Class AB Amp With Class Driver

Discussion in 'Analog & Mixed-Signal Design' started by newbie2019, Jun 13, 2019 at 5:22 PM.

  1. newbie2019

    Thread Starter Member

    Apr 5, 2019
    91
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    I found the following circuit online. Like most circuits found online or in textbooks,
    it does not work. I know how to design a simple class ab amplifier but can't find
    out how to add the class driver to the design. Note: I added the two .33 Ω emitter resistors to prevent thermal runaway. Later I would add negative feedback.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    To start with, the DC bias is off.
    The voltage at the output to C2 should be about 1/2 the supply voltage or 6V.
    Try reducing the value of RB11 from 100kΩ to about 35kΩ.
     
  3. newbie2019

    Thread Starter Member

    Apr 5, 2019
    91
    2
    Thanks man that did the trick. The THD is like 1.8%. Where would I add the circuitry
    for the negative feedback? Or is there feedback provide by deriving the Q1 base
    bias from the output?
     
  4. Zeeus

    Member

    Apr 17, 2019
    288
    10
    Let's start with this..not nice gain

    and bandwith calculation is fun but can't do : learning

    you understand circuit?

    oh and change the output transistors (they are not on spice )..3904 and 06 not good for output yeah but TIP not on spice



    what's THD? and that Vcc sign..Please how did you do it?

    Thanks
     
    Last edited: Jun 13, 2019 at 7:07 PM
  5. crutschow

    Expert

    Mar 14, 2008
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    You add this command and then look for the results in
    View -Spice Error Log.

    upload_2019-6-13_16-42-16.png

    Vcc is simply the added NET (node) name for the power supply rail.
    Any node with that name is connected together.
     
  6. Zeeus

    Member

    Apr 17, 2019
    288
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    what's the THD for? sorry

    and yeah it's node but how to add?

    like want to create a node name like "Vas" which is 4V then connect to different points but don't know how to create it

    edit : think I see how to do now

    Also, please view this
    https://forum.allaboutcircuits.com/...-analog-circuits-designs.160671/#post-1403366

    Thanks
     
    Last edited: Jun 13, 2019 at 7:59 PM
  7. crutschow

    Expert

    Mar 14, 2008
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    No, that provides only DC (bias) feedback.
    Any AC feedback is shunted to ground by he zero source impedance.

    To get some AC feedback, add a 5k resistor in series with the input source, and add a 1000uF cap from the emitter of Q1 to ground (to keep the overall gain about the same).
    That should drop the distortion to about 1%.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    It's Total Harmonic Distortion of the output signal (a measure of it's accuracy as compared to the input).
    Go to Edit / Label Net, write in the net name, and then attach it to all the nodes that you want connected with that name.
    The Port Type puts a box around the name if you want, such as In or Out.

    upload_2019-6-13_17-1-40.png
     
  9. Zeeus

    Member

    Apr 17, 2019
    288
    10
    okay..give you like...
    Please go here and give expert answer
    Thanks
    https://forum.allaboutcircuits.com/...-analog-circuits-designs.160671/#post-1403366
     
  10. newbie2019

    Thread Starter Member

    Apr 5, 2019
    91
    2
    Thanks again crutschow.
     
  11. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But how would adding the Vbe-multiplier will improve the amplifier? Do you know what Vbe-multiplier was supposed to do in this circuit?

    Try to read this very cerfully
    https://electronics.stackexchange.c...er-gain-than-input-and-distorte/262302#262302
    https://electronics.stackexchange.c...ng-out-the-bottom-of-this-push-pull-amplifier
    https://electronics.stackexchange.c...t-buffer-for-low-current-op-amp/330205#330205
    http://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/
     
    Last edited: Jun 14, 2019 at 10:37 AM
    Zeeus likes this.
  12. Zeeus

    Member

    Apr 17, 2019
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    Seems you have something new every time
    This circuit, dc output is not Vbe ?

    About to read properly
     
  13. Jony130

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    Feb 17, 2009
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  14. crutschow

    Expert

    Mar 14, 2008
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    That's just a transistor to generate a bias voltage that takes the place of the two diodes in the first circuit.
    You can replace the two diodes in that circuit with the transistor and two associated resistors, but I see no particular advantage of the transistor over the diodes.

    The two dodes have the advantage in that there combined temperature coefficient will roughly match the output transistors' temperature coefficient if the diodes are glued to the transistor housings.
    That helps keep the bias current constant as the transistors heat up.
     
  15. Jony130

    AAC Fanatic!

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    But Vbe-multiplier can also do that. Because Vbe-multiplier not only multiplies the Vbe voltage but also the transistor temperature coefficient.
     
  16. crutschow

    Expert

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    But for only one transistor.
    There are two coefficients in series in the output.
     
  17. Ylli

    Active Member

    Nov 13, 2015
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    There is *no* AC negative feedback in that circuit. You could try adding a small resistor (1K) in series with C1.
     
  18. newbie2019

    Thread Starter Member

    Apr 5, 2019
    91
    2
    Here is another circuit that is no good. The input signal is 10mV while the output
    is only 45mV. Av = RB3 / RB4 = 1K / 100Ω = 10. The output power is almost zero.
    I am trying to find one that works right so that I can see how it was designed. No
    sense studying a circuit that is no good.
     
    Last edited: Jun 14, 2019 at 4:04 PM
  19. Jony130

    AAC Fanatic!

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    "works right"?? Define what you mean this statement?
     
  20. newbie2019

    Thread Starter Member

    Apr 5, 2019
    91
    2
    The input signal is 10mV while the output is only 45mV. Av = RB3 / RB4 = 1K / 100Ω = 10. The output power is almost zero.

    You could get more power out of a 3904 in class A.
     
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