Class AB push pull amplifier with VBE multiplier biasing

Discussion in 'The Projects Forum' started by zsquare, May 8, 2012.

  1. zsquare

    Thread Starter New Member

    May 8, 2012
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    I have to design a Class AB push pull amplifier with VBE multiplier biasing and using a pnp current mirror to bias the VBE multiplier.

    I am using OrCAD, with Q2N2222 and Q2N2907A βf=50 and Is=14.34f for both types of transistors.

    I attached the schematic for circuit that i have so far, but it's not working, as the output at R6 only shows the bottom half of the wave and it's not really amplifying the input.

    Any help is appreciated.
     
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    Last edited: May 8, 2012
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Your current mirror is connected between GND and -15volts. In order to turn on Q6, it's base must get higher than GND. The current mirror will not allow it.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    What is "multiplier biasing"? :confused:
     
  4. zsquare

    Thread Starter New Member

    May 8, 2012
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    The VBE multiplier biasing means the biasing current is from the current mirror.

    I made the change to the current mirror and this is what i got as for vo vs. vin.
     
  5. #12

    Expert

    Nov 30, 2010
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  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    How about grounding the negative terminal of V5? That should allow the current mirror output to rise above ground.
     
  7. zsquare

    Thread Starter New Member

    May 8, 2012
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    putting that ground helped a lot, as the current mirror does give me the bias current that i need, but my output wave just seems to be offset by a constant value, and is not really amplifying the input.

    Should the problems be that my resistor values at R1 and R2 are off?
     
  8. #12

    Expert

    Nov 30, 2010
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    It isn't a voltage amplifier. It's a current amplifier.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    There is no bias on the collector of Q4.
     
  10. zsquare

    Thread Starter New Member

    May 8, 2012
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    ahah, you are right, i misread the question that i have to answer. It only asked me to show vo with a peak swing greater than 5V, not a gain of greater than 5.
     
  11. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    When Q3 is "on", ie. conducting, it's Vbe is about 0.7 volts. That is the only thing between Vin and Vout. A little more complicated for Q6 conducting, but still can be easily checked.
     
  12. supertroopa86

    New Member

    Apr 9, 2012
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    Been working on the same project, how did you come up with the values for R1 and R2?
     
  13. #12

    Expert

    Nov 30, 2010
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    It's kind of set R2 and adjust R1 as you go. The point is to get the right idle current for Q3 and Q6 and it depends on the output transistors you use. Q1 is also supposed to be thermally coupled to the output transistors.
     
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