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Design >> RF Design >> Dependency of inductor value in a CS LNA
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Message started by sabawat on Dec 3^rd, 2009, 6:32am

Title: Dependency of inductor value in a CS LNA
Post by sabawat on Dec 3^rd, 2009, 6:32am

hi, all

while reading a paper on a multi-band lna design, i came across this sentence...

"in short channel MOSFET the value of the inductor( which will be present at the source of MOS device in CS configuration) is almost independent of the bias current and device width, where as, in long channel it depends on bias current and device width"

can anybody explain how this value of inductor varies in accordance with channel length?

Title: Re: Dependency of inductor value in a CS LNA
Post by loose-electron on Dec 3^rd, 2009, 5:47pm

It doesn't.

They are hinting at the fact that the capacitance changes at the input of the transistor(C looking into the gate)

If C changes then the need for a different L become evident.

Title: Re: Dependency of inductor value in a CS LNA
Post by aaron_do on Dec 3^rd, 2009, 6:54pm

Hi,

I believe they are talking about the degeneration inductor, L_s, in an inductive degeneration LNA. L_s is chosen to be equal to R_source/ω_T where R_source is usually 50 ohm. I'm not sure why they say it is independent of biasing and device width for a short-channel device however.

For long-channel, ω_T = μ_n(V_GS-V_TH)/L² [Razavi, Design of Analog CMOS Integrated Circuits, ch2, pp. 40] so if anything, the choice of L_s should be independent of the bias current and device width for a long-channel device...unless i've misunderstood something...

cheers,
Aaron

Title: Re: Dependency of inductor value in a CS LNA
Post by Mohamed.eissa on Dec 10^th, 2009, 11:33am

Hello
It will be help full if you can send me this paper or where can i get it, but what I know about that is:
Ls=( Rs/wt)=(Rs*Cgs/gm) = Rs*(Cgs*L/μCox*W*Veff)
if we assumed that Cgs=Cox*W*factor
so Ls = Rs*(factor*L/μ*Veff)
so I guess it depend on the Veff so changing the Width and current by same value keeping Veff constant will mean the Ls value need not be changed,
But any change in Veff or L will lead to a need in Ls change for optimization.
But about short channel and long channel effect on these relations I am not sure about that I have to check it.

Regards

Title: Re: Dependency of inductor value in a CS LNA
Post by sabawat on Dec 11^th, 2009, 1:12am

hello mohamed..

i have attached the corresponding paper. it is section 4 of that paper.please find it.

Quote:

I have deleted the attached paper. Please only attach papers that you personally own the copyright to.
The paper was ...
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 1, JANUARY 2002;
Concurrent Multiband Low-Noise Amplifiers—Theory, Design,and Applications;
Hossein Hashemi and Ali Hajimiri
-Ken Kundert

Title: Re: Dependency of inductor value in a CS LNA
Post by yvkrishna on Jan 19^th, 2010, 11:03pm

hi,

i found it on google scholar..

http://www.chic.caltech.edu/Publications/MTT_Concurrent.pdf