Here it is shown how to build a simple single transistor FM Transmitter. The transistor used is 2N3904 general purpose transistor. Other than this single transistor we need just few passive components and battery.
The following shows the FM transistor build with one transistor.
The corresponding schematic diagram is shown below.
In the above circuit diagram, the audio is enters the FM transmitter via the electret condenser labelled MIC1. The resistor R1 provides the voltage required for the electret microphone to operate. The audio from the microphone transducer into the base of the transistor via the coupling capacitor C1. The transistor is biased using emitter biasing technique which was explained in the tutorial How to design emitter biased BJT amplifier.The resistor R2 is for biasing the base of the transistor, that is to turn on the transistor so that it operates in the active region of operation. Similarly we bias the emitter of the transistor using the resistor R3 so that there is negative feedback and the output is stable to compensate the changing beta of the transistor due to environment temperature. The emitter bias resistor R3 is also ac bypassed using the bypass capacitor C4. L1 is air core inductor made up of 5 turns of laminated copper wire with 5mm inner diameter. The capacitor C6 is a variable trimmer capacitor of 10pF to 40pF. The inductor L1 together with C6 and C3 forms an oscillator in colpitts oscillator configuration. This oscillator is designed to oscillate in the FM frequency range 86MHz to 106MHz. The capacitance of the trimmer capacitor can be changed to get different frequency in the FM radio frequency range. The capacitor C5 is a coupling capacitor
that couples the generated FM signal to the antenna. The antenna is a
25cm wire.
