This four stage FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected to the input of the oscillator.
Four stage FM transmitter circuit
Working of the circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna. Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also, adjust trimmers VC2 through VC7 to get maximum output at maximum range.
Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the frequency generated. You can also use a 12V battery to power the circuit.
Construction & testing
Assemble the circuit on a general-purpose PCB. Install the antenna properly for maximum range. Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively.
This transmitter is meant only for educational purposes. use of this transmitter with the outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.
The article was first published in January 2010 and has recently been updated.