Circuit : Kamran Ahmed - UK
An low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier.
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
IC1 Dual Op-amp LM358
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
Q1 is configured as a Clapp oscillator. Frequency modulation results from
the audio voltage changing the transistor's base-emitter capacitance.