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Switchable biasing Bob85 style

Discussion in 'Home Brew' started by DIRTYWHITEBOY, Dec 24, 2008.

  1. dudmuck

    dudmuck Active Member

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    nothing could be said that hasnt already on wikipedia.
    Decibel - Wikipedia, the free encyclopedia

    But i'll just say that dB is used expressing gain, whether its an amplifier or antenna or whatever.
    And its just simpler to figure the gain when your power is in dBm instead of watts.


     

  2. mackmobile43

    mackmobile43 Jock Supporter

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    I guess there's more than one way to skin a cat, eh?

    Do you think "Joe Plumber" could figure off the top of his head if I were to tell him over the radio that my rig produced 3.4% more gain in class ab over class c at one 1 watt input? Did'nt think so, but for all practical purposes dbm gain is useful, but not practical for most users.
     
  3. dudmuck

    dudmuck Active Member

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    That an understandable mistake to think 13.4dB is 3.4% more gain than 10dB.
    But instead, 13.4dB gives you a bit more than double the power you'd have compared to having only 10dB gain.

    Think of dB another way.
    Consider the JoGunn Smokin Gunn II antenna, they say it has 12dBi gain.
    This means it takes a 4watt CB signal (which is 36dBm) and simply adds 12 to it to make 48dBm ERP, which is 63 watts ERP.
    So a 4watt CB with a smoking-gunn-II is the same as a 63watt CB on a isotropic antenna.
    (isotropic is just a fancy word for an omnidirectional antenna)
     
  4. mackmobile43

    mackmobile43 Jock Supporter

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    What ever (blows your skirt up)is a laymens term that states I could not care less so I'll stick to how I've stated it and the results are as I have posted them. :love:
     
  5. Bulldog357

    Bulldog357 Audio Wrecker

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    Blowing the dust off the thread... Class C is more efficient and is appropriate for High Power design if linearity isn't your first concern. [see N. Dye, H. Granberg, Radio Frequency Transistors, 2nd Ed].

    Mack's test proves a good point; at a given amperage with a specific power in the AB bias will produce more watts.

    What's being lost in your efficiency argument Booty is, for a fixed amount of amperage (i.e. you have one alternator that produces 200 Amps, or a single power supply that produces 100 Amps) the efficiency of Class C will give you more watt's ultimately than a class AB.

    If we wanted to accurately measure Macks test and compare apples to apples we'd also need to measure the amount of amps drawn at the time of the test. What we should see is the Class C amp requiring fewer amps to produce nearly identicle watts out - leaving more capacity that will eventually surpass total power out that can be produced by the AB platform.

    If efficiency was of no concern; than amp builders would all use Class A and AB and AB1 would be the red-headed step children of RF design...
     
  6. mackmobile43

    mackmobile43 Jock Supporter

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    Sigh! alas I have no way of measuring the amperage drawn. sorry but I'm sure it's greater in class ab than c which should be of no consequence to the end user as he's already decided to use this class and should be prepared to power it with it's necessary current needs.

    Efficiency equals the amount of money you have to blow.
     
  7. Bulldog357

    Bulldog357 Audio Wrecker

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    Hey Mack,

    It's been a while since you built that circuit; are you still using it? How has it held up?
     
  8. mackmobile43

    mackmobile43 Jock Supporter

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    Yes I still use it when dx is going, I have not had a single issue with it since it's installation.

    I've had 30 minute conversations and not seen any deviation in power or audio quality and the temperature of the amplifier seems to stay constant so the thermal tracking seems to be holding steady.
     
  9. eagle1911

    eagle1911 Active Member

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    I know this is an older thread, but what are the odds that a schematic of these boards is available?
     
  10. eagle1911

    eagle1911 Active Member

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    Since no schematic has presented itself, are the boards themselves available for purchase?
     
  11. bob85

    bob85 Supporting Member

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    the boards were based on motorola application notes and modified, they were a prototype, only a few friends got them, i don't have any for sale,
    you can find the basic lm723 circuit on communications concepts website;)
     
  12. eagle1911

    eagle1911 Active Member

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    Thanks for the reply and for pointing me in the right direction.. I'll have a look at my various Motorola app notes and paperwork, I'm sure I've got the one you're referring to.
     

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