2n3904/PN2222A 150mW Stable FM Transmitter

Hi Harry,
Can you publish the design on your website?

Title should be "150mW FM TX using common parts"

Pic of the prototype. It was made with ugly bug construction technique on a piece of copper clad board. The large groundplane is ideal for RF circuits. The TX is feeding a 50ohm dummy load and output power is close to 200mW. I've also added an electret microphone which is biased by a 22K resistor from 12V and its output feeding the audio input of the TX via a 100n series capacitor. Audio is loud and clear.

Transistors T6, T7, and T8 can be glued to the copper clad board itself for heatsinking.

RF amplifier section. L4 = 3T, 20SWG on 8mm dia. air core.

L2 = 5T 20SWG 8mm dia. air core.

With DIP switch on, stretch coil L2 with your fingers for maximum output power (maximum voltage at point V = 3.8V for 180mW RF output into 50ohm load).

VC1 has been replaced with a 27p capacitor. This makes the design trimmer capacitor free!

T6 requires a small heatsink. A coin was superglued on top of it. T7 and T8 worked fine without one but output power fell by 25mW or so as T7 and T8 warned up. Another coin can be glued to them to keep them cool. Both transistors in the output stage are biased for 20ma standing current each (Class AB operation) for good gain and to minimize harmonics.

All RFCs are 70-100 turns of thin enameled copper wire on 10K - 33K 1/4W carbon film resistors. 1/2W resistors can be used to wind the RFCs.


This FM TX is far more stable than other FM transmitter circuits available on the internet. Oscillator current is kept low (3ma only) to reduce internal transistor heating and very little power (0.3mW) is drawn from the oscillator. This and the use of NP0 capacitors in the oscillator section greatly enhances stability. The three 5V6 zener in series is used as a varactor diode. Using a single 16V is not recommended as a single zener diode is also prone to thermal drift.

Gain of the amplifier was a flat 25dB from 88MHz to 99MHz and gave an output power of 180mW into a 50ohm load. Power output level remained unchanged when feeding a 75ohm dummy load.

No hard to find RF transistors have been used. 2n3904/PN2222A are cheap and universally available.
The goal was to design an FM transmitter using common parts with excellent frequency stability and an usable power output of 150mW . The prototype is delivering 0.18W at 99MHz after the 3 pole butterwoth low pass filter and meets the original design requirement.



Note: Use only PN2222A (Ft = 300MHz) for T7 and T8 PA stage. 2N2222 will work fine but power output will decrease as its Ft is only 250MHz.

The 100ohm resistor between T2 and T3 was increased to 220ohm for better frequency stability. The resistor represents a constant load to the oscillator via T2.

FM oscillator section

L1 = 0.15uH (9T 26SWG on 2mm dia. plastic former. Secure the coil with several layers of superglue).

C4 - C7 = NP0 type.
Stability is excellent. Center frequency should be 98 - 100MHz. VR1 provides 1.5MHz fine tuning. 50uS pre-emphasis is included.

Update: R6 should be reduced from 4.7K to 1K. This further improved frequency stability of the transmitter. Keep all leads short.