Question about Class B audio amplifier

Discussion in 'General Electronics Chat' started by simpsonss, Feb 23, 2010.

  1. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    hi,
    I'm trying to learn about this so i decide to do the following experiment.
    http://http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

    [​IMG]
    The first circuit i construct is as above. From the explanation i will get a distortion waveform at the output part. But what i get is a nice sine wave. :confused:I had inputted 1Khz 2Vp-p sine wave at the input part. The only distortion is get is when i turn the volume (potentiometer) higher. Can someone help me on this.

    thanks.
     
  2. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    anyone can help?
    thanks.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The crossover distortion is worse when the signal at the output of the opamp is near 1.4V p-p.
     
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  4. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    If i disconnect the output pin(pin1) of the OpAmp which going to the base of the TIP42C transistor but still connected to the base of the TIP41C transistor, and then i have a look on the output waveform should i get a half cycle sine wave? The purpose for me to do this is to more understand on the ON OFF of both transistor.

    thank you.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Do not disconnect one output transistor because then the remaining transistor might have a reverse emitter-base voltage that exceeds its max allowed reverse voltage that is only 5V.
     
  6. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    so is it no other way to have a look on just a half cycle wave of the sine wave at the output part?
     
  7. Audioguru

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    Simply look at the output. The positive-going part is from the NPN transistor and the negative-going part is from the PNP transistor.

    In a simulation program you can separately look at the current in each output transistor that flows for its on-polarity.
     
  8. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    so do u mean that i cant display out only the positive-going part by just disconnect the -12V which going to the PNP transistor. Am i correct?

    thank you.
     
  9. Audioguru

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    If you disconnect the -12V going to the collector of the PNP transistor then the base signal of the NPN transistor could go more negative than -5V which is the max allowed reverse-bias emitter-base voltage causing avalanche breakdown of the junction and damaging the transistor. Look at the datasheet of the NPN transistor to see the Absolute Maximum Emitter-base Voltage.

    If you disconnect the +12V to the collector of the NPN transistor then the PNP transistor might be damaged.

    But if you keep the peak signal level less than the max reverse voltage rating then you can disconnect a collector to see half the waveform.
     
  10. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    hi, thanks for the reply. But can u explain a bit more about that?How could i keep the peak signal below the max reverse voltage?

    thank you.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    By simply turning down the level of the input signal so the reverse voltage of the emitter-base junction does not exceed its max allowed rating of about 5V.

    The datasheet for every transistor has a max allowed voltage rating listed for its reverse-biased emitter-base junction. Some transistors have a 4.5V rating (2N5089), some have 6.0V (2N3904) and other transistors have 7.0V (2N3055).
     
  12. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    Do u mean that tune the input signal that i inputted to 1 or 2vp-p. Is it like that?

    thanks.
     
  13. Audioguru

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    Dec 20, 2007
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    Yes, but you can also adjust the gain control pot to reduce the output level.
     
  14. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    hi guru,

    By refering to the worksheet experiment. Could this be a push-pull amplifier?thanks.

    ABCD0012.jpg
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    It is the same circuit as before except it has negative feedback, is biased for class-AB and has output transistors that have some voltage gain.
     
  16. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    hi guru,

    could 2 pairs of push-pull amplifier become a quasi-complementary amplifier?
    The characteristic of a push-pull amplifier is only one transistor will be switched on at a time and NPN for the positive waveform while PNP for negative waveform. Am i correct?

    Since it seems like very less info about the characteristic of quasi-complementary amplifier in the net. I would like to ask some guidance and info from guru. Any idea about quasi-complementary amplifier?

    thank you.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    Instead of guessing, why don't you look in Google?
    Many articles show that an old quasi-complementary amplifier uses the same transistor for both output transistors instead of complementary transistors like used in the last amplifier you showed.

    I have an amplifier that was made in 1964 and it uses a quasi-complementary circuit using NPN 2N3055 power transistors for both output transistors. Matching high power PNP transistors were not made yet so only the medium-power driver transistors were complimentary.
     
  18. simpsonss

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    Jul 8, 2008
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    Does quasi-complementary better than push-pull or it depends?
     
  19. Audioguru

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    Pure complementary and quasi-complimentary are both push-pull.

    Modern amplifiers use pure complementary class-AB circuits for very low distortion.

    46 years ago when good PNP power transistors were very expensive or not available then they used quasi-complementary class-AB circuits that worked but not as well as pure complementary circuits today.
     
  20. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    Do u have any simple quasi-complementary circuit that i could build and discover the output signal?

    thanks.
     
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