Homebrew Mosfet Amp Build

Discussion in 'Homebrew and Kit Projects' started by KF5MSU, Sep 20, 2016.

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  1. KF5MSU

    KF5MSU Ham Member QRZ Page

    I'm in the design stages of a Mosfet Amp Build. I'm having trouble figuring out how to bias the mosfets and how to setup a matching circuit. Not quite sure at this point which mosfets will be used in this build. I'm looking at either the IRF520 or the ERF7530. Any help with these problems would be greatly appreciated. This is my first build of course so I'm green behind the ears. Also not quite sure how to get the RF input into the Mosfets come to think of it.
  2. VK4HAT

    VK4HAT Ham Member QRZ Page

    Here is a good PDF on how to bias different transistors etc: http://vk4ffab.info/useful-docs/biasing-circuits-for-rf-devices/

    If going down the IRF path, 510, 530, 540 etc, here are a couple of tried and true amp designs to get you on the right track>


    I have built both of the above and had them work successfully, Google about for the WA2EBY article from some years ago and work from there. It explains a lot and will help you understand how the amp works and why he did what he did. Google for the PDF its on the net somewhere. There are 100 designs on the net, all essentially the same.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I bet that 100uf 25 watt cap is hard to find. :D
    WB1E, KF5MSU, KS4W and 1 other person like this.
  4. KF5MSU

    KF5MSU Ham Member QRZ Page

    Thanks for the info guys I appreciate it very much. I have been wanting to learn Amplifier design for quite some time now. I figure what a better way to do it than to just get my hands on it and get the right information.
    AF6LJ likes this.
  5. W0RIO

    W0RIO Ham Member QRZ Page

    You might be interested in the power amp section of my 10 meter beacon:
    The bias circuit uses an LM317LZ adjustable regulator.
    Unlike a lot of published circuits I've seen, the adjustment pot has a much finer control over
    a range of useful bias voltages.

    As for transistors, check out the Hitachi RD16HHF1 MOSFET and related parts.
    These have the source pin connected to the tab so you don't have to mess with
    electrical isolation from the heat sink and they are much harder to damage than the IRF devices.
    RF Parts sells them, among others.

    Also, be careful to not skimp on the size of the output transformer core, if it heats up
    then it is saturating and generating harmonics.
  6. BG6GLL

    BG6GLL QRZ Member

  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    It's a repacitor.

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