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.

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

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