Intuitive understanding of friis formula

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In summary, the conversation discusses the Friis formula and the impact of LNA placement on noise figure in a circuit. There is confusion about the role of the LNA and how it affects the overall noise characteristics of a system. The conversation also highlights the difference between Friis and amplifier chains and the importance of the first stage in noise amplification. Ultimately, the question remains whether the placement of the LNA has a significant impact on the signal output.
  • #1
sriecewit
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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?

Thanks,
Srini
 
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  • #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.
 
  • #3
No changes
 
Last edited:
  • #4
Quick comment,
The LNA will amplify everything coming from previous stages, including noise generated in previous stages.
 
  • #5
jambaugh said:
Quick comment,
The LNA will amplify everything coming from previous stages, including noise generated in previous stages.
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.
 

1. What is the Friis formula?

The Friis formula, also known as the Friis transmission equation, is a mathematical equation used to calculate the received power of a signal from a distant transmitter.

2. How is the Friis formula used in communication systems?

The Friis formula is used to determine the received signal power in communication systems, taking into account factors such as distance, frequency, and antenna gains.

3. What are the key variables in the Friis formula?

The key variables in the Friis formula are the transmitter power, transmitter and receiver antenna gains, and the distance between the transmitter and receiver.

4. How does the Friis formula account for signal loss?

The Friis formula accounts for signal loss by incorporating the distance between the transmitter and receiver, as well as the antenna gains, which can increase or decrease the received signal power.

5. What are some limitations of the Friis formula?

The Friis formula assumes free space propagation and does not account for obstacles or interference, making it less accurate in real-world scenarios. It also does not take into account factors such as atmospheric conditions or multipath propagation.

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