Differential Amplifier design

Discussion in 'Homework Help' started by Rumination, Mar 25, 2016.

  1. Rumination

    Thread Starter Member

    Mar 25, 2016
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    [​IMG]

    The figure shows a differential amplifier with a differential input vid = (vin_d_pos - vin_d_neg) between the bases of Q1 and Q2, and an output (out) at the emitter of Q4.

    I have to Determine R1, R2, R4, R5 and R6 so that the following requirements are met:

    R 1 = R 2
    Av = vout / vid = 100 [V / V]
    Vout = 0V when Vin_d_pos = Vin_d_neg = 0V.
    The output Vout must be able to deliver undistorted peak signal voltage in the range
    from vout-min to vout-max with RL armed.
    Quiescent current IC = 10 * Q3-Q4 IB

    ----------
    I have been given the values:
    Vcc-pos = 21 V
    Vcc-neg = 21 V
    R3 = 2000 Ohm
    RL = 49 Ohm
    I1 = 3,56 mA
    Vout-max = 8,8 V
    Vout-min = -8,8 V
    ----------
    How can I start? For I have difficulty getting started.
     
  2. Rumination

    Thread Starter Member

    Mar 25, 2016
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    My English is not so good. I have found this out:

    Vcm = 0 , Vin_D = 0
    VB1 = VB2 = 0 V.

    Ic1 = IE1 = I/2 = 3,65 / 2 = 1,825 mA

    VB3 = 8,8 V - R3 * Ic1 = 8,8 V - 2000 * 1,825 mA = 5,15 V.

    Ic3 = IE3 = (24 V - VB3 - 0,7 V) / R4

    But I don't know the value of R4?
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Hey but, your Vcc-pos is 21 V so VB3 voltage is equal to (if we ignore IB3):
    VB3 = 21V - Ic1*R3 = 21V - 1,825 mA*2kΩ1 = 21V - 3.65V = 17.35V
    Also notice that R4 and R5 "set" the Q3 voltage gain.
    I don't understand this ?
     
  4. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Yes, you are right. It's 21 V and not 8,8 V. I got confused. But what does this mean:

    "R4 and R5 "set" the Q3 voltage gain"?

    And just ignore, Quiescent current IC = 10 * Q3-Q4 IB.
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The Q3 voltage gain at first glance is equal to AvQ3 ≈ R4/R5
     
  6. Rumination

    Thread Starter Member

    Mar 25, 2016
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    But I don't know the value of either R4 or R5?
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    But you know their ratio.
    This is not a single answer to their correct values, only their ratio. You select the value to give the desired Q3 transistor bias current.
    For a real design this current would affect the frequency response which may be of interest.
     
  8. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Do you mean:

    Ad = gm * Rd ?
     
  9. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Now I have been working on this problem, but I really can't understand:(
     
  10. Jony130

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    Feb 17, 2009
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    You must start at the output. You have been given RL and Vout_max so from there you can find Ic4 and Ic3 quiescent current.
    And choose R4,R5R1,R1 to meet the voltage gain requirement at given quiescent current .
     
  11. crutschow

    Expert

    Mar 14, 2008
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    What's Rd?
    No, that gain equation only holds when there is no emitter resistor.
    The emitter resistor provides negative feedback and, for low values of gain (much less than gm*R5), the gain of Q3 is approximately R5/R4.
     
  12. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Can I do it like this:

    VB3=17.35V
    We know: VE3=(17.35+0.7)V=21-IC3*R4
    We also know: VB4=+0.7V=-21+IC3*R5.
    So by eliminating IC3 it gives a second equation for R4 and R5:

    18,05 = 21 - R4
    R4 = 2,95

    0,7 + 21 = R5
    R5 = 21,7

    Does it mean, that we have found the values of R4 and R5? And now we can find the value of Ic3?
     
  13. Rumination

    Thread Starter Member

    Mar 25, 2016
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    To find Ic3:

    Ic3 = IE3 = (21 V - VBE - 0,7 V) / (R4) = (21 V - 17,35 V - 0,7 V) / (2,95) = 2,95 / 2,95 = 1 mA.

    To find VB4:

    VB4 = -21 V + R5 * Ic3 = -21 V + 21,7 V * 1 mA = 0,7 V.
    Vout = VB4 - 0,7 V = 0 V

    To find Ic4:
    Ic4 = (0 - (-21 V)) / R6

    But I don't know the value of R6.
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But what about Vout-min = -8,8 V and RL = 49 Ohm ? Do you have any info about transistor current gain (beta /hfe) ?
    And a tip
    Ask yourself a question, why the output voltage is "clip" at -5V ?
    0.1.PNG
     
  15. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Does it mean, that I can find Ic4:

    Ic4 = V / R = 8,8 / 49 = 0,18 mA
     
  16. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Ic4 should be larger than this 0.18mA
     
  17. Rumination

    Thread Starter Member

    Mar 25, 2016
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    So what am I doing wrong?
     
  18. Jony130

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    Feb 17, 2009
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    It looks like you do not know (or understand) how to find maximum negative voltage swing at the load for a simply emitter follower circuit (Q4).
     
  19. Rumination

    Thread Starter Member

    Mar 25, 2016
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    Yes, you are right. I will try and find it in my book. I will be back.:)
     
  20. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    To help you with this problem look again at my post #14 and Ask yourself -- >What voltage at the T1 base will cut-off T1 transistor?
     
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