# Amplifier noise figure - RF Cafe Forums

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dohagan
 Post subject: Amplifier noise figure Posted: Tue Feb 20, 2007 1:43 pm
 Captain

Joined: Sat Mar 19, 2005 3:20 pm
Posts: 6
Hello,
I've got a variable gain amplifier connected directly to and Analogue-to-Digital converter (ADC). The input of the amplifier is terminated in 50 ohms and the gain is set to 20 dB. Measuring the noise at the output of the ADC (i.e. in Matlab) over a 200 kHz bandwidth, I get a value of -92 dB.

So, Pn = -92 dB = k+T+B+Fn+G

where Fn is the total noise figure of the system, G is the amplifier gain (20 dB) and B is the bandwidth (200 kHz).

I know the noise figure of the ADC itself to be 25.5 dB, but how do I find the exact value for the amplifier noise figure??

nubbage
 Post subject: Posted: Wed Feb 21, 2007 4:23 am
 General

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 343
Location: London UK
Hi
The overall noise factor of a 2 stage system will be dominated by that of the first stage, particularly, as in your case, where the first stage gain is 20dB.
First you have to convert dB NF to numerical NF (Fr), and dB gain to numerical gain (G, 100 in your case).
The overall NF is Fn = F1 +(Fr-1)/G
With G = 100, you can see the NF of the ADC has no importance, so Fn=F1.
The noise power into the ADC is just the expression you quoted, except Fn is the first stage NF, and the noise power is probably in dBW, depending on units used for the K value.
Inverting the expression then gives Fn.

sm7ovk
 Post subject: Posted: Tue Feb 27, 2007 4:41 pm
 Captain

Joined: Thu Jan 11, 2007 12:59 pm
Posts: 23
The formula is most often called Friis-formula, see more at www.producon.com/sm7ovk/friisformula.html

73
Jens, SM7OVK

OM
 Post subject: Posted: Sun Aug 10, 2008 4:40 pm
 Lieutenant

Joined: Sun Aug 10, 2008 3:41 pm
Posts: 3
To sm7ovk,

I regret to tell you that you are not right. You need to edit this page.

73!

smith08
 Post subject: Posted: Tue Sep 30, 2008 1:10 am
 Lieutenant

Joined: Tue Sep 30, 2008 1:06 am
Posts: 1
 where the signal to noise is measured by an ideal, noiseless receiver. ===================== smith Job Opportunities

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