LNA-Large Interference and Effect of Output Impedance

Discussion in 'Homework Help' started by prietess, Nov 30, 2010.

  1. prietess

    Thread Starter New Member

    Nov 30, 2010
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    Hi everyone....
    I am new to RF circuit design, and received my first task to design an LNA.
    There is this question asking:
    1. What will happen when the LNA receives a large interference signal (equivalent to 1dB compression point)? What is the effect on Noise Figure and S11?

    2. What is the effect of changing the output impedance to 200 Ohm(from 50 Ohm initially) on S21 and the voltage gain of LNA?

    I have tried to read some books by Razavi and Thomas Lee, but could not find some proper and good explanation.
    Please kindly help me.... I am really in need of help to complete the project which will due next week.....
    Thank you for your help.
     
  2. prietess

    Thread Starter New Member

    Nov 30, 2010
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    Anybody, please?
    Really need help from more experts like all of you.

    Thanks in advance.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Re you first question. Have you tried a search on IP3 [3rd order intercept] and LNA performance - e.g. return loss and noise figure?

    One would imagine a qualified conclusion being that both are degraded as a result of a high level of out-of-band interference giving rise to 3rd order inter-modulation products.

    http://en.wikipedia.org/wiki/Third-order_intercept_point
     
  4. prietess

    Thread Starter New Member

    Nov 30, 2010
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    Hi! Thanks for your kind reply.

    Actually I have all the values.
    fo=2GHz.
    Reverse Isolation is 40dB, and NF is below 2 dB at 2GHz.
    1db Compression point: -3dBm.
    IIP3 Point: 9.6dBm.
    Power Gain: 10dB.

    My friend told me that the biasing changes, but I wonder if there is a better and more complete explanation for it.
    Do you probably know?
    Thank you for your kind response!
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's not particularly clear what you need.

    If you are undertaking a design there are many examples of LNA design methods on the web. In the end it seems to involve accepting performance trade-offs for the various design goals.

    This site might be of help - at least from the overall design perspective.

    http://bmf.ece.queensu.ca/mediawiki/index.php/Introduction_to_LNA_Design

    or this

    http://www.eepublishers.co.za/images/upload/EngineerIt/EL_02_ASingle.pdf

    A very general discussion is also available here

    www.qsl.net/va3iul/LNA%20design.pdf

    Anyway - If you type in a search for "LNA design" and click on Similar on the first hit you'll get a pretty good response.

    Any design you undertake will probably require access to microwave simulation software to evaluate the design.
     
    Last edited: Dec 1, 2010
  6. prietess

    Thread Starter New Member

    Nov 30, 2010
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    Hi my friend!
    Thank you very much for your kind response again.
    I think you misunderstood something here.
    I actually have finished my design and simulated it using Cadence.
    The LNA has fulfilled all the requirements. My LNA is source inductive degeneration, which schematic is the same with that of inside Thomas Lee book LNA design examples section.

    What I am having trouble now is how to answer the questions above.
    1. What will happen when the LNA receives a large interference signal (equivalent to 1dB compression point)? What is the effect on Noise Figure and S11?
    Result showed that: NF increases and there is peak at the operating frequency. S11 changes very slightly.----->how do you explain these?

    Since I am new to RF, I really have no idea.
    Hope that I can receive some advice from you and other senior members.
    Thank you very much!
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    OK I get it now. One of the problems of trying to interpret the background & meaning of questions is when the OP provides information on what one might call the "drip-feed basis". Don't worry - it happens a lot. Eventually one gets the whole story by a convoluted process.

    From what I understand of the discussions about noise figure a significant factor is the input/source [50Ω] matching.

    Given this is dealing with an operating mode which is probably getting into the amplifier's non-linear region, it may be difficult to pin point a specific reason for the observed trends. It would probably require a well trained RF 'eye' with all the measurement data and plots (Smith Charts etc.) at hand to give an informed opinion. Bias point may well be relevant. Don't know anything about Cadence and whether it does small or large signal modeling or both ...?

    This is an RF related thread, so it's possible it might get better attention on the Radio & Communications Forum here at AAC. You could re-post there or better still, request the administrator to move your thread there.

    I'd still suggest reading through some of the links I gave. Issues like noise figure are clearly key considerations in the LNA design and the factors which influence such things are of necessity discussed by amp designers.
     
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