## Reactive power combiners

• 4
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.
Steve
• 1
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.
• 4
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..
• 0
Here is a related article to the topic.
• 4
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.
bold
italic
underline
strike
code
quote
ulist
image
url
mention
reveal