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  1. #1
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    RF front end

    I would like to start a discussion on RF front end. Is it possible to have 2 LNA's before a mixer? The answer is probably yes but my main concern is how to do so without having to worry about matching between the 2 LNAs?

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    Re: RF front end

    mainly in integrated circuit TRX front end , there is no matching circuits between LNA and mixer , the matching network is only used when the distance between the LNA and mixer is comparable to wave length

    so u donot need matching if the two LNAs are near eachother


    wish this help

    khouly



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    Re: RF front end

    I think you dont need any matching circuit if the output impedance of the LNA (typically 50Ω) is same as the input impedance of the other LNA and you might want to consider the frequency range you are operating and the microstrip trace impedance connecting the LNA's.



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    Re: RF front end

    You didn’t specify that this is an RFIC design.
    So, in a standard RF circuit design you need a proper match between two cascaded LNA’s to get the required Gain, Stability, Bandwidth and Noise.
    The matching impedance between two cascaded LNA’s could be different than 50 ohms, from the moment that is not interfaced with any external circuits.



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    RF front end

    1 year ago, there are many commercial WCDMA RFICs using 2 LNAs.



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    Re: RF front end

    Two LNA in RFIC is designed to handle 900MHz and 1900MHz. To get low noise, there is no way to use cascade LNA in RFIC.



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    Re: RF front end

    Quote Originally Posted by spweda
    Two LNA in RFIC is designed to handle 900MHz and 1900MHz. To get low noise, there is no way to use cascade LNA in RFIC.
    :( To get low noise, you definately need two LNAs.



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    Re: RF front end

    In reality the main goal of the LNA is to have a very low noise figure.
    However, as every thing in life you dont get anything without give some
    thing in return, it means the gain and stability of the first LNA play an important
    role. So, the first LNA would be low noise figure but low gain and the second
    moderate noise figure but high gain. An analysis of the chain for noise figure,
    stability, gain and intermodulation products is neccesary. Then the whole
    "system" of two LNA becomes more like a LNA and a first amplfier.



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