Please Help Me. I Have My Exam This Monday. - RF Cafe Forums

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esampathj
 Post subject: please help me. i have my exam this monday. Posted: Fri Dec 01, 2006 12:42 pm
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
any one can tell me how to Calculate the noise figure for a cascaded series of stage give the noise figure and power gain for the individual stages.
question is there are three amplifiers available. they give their gain and each of the noise figure. how can i calculate the overall noise figure.
thx
bye

Kirt Blattenberger
 Post subject: Posted: Fri Dec 01, 2006 1:58 pm

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 476
Location: Erie, PA
Greetings esampathj:

This is an easy one so even I can answer it for you. Both of these links show the cascaded noise figure equations, but in two different forms.

Compact form:
http://www.rfcafe.com/references/electr ... figure.htm

Expanded form (section 5.2):

Good luck on your test.

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esampathj
 Post subject: thx Posted: Fri Dec 01, 2006 9:39 pm
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
thx mr admin. i ll try.

esampathj
 Post subject: noise figure Posted: Fri Dec 01, 2006 10:11 pm
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
but i cant slove this question using that sourse. please can u help me to solve this.

the gain and noise figure of three amplifiers are given in table below.

amplifier A , gain = 6dB, noise figure = 2dB
amplifier B , gain = 10dB, noise igure = 4dB
amplifier C , gain = 8dB , noise figure = 3.5dB

calculate the overall noise figur when the amplifiers are cascaded in the order A, B , C

State the order of the amplifiers that would give the minimum averall noise figure.

calculate the overall noise figure that whould be abtained on B)

You are doing a great help for me. bcoz i dont have anyone to ask this question.
thx.

Kirt Blattenberger
 Post subject: Example worked Posted: Fri Dec 01, 2006 11:06 pm

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 476
Location: Erie, PA
Greetings esampathj:

OK, I will work out the first scenario as an example, but it is up to you to figure out which order of amplifiers yields the lowest overall noise figure. The rule of thumb for cascading noise figure is that the NF of the first stage adds directly to the system NF, and you want as much gain as possible at the front end of the chain because the contribution of subsequent stages is reduced by gain of previous stages.

The first step is to convert the decibel values to ratios:

amplifier A , gain = 6dB (3.981), noise figure = 2dB (1.586)
amplifier B , gain = 10dB (10.00), noise figure = 4dB (2.512)
amplifier C , gain = 8dB (6.310), noise figure = 3.5dB (2.239)

Then, plug all the values in to the recursive formula provided in my previous post:

nf = 1.586 + (2.512-1)/(3.981) + (2.239-1)/(3.981*10.00)
= 1.586 + 0.380 + 0.0311
= 1.997
NF = 10 * log (1.997)
= 3.004 dB

There you have it. Get to work. We oldtimers are well aware of students trying to get other people to do all their schoolwork for them.

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RF Cafe Progenitor & Webmaster

esampathj
 Post subject: thx thx thx i got the point. Posted: Fri Dec 01, 2006 11:16 pm
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
thax that mean first i have to convert noise figure to noise factor and then i have to apply that formula.
previous i directly applied it.
now i got the point.
thx lot.
u done me a greate help.

esampathj
 Post subject: about gain Posted: Mon Dec 04, 2006 7:31 am
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
are u sure that we have to convert gain also to the ratio before apply to the formula.

Kirt Blattenberger
 Post subject: Re: about gain Posted: Mon Dec 04, 2006 8:24 pm

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 476
Location: Erie, PA
esampathj wrote:
are u sure that we have to convert gain also to the ratio before apply to the formula.

I'd bet my life on it.

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

esampathj
 Post subject: thxs giving Posted: Tue Dec 05, 2006 7:05 am
 Captain

Joined: Fri Dec 01, 2006 12:38 pm
Posts: 8
 hi mr Kirt,today i sit on my exam and ur help is verry verry use full for me.any way now it's party time.after few days i ll come again.thx for everything.

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