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.

Thanks

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..

Here is a related article to the topic.

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.