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Does NFmin in cascode LNA depend on the negative feedback inductance Ls?

Discussion in 'Mathematics and Physics' started by anhnha, Feb 19, 2014.

  1. anhnha

    anhnha Member

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    Hi,
    This is a cascode LNA. Please help me with the question in picture. Thank you.

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  2. rumpfy

    rumpfy Active Member

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    Anh,
    You are asking a really simple question but the answer is seriously complex. It is just TOO difficult.
    Your question sort of assumes that it is easy to take any old device, get a simulation going, then plug in a range of values to get the best noise figure. It just dont work like this.
    The general principle of low noise amps is that the source and load impedances affect the transducer gain and noise figure. Generally for a first stage amp, this is designed for low noise and not maximum gain. It is a tradeoff between gain and noise.
    The use of the cascode arrangement is that the input impedance can be designed without being affected by the output impedance of the second stage and the input impedance of the second stage generally is better matched to the output impedance of the first stage for the low noise requirement.
    Your question moves from the assumption of an optimum design to what happens IF so and so is changed. It is getting to the low noise point that is THE problem.
    There are so many variables involved. I can tell you from the point of view of turret tuner design for TV band 3 designs that the work is painstakingly cut and try. The result for low noise is a compromise between bandwidth, adjacent channel rejection, sensitivity, coil design (using printed circuit coils), cross modulation, that to discuss all of these parameters is filled with traps and pitfalls. This type of design work is done in the screened room.
    When I was working in this area(1965), the cost of establishing broadcast services was high, but today, satellite services etc are now available. The really low noise stuff is often nitrogen or helium cooled and this stuff wasnt available 50 years ago. The sort of stuff used in space research or radio telescopes is where the leading edge of low noise amplifiers is today. It may involve studies of inductor values in drain circuits, but I feel it is more in low temperature electronics.
    I note your question was asked on a wednesday and today for me in OZ it is Sunday. To date this is your only hope.
    Hope I have helped you somewhat.
     
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  3. anhnha

    anhnha Member

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    Thank you, Rumpfy, for the thorough reply.
    I need to read it some more times before asking another question.
     

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