Simple 2-Transistor High-Power FM Transmitter (2SC2407 & 2SC3950)
Building a stable, high-power FM transmitter from scratch can often be challenging due to component availability and frequency drift. However, this simple 2-transistor circuit offers a reliable, low-cost solution using easily accessible parts.
Circuit Highlights & Transistor Lineup
First Stage (Driver): Uses the 2SC2407 transistor (Alternative options: 2N2219, 2N3866, or 2N2222 can also be used).
Final Stage (Power Amplifier): Uses the 2SC3950 power transistor (Alternative options: 2SC1971 or 2N353).
Tuning & Inductors: Uses readily available RF Chokes (RFC) for the transistor collectors—specifically 0.56 $\mu$H or 0.68 $\mu$H—eliminating the need to hand-wind complex coils. A 2–30 pF variable capacitor (or a fixed 20 pF capacitor) is used for tuning.
Audio Input & Biasing Tips
Source: The audio/RF input is taken directly from a Car MP3 Modulator.
Input Boosting: If the RF output power from the MP3 modulator is too weak to properly drive the first transistor stage, you can add forward biasing. Connect a 30k to 56k resistor from the positive supply rail to the transistor base to boost the incoming signal level.
Antenna Tuning Guide
PCB Labels: The antenna connection point is usually clearly printed on the PCB layout.
Using an FSM: If the antenna point is not marked, you can use a Field Strength Meter (FSM). Adjust your circuit until you achieve the peak FSM reading to ensure maximum RF power transfer to the antenna.
