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3V FM Transmitter for Short Distance 88MHz to 108MHz

3V FM Transmitter for Short Distance 88MHz to 108MHz

The important part of the circuit is formed of the Colpitts type oscillator. C3,C4,C5,C6,CD1-CD2 and L1 determines the frequency. BF982 and dual gate Mosfet are active parts in the oscillator. When the input impedance of the Mosfet gate inputs are high, LC tank is not affected.However transistors force the LC tank and cause phase shift.
Features of the transmitter: Supply Voltage:+3V DC Current: 21mA Output Frequency: 88MHz – 108MHz Modulation: FM Frequency Shift: ±30KHz Input: Electret microphone or other input audio source

Two driver stages are added to isolate the antenna from oscillator. First stage (BF199) amplifies the low signal of the oscillator and works as a constant load. The second stage (BFR90) amplifies the signal going through the antenna some more. A short copper wire can be used as an antenna here. Attaching a large antenna to this circuit is unnecessary because the output power is low

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