• UnknownEditor
    Suppose you are designing an SSPA by combining four GaN MMIC amplifiers. You are considering a reactive power combiner... if the amplifiers behave the same in phase and amplitude, they will see the same impedance match that the power combiner output provides (generally quite good). If a single amplifier fails, the whole assembly is going to catch on fire, right? But how non-ideal can the amplifiers behave before the start to load-pull each other? Is there a textbook or article on this topic? Or should we try to put some analyses together on this site to explore the limits of reactive power combining?

    Thanks for your thoughts.
  • madengr
    It probably depends on how a single amplifier fails. Say you had a 1/4 wave combiner (i.e. a Wilkinson without the resistor). If one amp fails with a short circuit at its output, that places an open circuit at the summing node, so the working amplifier just sees a 100 Ohm impedance mismatch. If the amp fails with a short circuit, the working amp sees an open circuit. If you added additional quarter wave lines at the amps, a short failure would present a short to the working amp. Perhaps a short is a better failure than an open since you have over-current (thermal problem) rather than breakdown from over-voltage.

    I’m sure there’s topologies that optimize degradation. I wonder if quadrature would be better.
  • UnknownEditor

    there will likely be some routing between the combiner and the amplifiers, so I consider the phase angle of the failure unknown, and possibly a strong function of frequency. The failure could be a short circuit somewhere back inside the amplifier, or a blown wirebond, or almost anything. For sure, adding the isolation resistor will help the remaining amplifiers see a decent match, with the caveat that the resistor does not blow up...

    I will take a look at the reflection coefficients that are seen by constituent amplifiers in a combiner, when the amplifiers have random phase and amplitude errors, and see what that tells me..
  • Michael Roberg
    Here is a related article to the topic.
    TrMTTMay92-2 (394K)
  • UnknownEditor
    Mike... that is an excellent article. At first glance I don't see any mention of the amplifiers load-pulling each other in a reactive combiner. I think that could be a significant effect.
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