Designing a Vbe multiplier

Discussion in 'Homework Help' started by fantabulous68, Feb 12, 2010.

  1. fantabulous68

    Thread Starter Active Member

    Nov 3, 2007
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    Does any1 know any useful sites that will assist me in answering this question
     
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  2. Audioguru

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    A transistor and two resistors at its base makes a Vbe multiplier that is used in audio amplifier circuits. One resistor can be adjustable for it to be adjusted for class-A or class-AB.
     
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  3. hgmjr

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  4. fantabulous68

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    Nov 3, 2007
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    yeah i did, but it just gives the output resistance of a Vbe multiplier.....& i do not want that.......

    I attached a diagram of a Vbe multiplier, I would like to design suitable values for all components in the diagram that will ensure the Vbe multiplier can cater for both Class 'A' and class 'AB' operation.

    Would appreciate some guidelines on how i should approach this....



    Another condition I missed in the attached doc: I iN > ID
     
    Last edited: Feb 12, 2010
  5. hgmjr

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    Do you have a diagram of the class AB and A circuits showing the Vbe multiplier you have in your sketch? From the sketch it appears that you have a handle on the basic operation of the Vbe multiplier.

    hgmjr
     
  6. fantabulous68

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    Nov 3, 2007
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    I dont have a diagram. Only the following question was given :

     
  7. Audioguru

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    You cannot design the Vbe multiplier because the schematics it will be used in are missing.

    If there are only two transistors then a class-AB Vbe multiplier will be set to about 1.4V.
    If the two transistos both have emitter resistors then you must know their value to determine how much voltage they will drop which is added to the voltage setting of the Vbe multiplier.

    If the output transistors are darlingtons then the voltage of the Vbe multiplier mut be nearly doubled.

    There are many different class-A circuits and there are many that do not need a Vbe multiplier. How can you set it if the circuit does not need one?
     
  8. fantabulous68

    Thread Starter Active Member

    Nov 3, 2007
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    VBB = VBE {(R1+R2)/R2}......that is the main equation for a VBE multiplier
    so if i know the bias voltage ie VBB, & we know VBE=0.7, then i can determine values for R1 and R2 with the ratio of the resistors.

    If the multiplier is biasing 2 darlingtons...then there are 3 bjts(including multiplier bjt),
    hence VBB=3x0.7=2.1V

    So how do I ensure the Vbe multiplier can cater for both Class 'A' and class 'AB'?
    Should I use a potentiometer & adjust it as long as Voltage>VBE

    class A- has a bias level that maintains an output current at all times

    Class AB output stage is biased to carry a quiescent current significantly less than half the maximum output current (as needed by a full Class A amplifier) but sufficient to keep both transistors running in Class A mode for small input signals.


    the current through it varies between 2ma and 8ma
    -->ID=8mA & IB=2mA since ID>>IB
     
    Last edited: Feb 13, 2010
  9. Audioguru

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    How does 2 + 2= 3?
    1) A darlington transistor has two base-emitter junctions in series.
    2) In a push-pull amplifier there are two darlington transistors in series.
    3) Then 2 + 2= 4 junctions which is 2.8V.

    Usually the upper resistor in a Vbe mulltiplier is made a potentiometer so that the bias current in the output transistors can be adjusted.

    I have never made a class-A room heater amplifier. I don't think they use a Vbe multiplier.
     
  10. fantabulous68

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    Nov 3, 2007
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    this is the 3 bjt circuit i was picturing when I was explaining---in attached doc
     
    • 3bjt.JPG
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  11. Audioguru

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    The Vbe multiplier is adjusted to make two base-emitter voltages in series, not three.
     
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  12. fantabulous68

    Thread Starter Active Member

    Nov 3, 2007
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    ok thanx Audio Guru......i had counted the Vbe of Q3 as well, & added that to X1 & X2 Vbe
     
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