This circuit provides an FM modulated signal with an output power of around 500mW. The input
Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset.
The oscillator is a colpitts stage, frequency of oscillation governed by the tank circuit made from
two 5pF capacitors and the inductor. ( Click here for Colpitt Oscillator Resonant Frequency Equation.)
Frequency is around 100Mhz with values shown.
Audio modulation is fed into the tank circuit via
the 5p capacitor, the 10k resistor and 1N4002
controlling the amount of modulation. The oscillator output is fed into the 3.9uH inductor which
will have a high impedance at RF frequencies.
The output stage operates as a class D amplifier , no
direct bias is applied but the RF signal developed
across the 3.9uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor
prevent instability and thermal runaway in this stage.
Paul K. Sherby