Simple amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Often in amateur radio practice, it becomes necessary to amplify a signal (for example, from a sensor) and in this case it is only necessary to provide a large gain with a minimum of parts used. In such cases, the amplifying stage, the circuit of which is shown in Fig. 1.

Only one limitation is imposed on the supply voltage of the stage - it should not be more than half of the maximum allowable value between the collector and emitter for the transistor used. Signal sources can be different - from a microphone to a preamplifier (or a radio receiver detector).

The input impedance of the amplifier (sometimes called the emitter-collector OS stage) does not exceed a few kilo-ohms, since the input signal goes directly to the emitter junction of the transistor. This requires the use of high-capacity C2 transition and C1 filter capacitors (tens and thousands of microfarads, respectively) if it is necessary to amplify frequency components below 100 Hz. The resistance of the resistor R1 is within 47 ... 120 kOhm.

The resistance of the resistor R2, on the one hand, should be as small as possible so that the useful signal falls mainly on the emitter load, but, on the other hand, it must be large in order to form at least the minimum required OS voltage transmitted through the capacitor C3. At the same time, if the sum of the resistance values ​​​​of resistors R3 and R4 is small (for operation in the output stage - tens, and sometimes even units of ohms), the minimum value of R2 is limited by the expression R2> Upit / Ik max where Ik max is the maximum allowable collector current of the transistor .

Thus, we can consider optimal conditions when 4(R3+R4)>R2>Upit/Ik max and R3=R4. If the amplifier is loaded with standard head stereo telephones, the sum R3+R4 and the equality R3=R4 will be determined constructively, and for the linear stage this sum should be chosen within 200 Ohm...2 kOhm. The capacitance of the capacitor OS C3 is determined from the ratio C3>1/2πfHR2, where fH is the lower limit of the reproducible frequency.

Currently, there are many cheap Chinese-made receivers on sale, assembled on one or two microcircuits and working on headphones. Although microcircuits of world famous companies are used, they also fail and it is not always possible to find a replacement. In such cases, the 3H amplifier can be assembled according to the circuit in Fig. 1 on one transistor of the KT315 series.

To do this, find the output of the receiver detector on the board, connect it to the volume control (having previously cut the printed conductors of the previous inclusion), and from its engine the signal is fed to the input of the described stage. Similarly, the headphone connection node is being redone. Extra parts are removed, and the parts of the new amplifier are placed in their place. In this case, the windings of the headphones will serve as load resistors R3 and R4 (see Fig. 1), and the OS capacitor is connected to their common point.

Another example of the application of the described stage is in the "electronic ear" of a beekeeper. Beekeepers have to listen to the processes taking place in the hive. From the sounds from the inside, they determine how wintering proceeds, there are no pests, when offspring appears. "Electronic ear" allows you to find out without opening the hive.

The scheme of the device is shown in fig. 2. Preamplifier DA1.1 is included according to the standard scheme. By changing the resistance of the OS circuit (resistors R3 and R4), you can adjust the sensitivity. Resistor R5 sets the required sound volume. The BM1 microphone is applied outside to the walls of the hive.

The device is powered by a Krona battery or a Nika or 7D-0,1 battery. Microphone VM1 - electret 34J9E (domestic analogue - MKE-332); you can replace it with a PDC telephone capsule, but this will somewhat reduce the sensitivity, which, however, is sometimes advisable. For the MPC capsule, the resistance of the resistor R1 should be reduced to 0,1 ... 3 kOhm. The current consumed by the "electronic ear" does not exceed 15 mA.

Those involved in car repair often use a medical stethoscope or even an ordinary stick as an auditory tube to identify a faulty node by the nature of the internal noise of the links of the mechanism. Obviously, with such a "device" it is not possible to crawl everywhere, and it is impossible to listen to the noise of the nodes while the car is moving. An electronic stethoscope removes these limitations.

The diagram of this simple device is shown in Fig. 3. It consists of two identical stages, connected in series by a signal. As in the previous device, the BM1 microphone is an electret, MKE-332 or similar (for example, MKE-333, MKE-389-1). The volume is controlled by resistor R6.

Headphones for a stethoscope should be chosen that are capable of reproducing at least 100 Hz. The previously produced TDS-2, TDS-3 or their modern counterparts from the commercially available ones work well. The autostethoscope with TDS-2 phones consumes 12 mA. It can also be powered from the car's on-board network, but through an effective protective filter that suppresses interference from a running engine.

Author: G.Sauridi, Ryazan

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