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Suppose that there is a 2 dBm interference signal and a -90 dBm signal of interest (SOI) at the input of LNA (Gain = 1, in linear region). Suppose that the P1dB of LNA is 1 dBm, so the LNA is saturated and can't amplify the input signal properly. My question is what the output of SOI would be like? Is it still -90 dBm? The interference and SOI are at the same frequency band.

tyrela
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  • You should definitely ask this question on the Electrical engineering stack exchange. The amateur Radio group is nowhere near answering this question. – Wireless Learning Apr 13 '22 at 07:49
  • OK, thanks for reminding me! – tyrela Apr 14 '22 at 03:39
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    FWIW I think this is a reasonable Q for this SE. If Amateur radio is anything, it is the practical application of theory for the purpose of education and problem solving. I recognize that this may also fit over on EE.SE (or even DSP.SE if the resulting signals were going to be sampled and inspected) but it is just as valid as a question related to the practical application of typical Amateur equipment. –  Apr 24 '22 at 15:39
  • It *could* have been asked at ee.se. However, it *is* on topic here because it is about *the technology of radio*. (Just don't double post. :-) – Mike Waters Apr 29 '22 at 21:05

2 Answers2

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As the amplifier gets closer to saturation, its gain reduces. This is because during the portion of time the transistor is saturated, it is not able to output any other signal. The signal of interest will therefore become weaker, or another way to think of it is the amplifier has less gain and proportionally higher noise figure.

The intermodulation products will also dramatically increase, faster than predicted by the third order intercept point (IP3) of the amplifier, because IP3 is characterized in the linear region and no longer applies as the amplifier is pushed into saturation.

S H
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  • So basically, when LNA get closer to saturation, the output SOI won't be the -90 dBm but decrease a little bit. Am I getting this right? – tyrela Jun 21 '22 at 02:00
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You would see higher order mixing products at the output of the LNA. The closest to the SOI would be third order products. 2f2-f1 and f2-2f1, where f1 and f2 are the frequencies of the two input signals.

Sam brown
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  • Let's assume there are band-pass filters on the output (or at least in the thing doing the eventual demodulation. Wouldn't those higher order products be filtered out? –  Jun 20 '22 at 23:59
  • No. the third order products are very difficult to filter out. – Sam brown Jul 04 '22 at 14:09