More on Biasing Class AB Output Stage

Discussion in 'General Electronics Chat' started by crazyengineer, Feb 18, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Okay so I here's what I learned about class AB output stage thus far

    1) Need a bias voltage between the transistors to eliminate if not reduce the dead band for the output stage
    2) One transistor turns on while the other stays off.
    3) Under DC biasing, each one can be treated as if you're biasing a current mirror
    4) To reduce thermal runaway, diodes are used as the bias voltage between the transistors pins (have to be switching) and to have emitter resistors as well.


    My main questions is if I matched the output impedance of the transistor to the impedance of my load, will DC current from the emitter be the same when it reaches my load or is it solely based on the current gain of the transistor I'm working with?
     
  2. nigelwright7557

    Senior Member

    May 10, 2008
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    Look up Vbe multiplier for biasing an amp.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    A modern audio amplifier has a lot of open-loop gain and a lot of negative feedback that reduces distortion to almost nothing and makes its output impedance extremely low (0.04 ohms or less). An audio amplifier with an extremely low output impedance has an excellent "damping factor" that damps the resonances of speakers for very good "tight" sound.

    Old fashioned vacuum tube amplifiers had an output impedance that was almost the same as the speaker impedance so the speakers were designed with internal damping that did not work well (sloppy boom boom sound or no bass sound).
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If you actually want them to track thermally, you use diode-connected-transistors to match against the amplifier transistors so they actually do match. Throwing a small diode in to track a transistor is a weak approximation at best.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Many little audio amplifiers have diodes squeezed against the output transistors for pretty good thermal tracking.
    But I agree that a transistor VBE multiplier bolted to the heatsink of the output transistors is much better.

    Today people use an audio amplifier IC instead of many separate parts.
    The transistors in an IC are perfectly thermally matched.
     
  6. PaulEE

    Member

    Dec 23, 2011
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    The LM3886, for example.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    I think that teachers do not understand electronics well enough to explain how an extremely good LM3886 amplifier IC works.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Yeah, I thought he was talking about discretes.
     
    crazyengineer likes this.
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I used a similar product I procured from national semiconductors audio group when I built my power amp for driving my stereo TV speakers. I built the speakers too, and incorporated the power supply inside one speaker with the amp screwed to the back. Been working great for 17 years. Sound is really good.
     
  10. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    I'm referring to discrete this time.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    But I prefer discrete.
    It gives the power I am hungry for.
     
  12. bertus

    Administrator

    Apr 5, 2008
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  13. PaulEE

    Member

    Dec 23, 2011
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    crazy,
    I'd consider buying a book called "microelectronic circuits" by sedra and smith. It does not have to be a brand new edition. It has an entire section dedicated to output amplifier stages and biasing of these stages. this:
    www2.engr.arizona.edu/~brew/ece304spr07/Pdf/VBE%20Design.pdf
    gives a pretty good breakdown
     
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