Miniature FM Transmitter #5

Parts List:

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
Circuit Notes:
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 Colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF ceramic capacitors and the L2 inductor.
(Click here for Colpitts 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 (L1) 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
Website: Paul's Website

Some substitutes for the 2N3904 are: NTE123AP, 2N4401, BC547(watch lead orientation), etc. The 7001 (Q2/Q3) oddly enough also reference to the NTE123AP. Although the 2N3904 is very common and easily available. -Tony

Back to Circuits page
Page copyright © 1995, Tony van Roon
Article copyright © 2001, Paul K. Sherby