Two years in the making – 70MHz 300w Amplifier

70MHz 300w MOSFET Linear Amplifier

Today, I finished a project I started over two years ago. It’s not that it took two years to build, it’s just that its taken me two years to get the parts together and build in my spare time! The upcoming 2nd 70MHz contest and PW 70MHz contest prompted me to get this finished.

Its a 300w 70MHz amplifier, using a BLF 278 MOSFET. Power supply is 50v @ 10A. The amplifier has 9dB attenuation at the PA input to bring the gain down to something usable, as without the attenuation, 2.5w input gives 300w output (21dB gain)! The input & output power, with 9dB ATT (total 12dB gain) is:

  • 5w = 100w
  • 7.5w = 150w
  • 10w = 200w
  • 15w = 300w

The amplifier uses ~7.5a @ 48v for 200w O/P, so it’s around 80% efficient. Bias is set to 750mA. The maximum power here in the UK on 70MHz is 160w, for which this amplifier requires 8w of input. Below is a picture of the inside. I will tidy up the cables at some point!

300w PA & LPF

300w PA & LPF

Bird 43 - 70MHz 250w+

I need a larger Bird element to measure the 300w output on the Bird 43!

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  1. Nice and clean inside solder cool project you made there…….

  2. Hi John. I have a couple of the BLF devices and it might be a good project for me to build an amp like yours. Do you have any details you’d be happy to share? Thanks, Simon

    1. Hi Simon.

      I bought the board without the MOSFET, so if you have some, it might be worth looking for a PCB less MOSFET on the band you want.

      Good luck, John.

  3. Where did Jon find tne LPF / Directional coupler PCB John?

    1. Hi David.
      I bought this from
      Good luck! John.

  4. Hi. Hey. Are you at some point giving up that 70Mhz power amplifier? I would be interested in buying it. Terho OH2FR

    1. Hi Terho.

      Well, yes maybe. I’ve been inactive for a while now, just don’t seem to have the time. Feel free to drop me an email!

      Cheers, John.

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