Biasing a Class AB with a transistor

Discussion in 'The Projects Forum' started by bennyq, Apr 14, 2015.

  1. bennyq

    Thread Starter New Member

    Sep 13, 2014
    6
    0
    Hi, Im trying to design a Class AB power gain stage with the following circuit. Im having trouble with achieving a load of 8ohms(5vpp).. I would like the transistor to bias the current gain stage as I would eventually like to place a darlington pair package that has a VBE on of 1V.. Can I achieve that with this circuit? Or should I consider trying to use diodes..
     
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  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,813
    That circuit is a mess.

    That is not how a transistor bias works.

    Google a transistor bias and post a reasonable result. We can work from there. Your circuit is not a reasonable starting point for a discussion.

    Start near the bottom of this one
    http://en.m.wikipedia.org/wiki/Bipolar_transistor_biasing
     
  3. bennyq

    Thread Starter New Member

    Sep 13, 2014
    6
    0
    Well this is my initial design, I'd want 1v preferably at the bases for turn on so voltage, 0.7V is a bit low. Is this design better?
     
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  4. JDT

    Well-Known Member

    Feb 12, 2009
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    Your second circuit is a simpler way (and not so good) of controlling the quiescent current in the output transistors. There are a few other things wrong as well but I will leave that for now!

    Take your drive capacitor to the junction of D2 and base of Q2. Use another 100uF capacitor to bypass the 2 series diodes. Will work but won't produce full output swing as the transistors run out of drive current as the voltage across the 1K resistors gets low.
     
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