A miniature FM wireless microphone circuit

The miniature FM wireless microphone circuit uses a capacitive three-point oscillator, which is simple, reliable and easy to start. The amplifier circuit composed of BG1 amplifies the weak sound signal detected from the small condenser microphone, reaches a certain amplitude, and then sends it to the capacitive three-point oscillator composed of BG2 to modulate the carrier signal generated by BG2, and finally outputs it from the antenna. The circuit adopts diode voltage stabilization technology, which can make the circuit work more stably. The circuit is as shown below: Component selection: BG1 uses 2SC9014 equivalent power transistor. BG2 uses high-frequency and high-power transistors such as 2SC9018, which requires β ≥ 100. If the amplification factor is too high, it will easily cause instability, and if it is too low, it will be detrimental to the vibration of the circuit. The MIC uses a miniature ultra-thin condenser microphone. L1 is obtained by wrapping 8 turns of Φ51mm high-strength enameled wire on the ballpoint pen core. It resonates in parallel with C4 and reaches about 100MHZ. The antenna uses a flexible wire about 80mm long. The entire circuit can be built on a circuit board with a size of about 3×2mm. An ordinary FM radio should be able to clearly monitor slight sounds within 5M around the microphone at a distance of 100M. In order to work stably for a long time, the circuit is powered by a 3V battery, and the whole machine consumes 8-10MA current.