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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> NFmin with/without Ls and Lg in cascode LNA https://designers-guide.org/forum/YaBB.pl?num=1392624891 Message started by baab on Feb 17th, 2014, 12:14am |
Title: NFmin with/without Ls and Lg in cascode LNA Post by baab on Feb 17th, 2014, 12:14am Hi, I decide to start my LNA over again and consider carefully all parameters. Here is my first question. My LNA is still the cascode. I read in some IEEE paper that the adding of Ls and Lg don't change NFmin of the overall circuit. I can't explain it in theory. For that reason, I will simulate the two cases and check if that statement is right. Below is two circuits for simulation. Case 1: cascode LNA without Ls and Lg Case 2: cascode LNA with the presence of Ls and Lg I run sp analysis with the variable Vgs sweeping from 0.4V to 1V and get NFmin from Direct Plot. Two curves of NFmin are ploted below. Please help me with the questions in the picture. Thank you. |
Title: Re: NFmin with/without Ls and Lg in cascode LNA Post by aaron_do on Feb 17th, 2014, 5:07pm Hi baab, I'm not sure if you figured it out, but I'm sure there's something wrong in your simulation setup. Aaron |
Title: Re: NFmin with/without Ls and Lg in cascode LNA Post by aaron_do on Feb 17th, 2014, 6:37pm Hi baab, In terms of whether or not Lg and Ls have an effect on NFmin, first you have to understand what NFmin is. Essentially we are finding the NF for every possible reactive network (call it network A) in between the source and the input, and calculating NF. Then we are selecting the minimum of those values and calling it NFmin. For Lg, its quite obvious then why it has no effect on NF. Since it is in between the source and the input, it can be considered part of network A, and only adds a series offset to the impedance. For Ls, I think its not as intuitive. You could do the math to prove it. Alternatively, if you think of inductive source degeneration as a form of negative feedback, you could follow Razavi's reasoning in his Analog IC book and say that the input referred noise is not affected by the feedback as long as the feedback network itself doesn't add noise (in this case it doesn't). Maybe somebody else has a more intuitive way of looking at it. Aaron |
Title: Re: NFmin with/without Ls and Lg in cascode LNA Post by baab on Feb 17th, 2014, 11:51pm Thank you, Aaron. Quote:
I don't know what is wrong with it. I tried various times and all of them give the same result. Quote:
Yes, I think I have understood it correctly. NFmin the the NF of the circuit with network A component's values appropriate for noise matching. Quote:
That makes sense. I will accept it for now. As for my understanding, input referred noises will change as network A component's values (let's assume that they are ideal L, C). So, it seems to me that NFmin also changes correspondingly. However, I know that is wrong. Quote:
I took a lot of time to prove it but no success. I like the idea you said in Razavi book. However, I just look it up in the book "Design of Analog CMOS Integrated Circuit" and can't find it. Could you tell me the name of the book and which page/section/chapter in which that part is mentioned. |
Title: Re: NFmin with/without Ls and Lg in cascode LNA Post by aaron_do on Feb 18th, 2014, 6:42pm Quote:
When you change network A, NF will change, but NFmin will not change. For example, suppose I have a transistor with no Lg and I check NFmin, and it corresponds to a matching network with a capacitor in shunt with the source equal to -jX, and an inductor in series with the transistor equal to +jY. After that, I add the inductor, Lg, with impedance equal to +jZ. So the NF will change. However, if I change my network A so that the series inductor is no longer equal to +jY and is now equal to +j(Y-Z), the NF will go back to NFmin. i.e. the effect of Lg was simply to offset the impedance where NFmin occurs. Quote:
You are looking in the correct book. I don't have it on me so I cant say which page. Its at the end of one of the chapters, and I think it is the chapter on feedback. regards, Aaron |
Title: Re: NFmin with/without Ls and Lg in cascode LNA Post by baab on Feb 18th, 2014, 11:17pm Hi, Aaron. Quote:
Excellent explanation! I got it. Quote:
You are right. I just found it at the end of the chapter on feedback. I am reading it. Hope everthign will be clear. :) |
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