Four Stage FM Transmitter

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built round transistor BF200 (T2), a driver constructed round transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected at the enter of the oscillator.
Circuits diagram :

Four-Stage FM Transmitter Circuits Diagram

Working of the circuit is inconspicuous. When you discuss close to the microphone, frequency-modulated alerts are bought at the collector of oscillator transistor T1. The FM alerts are amplified via the VHF preamplifier and the pre-driver stage. You may additionally use transistor 2N5109 rather than 2N2219. The preamplifier is a tuned class-A RF amplifier and the using force is a class-C amplifier. Signals are ultimately fed to the class-C RF energy amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground airplane 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 modify trimmers VC2 thru VC7 to get maximum output at most vary. 

Regulator IC 78C09 offers stable 9V provide to the oscillator, so edition in the provide voltage won't have an impact on the frequency generated. You can also use a 12V battery to energy the circuit.

Assemble the circuit on a general-purpose PCB. Install the antenna properly for maximum vary. Coils L1 via L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and seven turns of wire, respectively.  

EFY note. This transmitter is meant handiest for educational functions. use of this transmitter with outdoor antenna is against the law in most sections of the arena. The author and EFY might no longer be chargeable for any misuse of this transmitter.

Author : Pradee G.  - Copyright : EFY

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Wireless FM Transmitter

The wireless fm transmitter circuit described here has an extra RF power amplifier stage, after the oscillator stage, to raise the power output to 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this wireless fm transmitter can provide reasonably good signal strength up to a distance of about 2 kilometers.
The wireless transmitter circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicose diode circuit is included to change the frequency of the fm transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.
FM wireless transmitter circuit diagram For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet.
Coil winding details are given below:

L1 – 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 turns of 24 SWG wire near top end of L1.
(Note: No core (i.e. air core) is used for the above coils)
L3 – 7 turns of 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)

Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the wireless fm transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.
This fm wireless transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a licence is illegal in most countries.

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