FM Transmitter Circuit

Posted in FM transmitter, on 2015-08-20


An low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier. 

FM Transmitter Circuit

Parts List

  • R1 4K7
  • R4 150K
  • R7 3K9 (2K7)
  • R2 4K7
  • R5 220R
  • R8 120R (82R)
  • R3 4K7
  • R6 4K7
  • All resistors except R8 are at least 0.25W rated. R8 is at least 0.5W rated (the 0.6W metal film M-series from Maplin can be used for R1-R8).
  • C1 1n
  • C4 22uF
  • C7 10n
  • C10 1n
  • C2 4u7
  • C5 1n
  • C8 1n
  • C3 1n
  • C6 10n
  • C9 33pF
  • VC1 5-60pF
  • IC1 LM358
  • Q1 ZTX108


L1 is 0.112uH (this tunes to the middle of the FM band, 98 MHz, with VC1 at its centre value of 33pF).

L1 is 5 turns of 22 swg enamelled copper wire close-wound on a 5mm (3/16") diameter former. Alternatively, you can have a fixed 33pF cap instead of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 will give you a tuning range of 85 - 125 MHz, and a possible choice is the Philips type polypropylene film trimmer (Maplin code WL72P).

Two sets of oscillator bias resistors are given, the ones in the brackets give about 20% more RF power.

Mike is our favourite Omnidirectional sub-mini electret (Maplin code FS43W). Ant is a (lambda / 4) whip monopole (eg 76 cms of 22 swg copper wire).

Q1 is configured as a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor's base-emitter capacitance. 

If you like this page, please consider sharing


Source , Author: Kamran Ahmed - UK