Two transistor FM transmitter.

Description.

A lot of FM transmitter circuits have been already published here. This just another one, a simple two transistor FM transmitter.The first stage of the circuit is a preamplifier stage based on transistor Q1. This is a collector to base biased amplifier stage where resistor R2 sets the collector current and R1 provided the necessary collector to base bias. C1 is the input DC decoupling capacitor which couples the input audio signal to the Q1 base. C8 is the power supply by-pass capacitor. Next stage is the oscillator cum modulator stage built around transistor Q2. Electrolytic capacitor C2 couples the output of the first stage to the second stage. R3 and R4 are the biasing resistors of Q2. R5 is the emitter resistor of Q2. Inductor L1 and trimmer capacitor C5 forms the tank circuit which is necessary for creating oscillations. The modulated FM signal is available at the collector of Q2 and it is coupled to the antenna using capacitor C9.

 

Notes.

• The circuit can be powered from anything between 6 to 12V DC.
• Using battery for powering the circuit will improve the performance and reduce noise.
• A 9V PP3 battery is a good option.
• If you are going with a battery eliminator, then it must be well filtered and regulated.
• Trimmer C5 can be used for adjusting the transmission frequency.
• Antenna can be a 1m copper wire.
• L1 can be constructed my making 4 turns of 1mm enameled copper wire on a 10mm diameter plastic former.
• Trimmer capacitor C6 can be adjusted for obtaining the maximum range.
• Most of the components required for this circuit can be procured from your junk box.