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  1. #1
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    getting incorrect S parameters analysis for LNA design in Cadence virtuoso tool

    Hi,
    I am designing LNA for GPS applications (1.57Ghz) using inductive source degeneration topology, circuit is attached. I mathematically calculated the values of Cgs, gm for the gain transistor taking Q = 4 and Rs - 50Ohms. Using this i calculated the value of Lg, Ls & W/L ratio of gain transistor. I am flowing Id = 6mA through gain transistor using a current mirror and my gm/Id ratio is between 10-15.

    I implemented my design on Cadence Virtuoso tool at 180nm technology node (spectreRF version is MMSIM13.1).

    DC analysis i am getting is correct as my theoretical and practical values of Cgs, Gm, Id etc match but my S parameter analysis is totally incorrect with S12 = S21 = -3B, S11 = S22 = -10dB, NF = 2.5dB at 1.6GHz. My results are attached

    I request somebody to suggest what could possibly be wrong with my circuit.
    Click image for larger version. 

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    Click image for larger version. 

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ID:	143936

    •   Alt13th January 2018, 10:20

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  2. #2
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    getting incorrect s-parameters (S21 & S12) for LNA design

    I am designing LNA for GPS (1.57Ghz), i am getting reasonable S11 and S22 = -10dB and -14dB respectively but my S21 is always coming as equal to S12 (i.e. S12 = S21 = -3dB).
    Attached is my circuit and S-parameters analysis.

    Can somebody advise what could possibly be wrong.

    Click image for larger version. 

Name:	circuit.png 
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Size:	110.6 KB 
ID:	143969
    Click image for larger version. 

Name:	LNA-results.jpg 
Views:	4 
Size:	80.1 KB 
ID:	143970



    •   Alt15th January 2018, 05:48

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  3. #3
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    Re: getting incorrect S parameters analysis for LNA design in Cadence virtuoso tool

    S21 = S12 means no amplifier function at all, check bias point.



    •   Alt15th January 2018, 08:21

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  4. #4
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    Re: getting incorrect S parameters analysis for LNA design in Cadence virtuoso tool

    Isn't too much 10pF for C0 at 1.75GHz? It is only 9Ohm, and shunts the input.



  5. #5
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    Re: getting incorrect S parameters analysis for LNA design in Cadence virtuoso tool

    Hi FvM, please let me know how do i go about setting the bias point. It will be really helpful.

    Hi Frankrose, i noticed this mistake myself, i tried connecting the C0 left of R1 so that input sees a 10K resistor but no success.



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