input signal is applied. In this case, the multivibrator generates a pulse 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. In the initial state, the transistor VT2 is open, the LED HL1 is lit. It is now 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 will begin to discharge, the discharge current will flow through the resistor R5, keeping the transistor VT2 closed. As soon as the capacitor is discharged, the transistor VT2 will open again and the multivibrator will switch back to the "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 twice as long. Overcurrent signaling device (Fig. 1)
It happens that you need to monitor the current consumed by the load, and if it is exceeded, turn off the power source in time so that the load or source does not fail. To perform a similar task, signaling devices are used that notify of exceeding the norm of the consumed current. Such devices play a special role in the event of a short circuit in the load circuit. What is the principle of operation of the signaling device? To understand it will allow the proposed layout of the device, made on two transistors. If the resistor R1 is disconnected from the sockets X1, X2, the load for the power supply (it is connected to the sockets X3, X4) will be a circuit of the resistor R2 and the HL1 LED - it lights up, informing about the presence of voltage on the sockets X1 and X2. In this case, the current flows through the alarm sensor - resistor R6. But the voltage drop across it is small, so the transistor VT1 is closed. Accordingly, the transistor VT2 is also closed, the HL2 LED is off. It is worth connecting an additional load in the form of a resistor R1 to the sockets X2, X1 and thus increasing the total current, as the voltage drop across the resistor R6 will increase. With the appropriate position of the variable resistor R7 slider, which sets the alarm threshold, transistors VT1 and VT2 will open. The HL2 LED will flash and signal a critical situation. LED HL1 continues to glow, indicating the presence of voltage on the load. What happens if there is a short circuit in the load circuit? To do this, it is enough to close (for a short time) sockets X1 and X2. The HL2 LED will flash again, and HL1 will go out. The variable resistor slider can be set in such a position that the signaling device will not respond to the connection of a 1 kΩ resistor R1, but will “work” when a resistor of, say, 300 Ω is placed in place of the additional load (it is included in the set). Prefix "Colored sound" (Fig. 2)
One of the popular amateur radio designs is the light-dynamic installation (SDU). It is also called "color-music prefix". When you connect such a set-top box to a sound source, the most bizarre color flashes appear on its screen. Another design of the kit is the simplest device that allows you to get acquainted with the principle of obtaining "color sound". At the input of the set-top box there are two frequency filters - C1 R4 and R3C2. The first of them passes the higher frequencies, and the second - the lower ones. The signals selected by the filters are fed to the amplifying stages, the loads of which are the LEDs. Moreover, in the high-frequency channel there is a green LED HL1, and in the low-frequency channel - red (HL2). The source of the audio frequency signal can be, for example, a radio receiver or a tape recorder. To the dynamic head of one of them, you need to connect two wires in isolation and connect them to the input jacks X1 and X2 of the set-top box. While listening to the melody being played, you will observe LED flashes. In addition, it is easy to distinguish the "reaction" of the LEDs to the sounds of one or another key. For example, drum sounds will flash the red LED, and violin sounds will cause the green LED to flash. The brightness of the LEDs is set by the volume control of the sound source. Temperature indicator (Fig. 3)
Everyone knows the usual mercury thermometer, the column of which rises with an increase in body temperature. In this case, the sensor is mercury, which expands with heat. There are many electronic components that are also sensitive to temperature. They sometimes become sensors in devices designed to measure the temperature of, say, the environment, or indicate that it has exceeded a given rate. As such a temperature-sensitive element in the proposed layout, a silicon diode VD1 is used. It is included in the emitter circuit of the transistor VT1. The initial current through the diode is set (with a variable resistor R1) so that the HL1 LED barely glows. If you now touch the diode with your finger or some heated object, its resistance will decrease, which means that the voltage drop across it will also decrease. As a result, the collector current of the transistor VT1 and the voltage drop across the resistor R3 will increase. Transistor VT2 will start to close, and VT3, on the contrary, will open. The brightness of the LED will increase. After cooling the diode, the brightness of the LED will reach its original value. Similar results can be obtained if the transistor VT1 is heated. But the heating of the transistor VT2, and even more so VT3, will practically not affect the brightness of the LED - there is too little change in the current through them. These experiments show that the parameters of semiconductor devices (diodes and transistors) depend on the ambient temperature. Publication: cxem.net
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Comments on the article: Nicholas [up] ![[!]](https://diagram.com.ua/comments/smilies/excl.gif){kind=small} A very useful article, for example, it really helped me, and what is most important for rural radio amateurs is the simplicity of designs and the prevalence of parts! Thanks to the Author!
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