A simple radio microphone on a single transistor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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A schematic diagram of a very simple radio microphone, containing only one transistor, is shown in the figure.
Here, the carrier frequency generator in the VHF range (from 68 to 73 MHz) is assembled according to the capacitive three-point circuit due to the feedback capacitor C5 and the capacitance of the emitter junction of the transistor. The audio frequency signal from the electret microphone BM1 is fed to the emitter of the transistor. The oscillatory circuit L1, C4, which determines the generation frequency, is included in the collector circuit, and from it the modulated voltage enters the antenna. Power supply from 1,5 to 6,0 V is supplied from one or four STs-32 elements connected in series.
The current consumption is a few milliamps. Capacitors C1, C2 protect the radio microphone circuit from external interference and prevent high frequencies from entering the power circuit.
The loop coil L1 is wound on a cylindrical frame made of polystyrene or plexiglass with a diameter of 4 mm with a PEV-2 wire with a diameter of 0,56 mm in an ordinary winding with a pitch of 1,5 mm.
The carrier frequency is tuned by moving the turns of the coil.
The antenna is a piece of wire or a bar about 25 mm long.
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