This circuit provides an FM modulated signal with an output power of around 500mW. The input microphone pre-amp is built around a couple of 2N3904 transistors (Q1/Q2), and audio gain is limited by the 5k preset trim potentiometer. The oscillator is a colpitt stage, frequency of oscillation governed by the tank circuit made from two 5pF ceramic capacitors and the L2 inductor. 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.
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 (L1) which will have a high impedance at RF frequencies.
All resistors are 1/4 watt 5%
R1,R2,R8 = 1K C1 = 1uF/63V, electrolytic
R3 = 100K C2,C3 = 10nF, ceramic
R4 = 150K C4,C5,C9 = 4.7uF/63V, electrolytic
R5,R7 = 10K C6,C12,C13,C14 = 1nF, ceramic
R6 = 220 ohm C7,C8,C11 = 5pF, ceramic
R9 = 10 ohm C10 = 220uF/63V, electrolytic
P1 = 5K trimpot
Q1,Q2 = 2N3904 L1 = 3.9uH
Q3,Q4 = 7001, NTE123AP L2 = 1uH
D1 = 1N4002 L3 = aircoil, 8.5 turns air space
1/4 inch diameter