Class AB pushpull amplifier

Discussion in 'The Projects Forum' started by juniorece, Mar 17, 2008.

  1. juniorece

    juniorece Thread Starter Member

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    Class AB pushpull amplifier

    can anyone help me find and design a Class AB pushpull amplifier...:confused:
  2. beenthere

    beenthere AAC Fanatic!

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    Would you clarify your request? Do you want material to aid in understanding such amplifiers so you can independently design one, or do you simply want a schematic?
  3. juniorece

    juniorece Thread Starter Member

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    im very sorry beenthere.. well, first i want to understand how a class AB pushpull amplifier works.. and test/simulate a sample schematic..
  4. SgtWookie

    SgtWookie Expert

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    Attached Files:

  5. juniorece

    juniorece Thread Starter Member

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    thanks for the info sgt wookie... can anybody show me a diagram of a class ab pushpull diagram?? please..
  6. SgtWookie

    SgtWookie Expert

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    Did you look at the attachment to my last post?

    Did you read the Wikipedia article?
  7. juniorece

    juniorece Thread Starter Member

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    yep tnx master... i hope i can test it correctly..
  8. juniorece

    juniorece Thread Starter Member

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    it doesnt work in simulation... why is that??
  9. SgtWookie

    SgtWookie Expert

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    Did you change components, or miss a connection?

    As you can see, it works in the simulation I set up.

    Note that the junction of D1 and D2 is NOT connected to the junction of the emitters of Q4 and Q5.


    Post a screen shot of what you have so far.

    While you have the schematic on the screen, hit Ctrl+PrtScr
    Then start Paint (in the Accessories folder) and paste the image by pressing Ctrl+V.
    Save it as a type .png or .jpg, then attach it to a reply in Advanced mode.
  10. juniorece

    juniorece Thread Starter Member

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    mr. sgtwokie.. ive tried this several times but the output is a clipped sinewave.. but thanks i simulated a real amplifying ckts.. hope youll reply to this soon..;)

    Attached Files:

  11. thingmaker3

    thingmaker3 Moderator Staff Member

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    Input should go to the wiper arm of the potentiometer.;) Otherwise, the amp is over-driven.
  12. SgtWookie

    SgtWookie Expert

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    You have R5 connected improperly (R6 in the schematic I previously posted). In your circuit, R5 is supposed to be the "volume control", or input signal attenuator.

    As a result of R5 not being connected properly, the input from the signal generator is not being attenuated.

    This is causing the circuit to amplify the input signal to the point of "clipping", or exceeding the limits of the power supply "rails".

    If you connected R5 up as I've shown R6 in the previous schematic, and change your signal generator to output 200mV P/P, you should get approximately the same output that my simulation did.
  13. juniorece

    juniorece Thread Starter Member

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    whoa! stupid me..:D

    here is the updated one.. masters, thank you..

    here is my final question, how can i differentiate a class B and a class AB in just looking in the ckt??

    and this ckt is a class AB, right?:)

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  14. Audioguru

    Audioguru New Member

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    Your circuit is clipping because it is driven from the very low impedance of the generator instead of having about 50k ohms in series with its input from the volume control.

    Add a 50k resistor in series with C3 and turn down the generator's signal level.
  15. SgtWookie

    SgtWookie Expert

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    I believe you meant "add a 50k resistor in series with C3" ... which is basically what reconnecting R5 properly would do.
  16. Audioguru

    Audioguru New Member

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    Yes, in series with C3.
    The generator in the SIM probably has an extremely low output impedance. The circuit must be driven through some resistance.
  17. juniorece

    juniorece Thread Starter Member

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    i had done this but nothing particular happened.. the power gain decreases. but the waveform is still the same.
  18. SgtWookie

    SgtWookie Expert

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    If the waveform is still clipping, you need to increase the attenuation at R6. Change the % of the pot. First try 25%, then 75%. One should be better, the other worse.

    If you can't get a nice inverted & amplified version of your input waveform, then re-post your schematic and the input and output waveforms.
  19. juniorece

    juniorece Thread Starter Member

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    25% has a smooth waveform(less THD), 75% is not so good because i can see the "cross-over distortion" in the oscilloscope. what can i do to minimize or smoothen out cross-over distortion?
  20. SgtWookie

    SgtWookie Expert

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    Remember the two diodes in the base circuit? Those are there to keep the transistors right at the edge of being biased on. If you're seeing crossover distortion, try changing the diodes to a different model.

    The type I used, 1N4001, are power rectification diodes, and have relatively slow response times. You might try using a newer version, like perhaps 1N5817, which is a low forward voltage Schottky diode. You will need to use more of these in series, because they have a lower voltage drop across them.

    For example, at 100mA, a 1N4001 will have a Vf of approximately 0.75 (Fairchild Semiconductor datasheet, 1N4001-1N4007, Figure 2, Vf Characteristics).
    At the same 100mA current, a 1N5817 will have a Vf of 0.32 (ON Semiconductor datasheet, 1N5817 - 1N5819, Electrical Characteristics, Max Instantaneous Vf).

    Try replacing the two 1N4001 diodes with four, then five 1N5817 diodes in series. See what happens to crossover distortion. With four, it should be much worse. With five, it should be much better - except your current will increase due to the transistors being biased on a bit more.

    You could "fine tune" it by adjusting the current flowing through the diodes. Review the datasheets to see the Vf characteristics. As you will see, the Vf is not cast in concrete; it varies as a function of current.

    If the Vf of the diodes is perfectly matched to the Vf of the emitter circuits of the output transistors, the crossover distortion will be minimal along with minimal current usage. But in the "real world", no two transistors are identical (even if they have the same markings) - so Your Mileage May Vary.
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