Class AB Amplifier design help

Discussion in 'General Electronics Chat' started by EE_Eric, Apr 3, 2015.

  1. EE_Eric

    Thread Starter New Member

    Apr 3, 2015
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    Hey guys,

    I need help designing a class ab amplifier to power an 8 ohm 0.2W speaker to create an alarm sound. I'm outputting a sine wave at 150Hz and 5Vpp from an external DAC connected to my TI microprocessor.

    I have a couple NPN and PNP transistors, 2N3904 and 2N3906, which i plan on using.

    If I hook up my speaker strait to the waveform asis I get a very soft alarm sound with an AC current of 3.7-4.3mA's going through it. Since its a 0.2W speaker I figured i need to amplifier this current to around 150mA to get a nice sound.

    Questions 1: Is that the right logic?
    Question 2: Is a class AB amplifier what I should use to increase my current to 150mA?
    Question 3: If so, does anyone have a recommended schematic I should use?
    Question 4: How should I calculate the resistor and capacitor values I should use?

    Calling all analog wizards!
    Any help would be greatly appreciated!
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Why on earth would you want a class AB amp for a 200mW speaker in preference to a class A one?
     
  3. EE_Eric

    Thread Starter New Member

    Apr 3, 2015
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    I just thought it was a more elegant design, i guess, lol. So, you would recommend a class A amplifier then?
     
  4. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    I would recommend an LM386.
     
    Last edited: Apr 3, 2015
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    You will max out the 0.2 watts at .223 amps peak with a voltage of 3.577 P-P
    It will require a bit over 5 volts of supply to get there with this chip.
     
  6. Ramussons

    Active Member

    May 3, 2013
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    This?
     
  7. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Couple of problems there, Ramussons. 1) 16% crossover distortion (by voltage comparison) and 2) the output coupling capacitor is sized for 70% voltage delivery at 200 Hz. Target is 150 Hz. 3) The input capacitor has an impedance of 1061 ohms and that means the transistors will need a current gain of over 100. Again, not good for distortion.

    Still, the choice of circuits is up to EE_Eric
     
  8. Ramussons

    Active Member

    May 3, 2013
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    It's only an alarm. I don't think fidelity is of much importance; simplicity is.
    100 uF @ 150 Hz = 10 ohms - ok for output.
    At the most, the 1K at the input may require some trimming.
     
  9. EE_Eric

    Thread Starter New Member

    Apr 3, 2015
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    I need to create this amplifier using discete components
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  11. bertus

    Administrator

    Apr 5, 2008
    15,645
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    Hello,

    This sounds as homework.
    Is it a college project?

    Bertus
     
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