100w+?? Power Amplifier

No its not an April fools joke. I actually am going to build an rf amplifier capable of over 100w. Sacrilege i know for all the qrp’s out there. But there comes a time when everyone has to have a go at making some power. To date the most power I have made is a little over 10w and typically i would be happy with this kind of power. But for some unknown reason, the other day I decided while waiting for receiver bits, to design and layout a pcb capable of doing a few hundred watts or a little more. Actually, the board should handle 400w comfortably. The limiting factor being the T106 toroids wont handle much more than that on a good day.

The schematic can be viewed here: vk4hat_amp

Now, i do not claim credit for the design. A lot of it was directly copied from a 6m amp design by OZ1PIF http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm all i did really was add my own spin on it. Grounding the unused side of the Omron G2RL relays for better isolation and the relay itself was recommended by Mark VK5EME from https://www.minikits.com.au/ and other bits of inspiration were taken from people like LZ2JR http://lz2jr.com/blog/index.php/mosfet-linear-power-amplifier-50-watts/ from whom i learned about the FQP13N10 mosfet that lead me to a lot of Cb’rs who use that same transistor in a lot of their illegal amps. From what I have read, its a superior transistor to the IRF510 and should give better output. I will not be disappointed if i do not make a lot of power out of this, but somewhere between 100 and 300w should definitely be on the cards with a supply voltage over 24v.

They say short leads and short traces make everything better. You can see in the 3d view from my ecad how I plan on mounting the transistors. The tab on both the IRF510 and the FQP13N10 is connected to the drain, so i will, with the aid of tab under the mounting screws, soldered directly to the pcb. Other than going bonkers with the via’s its a pretty standard affair with a single 7th order lowpass filter. Yes, I am making this for one band only. I will likley made 2 of them, one for 40 and one for 20m, but making this multi band is just way to spendy. For each band you are adding $50 in relays, toroids and capacitors to the build cost. There is just no way to make power cheap. If the board turns out ok and the amp is a good thing and works. I will probably release the gerbers and let everyone have at it and build their own.


13 thoughts on “100w+?? Power Amplifier

  1. Looks like a nice project, will be interested to see how you get on.

    Just a small correction – the tab of both the IRF510 and the FQP13N10 is connected to the Drain, not the gate.

    1. Hi Terry, yeah proof reading is not a skill i am very good at HAHA. I have changed quite a bit since posting this and fixed some errors in the schematic.

      At the moment I am pondering adding feedback, but have not found a suitable answer to the question, do i treat the parallel transistors as one and have just a single feedback network, or do i really need four of them. I tried to simulate this in LT Spice but could not get my simulation to work. I can see myself reverting to old school and building something ugly style first to make sure its working as planned before i send off for boards to be made.

  2. Looking at OZ1PIF’s original, I note he has the capacitors across the output transformer that in this iteration at least you haven’t made allowance for on the pcb.

    As for feedback network, I would treat each bank of 4 as a single entity, so only one network each (assuming you actually need it in the first place).

    Also don’t forget the Gate stopper resistors (4R7), they stop the FET’s going into ultrasonic oscillation and blowing themselves to bits.

  3. Hi Rob,
    Just wondering if there’s a chance I could get the gerbers ? , or maybe the Kicad PCB file for this amp.

    I see that you have posted the schematic for it. but, I am not experienced enough with Kicad to recreate it .
    It would be a great learning experience for me .

    1. it is fundamentally flawed and I never got around to fixing it. There is a smaller version on here which does work. I think i released the files for it.

      1. Thanks for the quick response,

        If your willing , I would still like the pcb file if you are willing to share, to fool around with , I think I could learn from it.


        1. I would not give you something that does not work, it defeats the purpose and there is not much to learn from a broken design. Aside from that, i do not have the design filed for it, it was modified into something that did work. So there are no files for this anymore.

  4. Rob,

    Wondering what you meant by “fundamentally flawed”? How? We’re interested in hearing about your experience with a MOSFET amplifier with low Vds.



    1. The board design was flawed in a couple of ways, the mounting method for the transistors meant it was not easy to replace them when they exploded among other things. But that was the main one.

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