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Why Noise Figure amplifier measured is Negative in Multisim?

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I’ve simulated several amplifiers(such as common-emitter, common-collector, common-base, etc.) to get the NF parameter,but the results have shocked me: some data is Negative. In my opinion, NF should not be less than 0dB due to the noise generated by amplifier. I thought there was something wrong with my operation, so I download some file from NI website to study it ,but find a Negative result in NI Multisim User Manual which is -110dB with a sample Named “Noise Figure – RF Amplifier”. so the question is: Why Noise Figure amplifier measured is Negative in Multisim? I didn’t find any difference on definition of NF from Multisim Help, and haven’t researched any information about this question on Web either. It seems like Nobody discussed this question before. If anyone can give me some advice, I'd appreciate it cos I've been trying to figure it out for almost 2 weeks. Thanks!
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