Radio microphone on three transistors. Scheme, description

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Radio microphone on three transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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I propose a radio microphone circuit (see diagram) with very good parameters, which is easy to set up, stable (when the power is changed from 2 to 12 V, the frequency changes by only 0,1 MHz) and high "range" (200 m for a conventional Chinese receiver) .

Radio microphone on three transistors

The first stage on the transistor VT1 amplifies the signal from the condenser "button" microphone, and also sets the DC mode of the generator on the transistor VT2. As it, I used KT368, as the most stable in operation. The VT3 transistor amplifier operates in class C with high efficiency. When the supply battery is discharged below 5 V, VT3 closes, and the signal from the generator to the antenna goes through the base-collector pass-through capacitance.

These ratings of the radio elements were repeated many times, so the setting consists only in stretching and compressing the L1 coil to select the desired frequency. It will be useful to provide the circuit with an LED signaling the inclusion and sufficient supply voltage. A small increase in current consumption, approximately 2 mA, is compensated by the convenience of control. The circuit is powered by the Krona battery and consumes a current of about 15 ... 18 mA.

Coil L1 contains 8 turns of PEL 0,8 wire with a tap from the middle, wound on a mandrel with a diameter of 4 mm. Inductor Dr1 is wound on a ferrite ring K7x4x2 and contains 5 ... 10 turns of PEL 0,2 wire. For the antenna, 80 cm of wire with a diameter of 1-1,5 mm is taken and wound evenly on an AA battery. The whole design fits perfectly into a pack of cigarettes, the device can be picked up and there is practically no frequency drift. You can simplify the circuit by eliminating the RF amplifier. The current consumption is reduced to 5 mA, and the range is reduced to 50 m.

Capacitor C3 serves to prevent self-excitation of the radio microphone by RF and its capacitance is selected within 100 ... 1000pF. Resistor R6 determines the signal power of the master oscillator and the depth of its modulation by sound, and hence the sensitivity. So, with an increase in the value of this resistor to 1 kOhm, an increase in the sensitivity of the device to ambient sounds is noted. If the circuit is supposed to be used as a radio microphone, the resistance of the resistor R6 can be reduced to 100 ohms. The capacitance of the coupling capacitor C7 is chosen so small in order to reduce the influence of the antenna and the output stage on the frequency of the master oscillator. It is possible to increase the radiation power of the radio microphone and, as a result, the range by increasing the value of this capacitor to 10 pF, however, the effect of the antenna on frequency stability will also increase.

The master oscillator remains operational even when the supply voltage drops to 0,8 V! Therefore, if it is necessary to power the circuit from a low-voltage source, with a voltage of 3 ... 5 V, the output stage on the VT3 transistor should be switched to mode A. To do this, we put a 100 kOhm tuning resistor between the base and the plus of the power supply. Having set with its help the quiescent current of the output stage within 5 ... 10 mA and measuring the resulting resistance with an ohmmeter, we replace it with a constant one.

Author: D.Kibardin

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