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Calculate when an RF amplifier will block (desense)?

Discussion in 'Electronic Components' started by ForNewsPost, Jan 16, 2004.

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  1. ForNewsPost

    ForNewsPost Guest

    Is there a way to calculate when an amplifier will block by knowing its IP3?
    If not, is there any way to calculate the blocking capabilities of an RF amp,
    such as an LNA?


  2. Tim Wescott

    Tim Wescott Guest

    IP3 and blocking are related, but not closely enough to tie them together.
    Any two amplifiers that are built using similar parts and similar circuits
    will probably have these two parameters track, but you can do amazing things
    with feedback to hold off the 3rd order products until the amplifier is just
    about to shatter.

    I haven't had to calculate this parameter myself, so hopefully someone with
    real experience will answer. Try posting to --
    there are some real pros who lurk there (really real pros - guys who build
    radios and get paid for it).

    I'm pretty sure that for most amplifiers desense will start when the
    amplifier starts to clip. If you're getting a module with all the guts
    hidden then you're going to have a hard time figuring it out. If you can
    get your hands on a sample amplifier you should hook it up and crank up the
    input until the output starts to level off, or dig down into the specs if
    you have them. If you actually have a data sheet see if they either spec
    the 1dB compression point or if they plot the IP3 -- usually the IP3 plots
    also show the power out vs. power in, so you can look for the bend in that
    line and know.

    Failing that, for a class A (low-power linear) RF amp the _theoretical_
    maximum output power with sine-wave input is 50% of the supply power before
    it starts to clip. For a real amplifier that's not designed with reaching
    theoretical maximums the actual will be somewhat to much less -- I wouldn't
    be surprised if it were only 5% of the supply power, although I would be
    offended. So you can get an idea just looking at the supply current and
    voltage rating, but your milage _will_ vary.
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