K3 vs. K3S 10W PA

Discussion in 'General Technical Questions and Answers' started by KK5JY, Feb 6, 2018.

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  1. KK5JY

    KK5JY Ham Member QRZ Page

    I was comparing the schematic diagrams of the 10W "low-power" PA circuits between the K3 and K3S, and I noticed a change that made me curious. The K3 PA has three FETs -- one RD06 driver, and two RD15 finals in a push-pull configuration. The driver pushes the finals through a transformer.

    In the K3S, there are still the two finals in push-pull, but instead of being driven by a single low-power FET, the finals in the K3S (and in the KX3) are each independently driven by a "gain block" op-amp.

    Schematics are below. The power devices are marked in red. I'm wondering if anyone has an idea about the advantage of the latter approach vs. the former?

    Also, I have marked some small inductors in green, that are placed on the input and output of the driver stage of the K3. I'm curious if anybody knows why these are necessary in the K3, but not around the driver stages of the K3S?

    K3 PA

    K3S PA
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    Ask them!

    My guess is that they found a way to lower the parts count and board price by using the latter method. Note the reduction in parts and adjustments.
  3. KK5JY

    KK5JY Ham Member QRZ Page

    My last several experiences with E-craft "support" were so horrible that I have no desire to repeat them. And my experiences asking them about circuit details before that were less than helpful.

    The circuits aren't complex, but there are obviously some subtleties that are just new to me. I figured somebody here would be able to spot them with no effort.
  4. W6UV

    W6UV Premium Subscriber QRZ Page

  5. KK5JY

    KK5JY Ham Member QRZ Page

    I think I have figured out the center green inductor -- when combined with the 47pF caps above and below it, it forms a Pi network with a cutoff frequency somewhere around 80MHz. So apparently, it's a low-pass filter between the driver and final stages. I'm not sure about the other one, though.

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