The following is the circuit diagram of the FM wireless microphone. The high-frequency transistor V1 and the capacitors C3, C5, and C6 form a capacitor three-point oscillator. For beginners, we do not need to ponder the specific working principle of the capacitor three-point oscillator for the time being. We only need to know that this circuit structure is a high-frequency oscillator. The load C4 and L of the triode collector form a resonator. The resonant frequency is the transmitting frequency of the FM microphone. According to the parameters of the components in the figure, the transmitting frequency can be between 88 and 108MHz, which just covers the receiving frequency of the FM radio. By adjusting the L The numerical value (stretching or compressing the coil L) can easily change the transmitting frequency and avoid FM radio stations. The transmitted signal is coupled to the antenna through C4 and then transmitted out.
R4 is the base bias resistor of V1, which provides a certain base current to the transistor so that V1 works in the amplification area. R5 is the DC feedback resistor, which stabilizes the operating point of the transistor.
The frequency modulation principle of this FM microphone is to achieve frequency modulation by changing the capacitance between the base and emitter of the triode. When the sound voltage signal is added to the base of the triode, the base of the triode and The capacitance between the emitters will change synchronously with the size of the sound voltage signal, and at the same time, the emission frequency of the triode will change, achieving frequency modulation.
It’s just the opposite, the microphone picks up the sound and the speaker amplifies it.
Dynamic microphones use the principle of electromagnetic induction. The wire coil is mounted on the diaphragm, and then placed between the magnetic field of the magnet. As the sound pressure changes, it continuously moves in the magnetic field to generate an induced current. Convert sound signals into electrical signals. Its transduction principle is very similar to that of a speaker.
Audix dynamic microphone