Assignment

Discussion in 'The Projects Forum' started by dmag0006, Apr 5, 2011.

  1. dmag0006

    Thread Starter New Member

    Dec 8, 2010
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    Hi, I need to design an Audio amplifier and I was going to use the circuit in the attachment as my output stage. My BJTs are Darlington transistors and I was given a suggestion as to add two extra diodes besides the ones already there. However I can't fully understand why, could someone please explain? Also, would someone please give me any furthur tips as to what I can add to the circuit in order to improve it? Thanks in advance :rolleyes:
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you know the difference between Darlington transistors and standard BJT's (bipolar junction transistors) ?

    If not, have a look at this datasheet that includes the BD681:
    http://www.onsemi.com/pub_link/Collateral/BD675-D.PDF
    Look at the diagram on page 3 of that .pdf, and then figure out what the major differences are from this Darlington and a standard BJT.

    You might experiment with transistors vs Darlingtons in your SPICE simulator, just to get a few ideas on how they differ.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Your power transistors are connected directly between the power rails and ground, while the load resistor RL is connected to the signal source.

    Shouldn't RL be connected between the junction of the two emitters and ground?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    There is another configuration that is closer to a conventional Darlington Pair called a Sziklai Pair. Both resemble conventional transistors, the Sziklai pair even more so.

    Agreed, the RL should connected between the junction of the two emitters and ground.
     
  5. dmag0006

    Thread Starter New Member

    Dec 8, 2010
    17
    0

    The only difference between the Darlington connection and the 'normal' BJTs is the difference in Hfe....right? Since the Darlington connection has two BJTs inside. How can a change in BJT help improve my circuit?

    Should any current be passing through the diodes?

    Thanks
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    A Darlington drops 1.2V B-E, while a Sziklai Pair drops 0.6, like a conventional transistor. Both drop 0.6 CE at saturation, while a regular transistor drops around 0.1 or less.

    The diodes get the bias voltages near what the transistors need. Some current is needed to set the voltages, but remember that transistors are current devices.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Take another look at the Darlington internal schematic on page 3 of that document I referenced.

    Notice that from the base to the emitter, there are TWO base-emitter junctions. Base-emitter junctions act electrically like forward-biased diodes.

    So, a standard BJT has only one base-emitter junction, while a Darlington has two.

    This is why you need two diodes for the high-side Darlington, and another two for the low-side Darlington.
     
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