Anyone have any design tips for this old thread Class AB

Discussion in 'The Projects Forum' started by logans-electronics, Sep 9, 2009.

  1. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    I would like to build (in Multisim fist) an output stage using a TIP41C and a TIP42C. The schematics and designs prior to are of a Class B? or am I wrong. I understand the theory concept between the 2 designs but have trouble visually identifying which one is a B or AB.

    Or is the differnce between a B and a AB the way the bias currents operate?
    I noticed that the desing prior use diose in series to help with the "dead zone" of 1.4v or (+.7v-.7v)

    Would it be a bad idea to make an AB amp with a gain of 1 and use a LM833 op amp for the gain?

    Rail voltages are still being built....maybe somewhere around + and - 20v. Load would be 8 ohms...just looking for a cool project around 20-50 watts...nothing crazy.:)
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's confusing to continue with an old thread, like this one - http://forum.allaboutcircuits.com/showthread.php?t=10185&page=3

    Your question is not clear -
    If you mean to construct an amplifier that is essentially a buffer (G = 1) and then amplify the output with an LM833 op amp, it sounds odd. I might think to use the 833 op amp to act as a preamp and drive an AB amp that would power speakers.
     
  3. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Sorry for not being clear. I will start a new thread if it helps...
    I was considering using the LM833 OP amp as the preamp to provide the gain to the AB stage.
     
  4. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Feel free to drop this thread...I can start a new one tomorrow.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    + and - 20V is a total of 40V. The amplifier will have a voltage loss of about 8V at full output so the max across the speaker will be 32V peak-to-peak. Then the max RMS voltage is 22.6V and the power into 8 ohms is 64W.

    EDIT: 22.6V and 64W are wrong. The output is actually only 16W.

    But the peak current is 2A and the max base current of the TIP41 and TIP42 is 100mA (if their gain is minimum) which is far more than the opamp can provide.

    The opamp can drive darlington transistors or Mosfets.
     
    Last edited: Sep 12, 2009
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I am pretty confused about what you are trying to do here.

    However I would observe that many op amps are class B in output so if you are going to drive a pair of followers with a class B driver you cannot change this arrangement to class AB in the followers.
    It was usual to drive class AB followers with a class A driver when power amps were built from discrete components.
    Guru is correct in observing that you will need Darlington or Sizlaki pairs at the proposed power levels.

    So a tad more explanation, particularly expanding your important criteria, would help.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    No.
    Most opamps use a class-AB output for very low crossover distortion. The awful LM358 and LM324 opamps are low power and unfortunately use class-B at the outputs to save power and therefore produce 3% of crossover distortion.

    The LM833 dual opamp has a class-AB output for very low distortion but does not have enough output current to drive ordinary power transistors in an audio amplifier.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Are you saying; no, you can change a class B output to a class AB and maintain the feedback loop?

    Or are you saying; no, there are more class AB op amps than class B op amps in the world?
     
  9. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Wow!....This is great information...Could you share with me your formulas for your first paragraph above?:)

    I am taking baby steps to build a Class AB audio amplifier for the next 3 months. I am sorry for all the questions but, I really appreciate your help.
    Logan
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    1) The base voltage loss of each output transistor emitter-follower is 1.0V max.
    2) The output saturation voltage loss of the opamp driver is 3V.
    3) Therefore the max total voltage loss is 8V p-p.
    4) The total supply is 40V so the max output is 32V p-p.
    5) 32V p-p is (32V % 2.828=) 11.3V RMS.
    6) 11.3V RMS squared % 8 ohms load= 16.0W.
    7) The peak output current is 16V/8 ohms= 2A.
    8) The max base current of the output transistors is 2000mA/20= 100mA.

    I was wrong about the output power before.
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Is your aim simply to construct a Class AB amp or is it to study amp biasing?

    If you want to study biasing you can build a variable bias to the output pair so you can set it to Clas B or Class AB and see the difference. You can even build what used to be called 'sliding bias' where the change is effected by the circuit itself.

    Post if you want more.

    A general rule of thumb says that the output power of an amp is V^2/8R where V is just less than the inter-rail voltage and R is the load.
     
  12. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Let me put together some schematics and Mutlisim reports together so I may help you help me. I will post in a few days.;)
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Here is an example of an AB output power stage, you could scale it up for more power. This just shows the basic circuit configuration.

    I believe class B does not use a constant idling current in the output stage. Idling current wastes power, but reduces crossover distortion. Class AB uses the bias current.

    Here's a tutorial on it:

    http://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_22_ClassAB_Amplifier_08.pdf
     
    Last edited: Sep 15, 2009
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