Class-B output stage

Discussion in 'Homework Help' started by Rumination, Apr 4, 2016.

  1. Rumination

    Thread Starter Member

    Mar 25, 2016
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    [​IMG]

    The figure shows a class B output stage with two complementary output transistors Q3 and Q4 and two complementary driver transistors Q1 and Q2. The transistors are connected in Darlington connection for sufficiently high current gain from input to output at the output stage.
    R1 is inserted to increase the quiescent current in Q1 and Q2, and increases the speed of Q3 and Q4 when the transistors to turn off (Class B). RL is the load output stage to drive. Vbb1 and Vbb2 creates a voltage between the bases of the two symmetrical halves amplifier so that the quiescent current IQ in the output transistors can be controlled, it applies to VBB = Vbb1 + Vbb2 and Vbb1 = Vbb2. Rest current is the current flowing through the two output transistors collectors when RL is disconnected and Vout = 0V
    For transistors: Is-q1 = Is-q2 = 5 * 10^-14 A. and Is-q3 = Is-q4 = 2 * 10^-12 A.

    a) Calculate the maximum power dissipation in the output transistors PQ3 + PQ4.

    b) Calculate Vbb1 = Vbb2 so that the quiescent current in the output transistors are I_Q-X, and R1 as the quiescent current of the driver transistors (I_R1) will be equal to I_R1-X.
    β is ∞ for all transistors.

    c) Determine the maximum efficiency of the amplifier when the maximum output voltage is limited by the following (Vneg + 4V <= Vout <= V POS-4V).

    ----------------
    The given values are:

    Vpos = 24 V.
    Vneg = 24 V.
    RL = 7 Ω.
    R2 = 768 Ω.
    R3 = 768 Ω.
    I_R1-X = 27 mA.
    I_Q-X = 66 mA.
    Vout-X = 16 V.
     
  2. Rumination

    Thread Starter Member

    Mar 25, 2016
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    I have calculated the first question:

    a) P_max in Q3 and Q4:
    Vout = (2/ π) * Vcc = 0,64 * Vcc
    P_max = (Vcc^2) / ((π^2) * RL) = (24^2) / ((π^2) * 7) = 8,34 W.

    c) Can I determine the maximum efficiency of the amplifier with this table:
    [​IMG]

    Can you guys help me with the rest? The deadline is tomorrow night.
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    For part B you must use a Shockley equation and solve for Vbe3+Vbe4 at Ic = 66mA and next solve for Vbe1 + Vbe2 for Ic = 27mA.
    Then by help of a Ohm's law you can find R1 and Vbb1 + Vbb2 from KVL ( Kirchhoff's voltage law , II Kirchhoff's law ).
     
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  4. Rumination

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    Mar 25, 2016
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    I haven't heard about the shockley equation.
     
  5. Jony130

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    Feb 17, 2009
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    You have never seen this equation?

    Ic = Is * e^(Vbe/Vt)

    Ic = Is * e^{\frac{Vbe}{Vt}}
     
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  6. Rumination

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    Mar 25, 2016
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    Never :eek: But now I have. I know Ic, but what is Is.Vbe is 0,7and Vt is 24 V
     
  7. Jony130

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    Rumination likes this.
  8. Rumination

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    Mar 25, 2016
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    Okay. The thermal voltage is 26 mV. So I have to solve for Vbe:

    66 = (2*10^-12) e^(Vbe/26)

    27 = (5*10^-14) e^(Vbe/26)

    When I have the two Vbe, what to do next.
     
  9. Jony130

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    Solve for R1 using Ohm's law and next Vbb1 and Vbb2 with help of a II Kirchhoff's law.
     
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  10. Rumination

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    Mar 25, 2016
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    I found:

    66 = (2*10^-12) e^(Vbe/26)
    Vbe = 809,33

    27 = (5*10^-14) e^(Vbe/26)
    Vbe = 882

    Using Ohm's law to find R1:

    R = U / I = (809,33 + 882) / 27 mA = 62,64 Ω
     
  11. Jony130

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    Feb 17, 2009
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    Totally wrong, next time try to track the units and prefixes more carefully.
    This is also wrong. But why do you add Vbe3 and Vbe1 to get the voltage drop across R1 resistor?
     
  12. Rumination

    Thread Starter Member

    Mar 25, 2016
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    66 mA = (2*10^-9 mA) * e^(Vbe/26 mV)

    66 mA /(2*10^-9) mA = e^(Vbe/26 mV)

    3,3*10^10 = e^(Vbe/26 mV)

    Better?
     
  13. Jony130

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    Definitely much better now.
     
  14. Rumination

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    Mar 25, 2016
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    How can I isolate Vbe?

    3,3*10^10 = e^(Vbe/26 mV)

    I get it to Vbe = 629,7
     
  15. Jony130

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    Feb 17, 2009
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    Good, but what is the unit of this result ?? As for Vbe we can use maths and we can rearrange this formula Ic = Is * e^(Vbe/Vt) to get Vbe directly.
    Vbe = Vt * In(Ic/Is)
    So for Ic = 66mA and Is = 2pA we have Vbe = 26mV * In(66mA/2pA) = 629.714mV
     
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  16. Rumination

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    Mar 25, 2016
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    Yes. And the other Vbe, I get:

    Vbe = 702,386 mV.

    Using Ohm's law to find R1:

    R = U / I = (U) / 27 mA =

    Is my voltage 24?
     
  17. Jony130

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    Feb 17, 2009
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    OK

    Why 24V ?
    What is the voltage at the top end of a R1 resistor ? What is the voltage at the bottom end of a R1 resistor ?
     
  18. Rumination

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    Mar 25, 2016
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    I'm thinking it's Vpos = 24 V. and Vneg = 24 V.
     
  19. Jony130

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    If so, the voltage across R1 is equal to:
    VR1 = 24V - (-24V) = 48V which is of course not true. What about Q1 and Q2 Vce voltage is this voltage equal to zero ?
    Also notice that R1 is connected between Q3 and Q4 bases ? Do you see this ?
     
  20. Rumination

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    Mar 25, 2016
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    I'm sorry, I don't get it.
     
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