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LNA Output Matching Circuit

Discussion in 'Radio and Communications' started by qtommer, Jul 30, 2010.

  1. qtommer

    qtommer Member

    Jan 20, 2009
    I am desigining a LNA circuit and have done the input matching circuit as well as the biasing circuit.. I am now currently on the output matching circuit.

    The output impedance of the biasing circuit (input to the output matching circuit) is at 63.1-j63.4 ohms and Im matching it to a 50 ohm terminating load.

    I am currently using a LC network to match the output as well as to resonate the stray capcitance from the output of the biasing circuit.

    I cant seem to find a good compromise between the S11 and S22 parameters and would like to enquire whether using the LC topology is the best choice for this purpose.

    Opinions on this would be highly appreciated. THanks=)
  2. RCinFLA

    RCinFLA Well-Known Member

    Jul 23, 2009
    FLA, USA
    A series output cap of about -j62 ohms and a shunt parallel coil at output of device of +j56.5 ohms will match to 50 ohms real. Make the coil a little lower in inductance to be able to resonate out any additional stray capacitance.

    Just about any RF amp is going to have an interaction between input and output loadings due to S12 feedback. Optimum input match for maximum power gain may be different then match for best noise figure.

    Depending on frequency, type of device, and other requirements like dynamic range, different techniques can be employed. If you have some excess gain you can tolerate to reduce, one of the best things you can do to improve stability is to put some shunt resistance across output parallel coil. This will help overall stability and stabilize output match somewhat with little effect on input noise figure.
    Last edited: Aug 14, 2010

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