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    HAWKINSAR's Avatar
    HAWKINSAR Posts: 4, Reputation: 1
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    #1

    Feb 3, 2012, 12:22 PM
    An Amplifier has a input power 5mW the output power is 100mW what is the decibel to
    The nearest decibel
    joatbmon's Avatar
    joatbmon Posts: 3, Reputation: 1
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    #2

    Feb 5, 2012, 10:33 AM
    Since every 3db increase doubles the power out, 12 db would take you to 80mw. (starting at an input of 5 your doubles in power would be at the 3db,6db,9db,and 12 db mark, yielding 10mw,20mw,40mw,80mw)and then 20 additional mw would be achieved by another .75db increase. Since 20mw is 1/4 of 80, you would need 1/4 of 3db (.75) to get the additional 20mw. Been a very long time since I had to do calculations of this kind, so I could be completely off, but I don't think I am. I think your answer would be very close to 12.75db. Hence, 5mw in with a 12.75 db increase would yield around 100mw output. There are logarithms to give this type info, but I don't remember the formulas.
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #3

    Feb 6, 2012, 09:18 AM
    Hawkinsar - I responded to how to solve this type of problem here:
    https://www.askmehelpdesk.com/middle...ms-632931.html

    joatbmon - your answer is not a bad estimate. You'd be a little more accurate if you can remember that 1 dB in gain represents about a 25% increase in power. So to go from 80mW to 100mW requires about 1 more dB of gain above the 12 that gets you from 5mW to 80mW.
    joatbmon's Avatar
    joatbmon Posts: 3, Reputation: 1
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    #4

    Feb 7, 2012, 04:23 AM
    Yea, that's it. All I could remember was the 3db doubling the output. Thanks for jogging my memory and making me do some homework. I think I like this forum thing.
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