FM Transmitter Circuit with ZTX108


Fm transmitter circuit ZTX108

This is a small FM transmitter circuit. The circuit is using an op-amp chip LM358 as the audio preamplifier and a single power transistor ZTX108 as the RF amplifier.

Parts List:

R1 4K7
R2 4K7
R3 4K7
R4 150K
R5 220R
R6 4K7
R7 3K9 (2K7)
R8 120R (82R) 1/5W
IC1 = LM358
Q1 = ZTX108
C1 = 1n
C2 = 4u7
C3 = 1n
C4 = 22uF
C5 = 1n
C6 = 10n
C7 = 10n
C8 = 1n
C9 = 33pF
C10 = 1n
VC1 = 5-60pF

FM Transmitter Circuit Notes:

  • L1 is 0.112uH (this tunes for 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, it is possible to possess a fixed 33pF cap rather of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 provides you with a tuning assortment of 85 – 125 MHz, and also a possible option could be the Philips type polypropylene film trimmer (Maplin code WL72P).
  • Two sets of oscillator bias resistors are provided, the ones inside the brackets give about 20% extra RF power.
  • Mike is our favourite Omnidirectional sub-mini electret (Maplin code FS43W). The antenna can be a (lambda / 4) whip monopole (eg 76 cms of 22 swg copper wire).
  • Q1 is configured as being a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor’s base-emitter capacitance.

FM Transmitter Circuit source: http://www.zen22142.zen.co.uk/Circuits/rf/txcct.htm
Download the ZTX108 datasheet:
ยป Download Link

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