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.
Microphone preamp for the fm transmitter
Active components for fm transmitter circuit
88 108 MHz transmitter coil data