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# Question about input referred noise

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#### OMEsystem

I'm confused by input referred noise. If we use a input referred noise voltage
to model the noise of amplifier. Does this voltage value chage if the source
resistance is from 50 to 100ohms? Assume the input impedance of amplifier
is not high impedance.

#### GDF

##### Full Member level 3
input referred noise definition

I have additional question, what's difference between so called "input referred noise" and "input equivalent noise voltage" in noise two-port model?

Are they identical?

#### suhas_shiv

##### Full Member level 2
input-referred noise

It depends. Sometimes the input referred noise is calculated by considering the source resistance as a part of the amplifier. Sometimes it is not. An amplifier noise model has 2 sources, Vn and In. So hence it depends on how you model your amplifer Noise. if you consider source resistance as a part, then the Input referred noise changes when you change the source resistance.

#### GDF

##### Full Member level 3
input referred

When we will include source resistance, and when won't?

#### suhas_shiv

##### Full Member level 2
input referred noise

Well actually it depends on whether you derive the formula for Vni(Input referred noise with or without Rs(source resistance). Ultimately Rs affects Vni. There is a minimum Rs that gives minimum noise but since Rs is fixed you have no choice. Some books take Rs into consideartion while some don't when deriving Vni.

Refer the following book for any noise related questions. I found it very helpful when designing a LNA.

LOW NOISE ELECTRONIC SYSTEM DESIGN
Author: MOTCHENBACHER
ISBN: 04715-7742-1

#### GDF

##### Full Member level 3
input referred noise model

suhas_shiv said:
Well actually it depends on whether you derive the formula for Vni(Input referred noise with or without Rs(source resistance). Ultimately Rs affects Vni. There is a minimum Rs that gives minimum noise but since Rs is fixed you have no choice. Some books take Rs into consideartion while some don't when deriving Vni.

Refer the following book for any noise related questions. I found it very helpful when designing a LNA.

LOW NOISE ELECTRONIC SYSTEM DESIGN
Author: MOTCHENBACHER
ISBN: 04715-7742-1

Thanks, I have to say the input referred noise is really confusing.
I didn't see the clear definition so far.

#### suhas_shiv

##### Full Member level 2
input referred noise voltage

Assume you have a noisy amplifer A. It adds some noise to the signal(which might already have some noise in it) being processed by it. You measure the total output noise, say Vno.
Now imagine amplifier A being noiseless. What is the amount of noise(Vni) that should be present at the input of this noiseless amplifier to get the same output noise(Vno) is called the Input referred noise.

Read the book I mentioned. It will clear you doubts regarding Vni.

input referred noise and input equivalent noise are the same.

Points: 2

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