Circuit : Paul K Sherby, Belleville, Michigan
A four transistor transistor with FM modulation. Output power about 500mW-
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.