4 Stage FM Transmitter
This is the FM transmitter circuit which apply 4 radio frequency stages, that are a VHF oscillator designed around transistor BF494 (T1), a preamplifier designed around transistor BF200 (T2), a driver designed around transistor 2N2219 (T3) and also a power amplifier designed around transistor 2N3866 (T4). A condenser microphone is wired at the input of the oscillator.
The Operational of this FM transmitter circuit is very simple. At the time you speak close to the microphone, frequency-modulated signals are produced at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and also the pre-driver stage. You’ll be able to also use transistor 2N5109 as a placement of 2N2219.
The preamplifier works as a tuned class-A RF amplifier while the driver works as a class-C amplifier. Signals are finally fed towards the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna.
Regulator IC 78C09 gives you stable 9V supply to the oscillator, so variation in the supply voltage is not going to have an impact on the frequency produced. You are able to also use a 12V battery to power the circuit.
Use a heat-sink with transistor 2N3866 for heat dissipation. Alter the trimmer VC1 connected across L1 to produce frequency within 88-108 MHz with care. Also alter trimmers VC2 through VC7 to obtain maximum output at maximum assortment.
Assemble the circuit on a generalpurpose PCB. Set up the antenna correctly for optimum range. Coils L1 through L5 are built with 20 SWG copper-enamelled wire wound over air-cores that have 8mm diameter. They will have 4, 6, 6, 5 and 7 turns of wire, respectively.
This 4 stage FM transmitter tested by Electronics For You mag and should be work. You can download this circuit in PDF version from the following link:
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