40W FM transmitter

Here’s a 40W amplifier for band 2 FM transmitter applications (88 to 108MHz), taking an input of 0.5 to 1 Watts of RF power.

This is a two stage design using 28v devices – a 2N3375 driving a 2N5643 for the final stage, but there are many other devices that will work. I used these 2 transistors just because they were cheap at the time.

If any of the variable capacitors are at full capacitance you can pad them out with a fixed ceramic capacitor of suitable value. Extra capacitance also might be needed on the base of the transistors (I had to add 3 100pF capacitors on the base of the 2N5643) depending on the frequency you are operating on.

This is probably the most basic RF amplifier design and can be build using copper clad board with pads cut out using a cutting tool – see the picture below. The transistors are bolted to a piece of right angle aluminium that’s fixed to the metal chassis to dissipate heat effectively.

40W amplifier for FM transmitter – circuit diagram

Fitted into a complete FM transmitter

I added a basic linear power supply with toroidal mains transformer along with forced cooling from a brushless DC fan. The amplifier is driven with a PLL FM transmitter driver board, delivering 0.5 to 1Watts of power to the amplifier. For more information on this driver board see here: 1W PLL FM transmitter overview