Homemade 50W Amplifier

Discussion in 'Homebrew and Kit Projects' started by KK9JEF, Jul 14, 2018.

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  1. KK9JEF

    KK9JEF Ham Member QRZ Page

    I'm certainly not the first to walk this path, but I made some good progress tonight on a 50W portable amplifier. It's very much in the vein of the classic WA2EBY IRF510-based amp, with some details borrowed from G0KLA and VU2ESE's related projects. 5W in, 50-60W out.

    My only real novel idea is to insert a switch to allow the FETs to be driven either from a single 13.8V source or two in series, to provide multiple options for portable operating. At 13.8V, it puts out about 15W.

    Lots of work yet to be done, but I made a quick progress video, just so exciting to pump 50W out of a homemade circuit for the first time, without anything blowing up:

    N1OOQ, AC0OB, KB4QAA and 1 other person like this.
  2. KK9JEF

    KK9JEF Ham Member QRZ Page

    Started working on a PCB for this project today, including schematic capture. I did this as a live stream, so the video's almost two hours long (!), but might be of some interest. I got through the full schematic capture but didn't get to the actual component layout

    First time trying a live stream - pretty easy to set up, and fun!

  3. WB0RIO

    WB0RIO Ham Member QRZ Page

    That looks like a fun project, keep in mind that the output impedance of the amplifier will change when the VDD voltage is doubled so you
    may need a way to tweak the values on your low-pass output filter when you change the power supply voltage.

    A lot of MOSFET RF amp designs use fixed-component LPF circuits, I've always liked the ability to adjust the LPF capacitor values.

    When I built the amp on this project: http://www.solorb.com/elect/hamcirc/28mbeacon1/index.html
    I was able to adjust the LPF for much higher output power while keeping the harmonic content within FCC regs (-43dB).
    KK9JEF likes this.
  4. KK9JEF

    KK9JEF Ham Member QRZ Page

    Things are still moving along on the 50W amplifier. Here's the schematic (as captured above):
    KK4CUL likes this.
  5. KK9JEF

    KK9JEF Ham Member QRZ Page

    Still chugging along with the QRP Amplifier. First draft of the PCBs arrived this week and assembly is under way.

    I’ve made a couple of additions to the schematic since the original layout, including a relay-activated indicator (R27 and its LED) and an RF-output sensing LED (from C14 to its associated LED to ground) along the the lines of VK3YE’s recent project. There’s also a space on the PCB now for a low pass filter with the same footprint as Hans Summers’ LPFs over at QRP-Labs. Not that you’d necessarily want to reuse a QRP LPF for a 50W amp, you’d be in danger of putting too much voltage on the caps, but that would be a simple change.

    Here’s the schematic as it exists now:

    And here's the current PCB layout (layout, unpopulated, populated):




    I’ve already got a little laundry list of things to modify for a second rev of this board, including, in no particular order:

    • Swap the Diode placements the vertical to preserve board space
    • Add footprints for alternate relay packages
    • Add footprints for alternate trim-pot packages
    • Re-think component designators for clarity
    • Add bypass jumpers for the 3dB input pad and the LPF.
    • For some reason, the none of the component values printed on the silkscreen, will need to sort that out
    • I’m not sure if I screwed up how to designate a cutout or if JLCPCB doesn’t do them for its bare-bones PCB service, but I’d like not to do the next set with a drill press and a nibbler.
    W1BR, WB0RIO, KB4QAA and 1 other person like this.
  6. VK4HAT

    VK4HAT XML Subscriber QRZ Page

    Was the cutout specified in the outline and had no fill? I can see it has a plated through hole, so i guess they assumed it was a filled area, not a cutout, hard to drill a hole in a cutout LOL. I always add text in my cutouts and make sure there is no fill.

  7. KK9JEF

    KK9JEF Ham Member QRZ Page

    VK4FFAB, thank you for the suggestion - I can see how it would have been unclear in my gerbers that the center area should be cut out. When I spin up Rev2, I'll try your suggestion.
  8. N7EKU

    N7EKU Ham Member QRZ Page

    Hi Jeff,

    One thing I might do is the try to test this board fully before moving on to another revision. I saw you left out the inductors on the gates which were intended for input impedance matching on the upper bands, so this may be a concern. Also, the trace length, to me, seems a bit long and of uneven lengths both on the input to the finals and on the outputs; this may affect amp performance also.


  9. VK4HAT

    VK4HAT XML Subscriber QRZ Page

    50 ohm traces also on all the RF paths also would be a nice touch as well as having the mounting holes connected to ground, so using brass standoffs connects everything to chassis.
  10. K6BSU

    K6BSU Ham Member QRZ Page

    At HF, stripline traces are not necessary. Some concern about the small traces in the high current places. Is there enough cross sectional area in those traces for current required at 50 Watts?

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