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    Low Noise Amplifier output match

    Hello all,

    I am designing a Common source Cascode low noise amplifier. How do we match the circuit at the output to 50 ohms ?
    As I understand , we use an inductive load to get rid of parasitic capacitance of the transistor. So, we are only left with the intrinsic resistance of the transistor. We can not use the matching network to match resistance to 50 ohms . we have to have reactive part to have matching network to match to 50 ohms.

    Please help/advice.

    Thank you

    •   Alt22nd April 2013, 21:56

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    Re: Low Noise Amplifier output match

    depends on bandwidth, output level and source impedance
    what is it?
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
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    Re: Low Noise Amplifier output match

    I am testing Common source Inductive degeneration LNA performance for low power dissipation. ( 200uA current from 1.2V). I am not designing it for particular application. Initially I used Load inductance with a blocking capacitor with the port at the output with a resistance of 2k ( assuming that LNA is going to feed the input of the mixer which will have very high resistance). I got the gain around 11dB. Then I realize that a stand alone LNA should be matched to 50 ohms. Lets say for a moment , I want my LNA to be narrow band. How should I design output matching network?
    I guess my main question is that if I am using an inductance at the load which will resonate out the parasitic capacitance of the transistor then why do I need matching network ?

    Please advice.

    Thank you



    •   Alt22nd April 2013, 23:37

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    Re: Low Noise Amplifier output match

    best way is stripline transformer using copper and appropriate substrate with low loss tangent such as Getek FR4 or Rogers teflon.
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975


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    •   Alt23rd April 2013, 01:07

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    Re: Low Noise Amplifier output match

    I am sorry , I did not understand what you are suggesting. ( I am using BiCMOS , 120nm process, 2.4GHz frequency)

    Thank you.



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    Re: Low Noise Amplifier output match

    there are too many methods, coil transformers, single ended, differential, stripline, microstrip

    What is center frequency, output level and source impedance that you will be generating?

    Here is an example of on chip design

    https://www.google.ca/url?sa=t&rct=j...45512109,d.b2I

    Yes I am asking you to clarify what you want to match.....
    Last edited by SunnySkyguy; 23rd April 2013 at 01:49.
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
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    Re: Low Noise Amplifier output match

    Center frequency is 2.4GHz . I guess , I did not understand your question. Are you asking about the impedance looking into the circuit after adding the matching network ? (or ) Are you asking about the inherent resistance of the circuit ?

    Thank you.

    - - - Updated - - -

    I just found out my mistake. My output resistance of the amplifier is 9 ohms ( normalized to 50 ohms ). I just looked at the smith chart and realized that I can simply add an L match network to match it to 50 ohms. Am I correct?

    Just confirming one last thing, to measure the output impedance of my LNA , do we use psp analysis ?



    •   Alt23rd April 2013, 02:01

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    Re: Low Noise Amplifier output match

    I think you are confusing your ownself and me also
    the moment you insert an inductor to balance parasitic capacitance, you are actually inserting a "matching Network" (that is the answer of your question "why do I need matching network ?" )
    next if it is narrow band then surely matching would be comparatively easy.
    in order to design a matching network you have to have the values of impedances at its both ends, 50 ohm at one side and the impedance your transistor is offering on other side.
    just insert these values in Smith chart simulator ( ADS has got one) and you can have your matching network automatically or either by designing/inserting your own values and components that can easily available practically, .
    you can know your output impedance by S22 simulations, take the impedance at 2.4 GHz of your band and match it to 50 ohm.
    If you Believe everything you Read, Better Don't Read!


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    Re: Low Noise Amplifier output match

    Hi Smooth criminal,

    Thank you very much for your reply. I think I follow the concept now.
    As you just said " In order to design a matching network you have to have the values of impedances at its both ends, 50 ohm at one side and the impedance your transistor is offering on other side." Now which analysis is the best to measure the impedance that my circuit/transistor is offering ? Is it SP ( small signal ) or PSP ( large signal) and why ?

    Thank you.



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    Re: Low Noise Amplifier output match

    For this purpose as i said, S parameter analysis are done, S-parameter analysis assumes that all the network is linear thus it is a small signal analysis,
    as far as Large signal analysis is concerned, it is used to study the nonlinear behavior of a circuit/ device and analyzing and creating a nonlinear response is very difficult, moreover we can solve our problems , simply with this small signal analysis so why to bother for trouble!!!
    If you Believe everything you Read, Better Don't Read!



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