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Design >> RF Design >> Noise Figure of Cascaded Stages with no Interstage Matching
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Message started by DDC on Apr 23^rd, 2012, 11:22pm

Title: Noise Figure of Cascaded Stages with no Interstage Matching
Post by DDC on Apr 23^rd, 2012, 11:22pm

Hello,

I want to calculate NF of some cascaded stages, but the problem is that there is no interstage matching and so it is not possible to calculate available Gain, we only deal with voltage gain in this case. Can somebody give me some literature where this problem is discussed ?
Thanks for response.

Title: Re: Noise Figure of Cascaded Stages with no Interstage Matching
Post by RFICDUDE on Apr 24^th, 2012, 4:01am

You haven't provided enough information to answer your question.

Is your problem cascaded blocks with known NF and gain, or is your problem at the transistor level where you have cascaded circuits and you need to find the total NF?

If you have blocks with known NF and gain (and cascading them doesn't impact gain) then the problem is easy because you may be able to use the cascaded NF formulas.

If you have circuits and you need to compute total NF then the basic procedure is to refer each noise source (voltage and/or current) to the input where it is compared to the source noise at the input.

I good book on circuit noise is
Low-Noise Electronic System Design
C. D. Motchenbacher, J. A. Connelly

Title: Re: Noise Figure of Cascaded Stages with no Interstage Matching
Post by DDC on Apr 24^th, 2012, 6:10am

Hello,

I have many blocks with known NF and Voltage Gain only. I want to calculate total NF when there is only a special voltage gain of each block, because there is no matching between the stages. They are only matched at the input to 50 Ohm.

Title: Re: Noise Figure of Cascaded Stages with no Interstage Matching
Post by RFICDUDE on Apr 25^th, 2012, 4:47am

From what you describe I would think that you simply need to find the input referred noise voltage for each noise source. If all you have is NF and voltage gain then I would think that the cascaded equations still hold, but without seeing the exact problem and knowing the context I cannot tell if there are some considerations that are missing.