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I am looking at the QPA2609 amplifier's datasheet. I noticed that the Output Return Loss is about -20 dB at 13 GHz and the reverse isolation is around -60 dB at 13 GHz.

My understanding is that output return loss (dB) should tell me the ratio of the power incident on the output to the power reflected back to the load and reverse isolation tells me how much of the power incident at the output appears at the input port. My question though is how can both of these numbers be so low? Where is the power incident on the output port all going? Is it dissipated by the LNA?

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  • \$\begingroup\$ Are you sure the "loss" and "isolation" are negative, implying they are amplified? (Is the datasheet sure? It's shocking how often this is gotten wrong..) \$\endgroup\$ Commented Aug 18, 2023 at 18:07
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    \$\begingroup\$ @TimWilliams "here is how I address the problem. Notice I label the left axis -Return Loss. The - is the design review smack-down; pardon my wrestling terminology. Normally it's not even noticed, but if someone complains about return loss being negative, I smack them down with -. " \$\endgroup\$ Commented Aug 18, 2023 at 20:53
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    \$\begingroup\$ @比尔盖子 Dear god, it actually comes (in part) from an EIA standard getting it "wrong"? No wonder so many else do. :facepalm: \$\endgroup\$ Commented Aug 18, 2023 at 20:58

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The output return loss is essentially how well matched the output impedance is while the isolation is the amount of signal going from the output to the input.

How can both of these numbers be so low? The matching at -20dB is nothing to write about. It is a good matching. It can be achieved quite easily with a matching network. The isolation is also in the ballpark you'd expect an amplifier to be.

Where is the power incident on the output port all going? It is mostly dissipated by the amplifier. A small part will pass through but most of it will be dissipated by the amplifier itself. That is one of the reason you should never leave an unterminated amplifier on.

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