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Fm transmitter with MOSFET

2016-04-04 15:32  
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This mosfet fm transmitter circuit is designed to be used for aligning FM radio receivers. It has a microphone amplifier with automatic gain control. The FM modulation is a voltage oscillator and the final stage is built with MOSFETs. A lowpass filter at the output assures the transmission of a clean 10 … 50 mW FM signal. IC1 works as an amplifier supplying a negative control voltage to FET T1. The amplitude of the amplified audio signal can be adjusted through P1. The carrier frequency is generated by the oscillator T2.

The oscillation is frequency modulated through the internal capacitance changes of the double diode D4/D5. The frequency modulated appears at the drain terminal of T2. This signal is then amplified by T3 and passes through the lowpass filter before finally arriving at the antenna.

FM Transmitter Circuit Diagram

fm transmitter schematic
Microphone preamp for the fm transmitter
mic preamp fm transmitter
Active components for fm transmitter circuit

T1 = 2N3819, 2N3823, 2N5397T2 = T3 = 3SK35, 3SK37IC1 = 741

88 108 MHz transmitter coil data

L1 = 6 turns 0.8mm magnet wire around T50-12 ferrite ring, tapped at 2nd turn from groundL2 = same as L1 but tapped at 3rd turn from groundL3 = same as L2 but tapped at 1st turn from groundL4 = 4 turns 0.8mm magnet wire in a ferrite core
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