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    Why an input inductor required in all GPS LNA?

    Hi,

    In almost all the datasheet of GPS LNA i have read so far, all of them say following

    "Output is matched to 50ohm, Input just needs a matching inductor". They even write it highlight this in the features of chip on front page that it just needs one inductor for matching. They also specify the value of inductor which is mostly under 10nH.

    what i dont understand is
    1. when they could match output to 50ohm , why did not they match the input as well and make it standard 50 ohm input and 50 ohm output?
    2. and how exactly we users of the chip are supposed to perform the matching part? do we assume that after adding the suggested value of (lets say datasheet says 9.1nH) , it will become a 50ohm input?

    Some example datahsheets are
    https://datasheet.lcsc.com/szlcsc/Sh...NR_C146262.pdf
    https://www.infineon.com/dgdl/AN250_...30210e89596b04

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  2. #2
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    Re: Why an input inductor required in all GPS LNA?

    No. Low Noise Amplifiers which are used in GPS applications need Very Low Noise Figure due to very low level signal strength.Therefore Input Matching is done to achieve this Possible Lowest Noise Figure.



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  3. #3
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    Re: Why an input inductor required in all GPS LNA?

    Chip designers could implement a 50 Ohm input by a bias resistor, common-gate amplifier's input transconductance, etc. But this way is a waste of sensitivity or/and power.
    Discrete inductance is technically noiseless because its series resistance is very low, it has got acceptable Q factor, and same is true for integrated capacitors. But IC developers cannot implement inductance on the chip, because it could be huge in area, the chip cost would be huge too.
    And the IC manufacturers also don't like bigger external component numbers, because cheaper applications are better, so they try to minimalize the components, and for input matching usually a simple LC network is enough.
    If you build an LC matching network from external/discrete inductance and internal capacitor (cheaper actually then use discrete capacitor) and you match the antenna with a bigger resistive input impedance on the chip then the LC network will gain/boost your RF signal. Without adding noise to it! It is at the very begin of the receiver chain, so it will determine the system's noise figure.
    This is why a simple inductor is so elegant. Good for the manufacturer, enough for the customer.
    10nH btw comes from the tolerance of the cheap SMD inductors, the GPS center frequency and the GPS bandwidth.
    "Try SCE to AUX." /John Aaron/


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  4. #4
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    Re: Why an input inductor required in all GPS LNA?

    I bet you r a little bit new to LNA design, a simplest answer regarding this stuff is to reduce significantly overall NF of this amplifier, because NF = 1 + gmRs*y*(w0/wt)^2, and Rs (= 50) s chosen equal directly to the real part of input impedance , but unfortunately the parasitic resistor of your input inductor (that s mentioned in datasheet) dominates the thermal noise of whole NF. If it is an off-chip, then R will be low (Q>100), SMD inductor), and then results in a low NF for your PA.
    Otherwise, you are able to pick out that input inductor OFF CHIP, but it shows off a bad Q (<15) So rarely we have so far employed that guy for a good LNA performance.
    For more insight into LNA, I encourage you a good book for reference: RF MICROELECTRONICS (RAZAVI) - CHAPTER 5, this book provides a good intensive n practical view of designing LNA for a beginner
    Have fun !



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    Re: Why an input inductor required in all GPS LNA?

    It is definitely possible to put the small inductor inside the LNA. There are two reasons why they don't do it:
    1. Internal inductor will increase the chip area, complexity and then the cost;

    2. Exact impedance matching depends also on the PCB stack-up and layout. Even the manufacturer could provide perfectly matched chip and evaluation board, the end users may not use the same PCB stack-up and layout. So external impedance matching network is still needed, put a perfect impedance matching network inside the chip is a sort of waste.



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