FM transmitter circuit using C9018 transistor

FM transmitter circuit is a quite fun project for electronics beginners, so here's a circuit with the C9018 transistor.

In a previous post I've wrote about a stable and simple miniature FM transmitter circuit, anyway it was a good one.

This circuit uses the C9018 high frequency transistor, based on a different spin of the common base Collpit's oscillator.

Single transistor FM transmitter circuit diagram

The circuit is rather simple, uses only one transistor and few passive components.

Here the RF feedback is taken from the center tap of C2 and C3, close to original Collpit's oscillator.

You can calculate the effective capacitance of the tuned LC circuit by determining the series capacitance of C2 and C3.

If both C2 and C3 are equal, the effective capacitance will be the half.

FM transmitter circuit Part list and construction

Here's the component list,

1. Q1 - C9018 NPN transistor
2. R1 - 470 Ohm, 1/4 watt
3. R2 - 10 kOhm, 1/4 watt
4. C1 - 2.2 nF ceramic disc capacitor (222)
5. C2 - 30 pF ceramic disc capacitor, (30)
6. C3 - 30 pF ceramic disc capacitor, (30)
7. C4 - 10 pF 30 pF ceramic disc capacitor, (10)
8. C5 - 1 nF 30 pF ceramic disc capacitor, (102)
9. C6 - 100nF 30 pF ceramic disc capacitor, (104)
10. L1 - 16 turn on  3 mm diameter

This circuit uses only 10 parts including the coil. The coil is winded on a 3 mm drill bit, 16 turns of 0.4 mm (28 S.W.G) enameled copper wire.

It's very easy to construct the circuit on a strip board, just few components to solder. Always try to minimize the track length.

My prototype without the audio source connected, small enough to fit on a matchbox. Fix the inductor with some wax for better frequency stability.

This circuit runs on a 3.3 Volt regulated power supply.

Testing the FM transmitter circuit

This FM transmitter circuit performed well in terms of frequency stability, almost zero drifting after about 4 hours of continuous operation.

But still digital FM receivers can't tune to this transmitter, as it's not a PLL based design.

There's no antenna connected to the transmitter, but you can easily receive clear signals from 10-12 meter distance. After that, the signals starts fading rapidly.

Conclusion

So, that's all for this FM transmitter circuit project. As the transistor supports very high frequency range, you could expect significant amount of harmonics along with the main frequency. That's basically you can listen to this transmitter even in the commercial VHF/UHF TV frequency spectrum. Which is undesirable in this case.

If you have any suggestion or question, just drop a comment.