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Intuitive understanding of friis formula

  1. Jul 21, 2011 #1
    Hi Guys,

    Can anyone help me with the intuitive understanding of the friis formula?

    I'm not able to contemplate the fact that when you put LNA in the front-end of a circuit, overall noise figure is less.
    The same hardware, you put it after a few circuitry, the noise figure goes up.

    How can the same component cause so much change in noise characteristics of a system when you change its position?

  2. jcsd
  3. Jul 23, 2011 #2
    Wrong answer to wrong thread

    Friis is about transmitters/receivers and not noise in amplifier chains. Two different things.

    In Friis, you have 1/4pi R2, which should scream out the obvious: solid angle at a distance. That with antenna gain should be pretty clear: 1/R2 losses with antenna performance.

    A amplifier chain is only as quiet/non-noisy as the first stage because noise is uncorrelated so you can't magically remove it once it's there so you will be simply amplifying the noise and the signal after the first stage. So the SNR has to be on the first stage or the LNA goes first.
  4. Jul 23, 2011 #3
    No changes
    Last edited: Jul 23, 2011
  5. Jul 23, 2011 #4


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    Gold Member

    Quick comment,
    The LNA will amplify everything coming from previous stages, including noise generated in previous stages.
  6. Jul 23, 2011 #5
    yeah. So practically speaking, does it really matter if you put it in the beginning or later? Its the same hardware and same signal flowing. So you would eventually get the similar signal output right? I'm still not convinced with the mathematics of friis formula.
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