Emitter-Follower Amplifier

Discussion in 'Homework Help' started by Chinh Thuy Nguyen, Dec 21, 2016.

  1. Chinh Thuy Nguyen

    Thread Starter New Member

    Dec 21, 2016
    2
    0
    Hi everybody,
    I'm meeting a trouble with Emitter-Follower Amplifier circuit. That is in two attached pictures.
    What is the meaning of two capacitors that I highlight by the red circle?
    As well as, what is the "self-trigger" in Amplifier circuit?
    Thank you!
    IMG_0512.JPG IMG_0513.JPG
     
  2. MrChips

    Moderator

    Oct 2, 2009
    18,891
    6,051
    The first circuit does not have an emitter follower.
    The second circuit has an emitter follower in the second stage.

    The capacitors you have circled in red change the response at higher frequencies.

    Is this homework?
     
  3. Chinh Thuy Nguyen

    Thread Starter New Member

    Dec 21, 2016
    2
    0
    Oh sorry, I have a mistake with the picture 1.
    Yes this is my homework :)
     
  4. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    2,490
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    Do you know what RL means? In the 2nd circuit, if a capacitor is in series with RL, what can you say about the current seen by RL?
     
  5. crutschow

    Expert

    Mar 14, 2008
    22,518
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    In the first circuit, the common-emitter output stage has the DC bias point (DC gain) determined by R8, and the high frequency AC gain determined by R4 and C.
    The low frequency corner for the transition from the DC gain value to the AC gain value is determined the the R4C time-constant.

    In the second circuit Ci, Cc, Ce1 and Ce2 are DC block capacitors that pass the AC signal but block the DC bias voltages.
    Ce1 bypasses the emitter of Q1 so that Q1's gain is maximum (not reduced by the degeneration effect of Re1).
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    In that case, moved to Homework Help.
     
  7. Zeeus

    Member

    Apr 17, 2019
    376
    12
    Nice...So in the first circuit, he's trying to invert the signal twice..Although why use current source as input? Isn't Rout of current high?

    Same as second circuit..In the second stage, bias resistors (120k||20k) * 10 > beta * Rin(270 + 120 + re)..(This is not bad design?)

    Thanks..was thinking about this before to make (invert twice)
     
  8. Zeeus

    Member

    Apr 17, 2019
    376
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    hmm..Don't understand
     
  9. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hello,

    As a first approximation, look at all the capacitors as a short for AC analysis, and as a DC voltage source for DC analysis.
    That will tell you what they are for qualitatively.
    To get quantitative results you have to do a full analysis.
     
  10. crutschow

    Expert

    Mar 14, 2008
    22,518
    6,598
    Below the R4C corner frequency it series impedance starts increasing and the gain is eventually determined by the value of R8 at DC.
    At higher frequencies the impedance becomes equal to R4, which then determines the AC gain.
     
  11. Zeeus

    Member

    Apr 17, 2019
    376
    12
    The first image, the cap highlighted in red, please what's polarity?

    Thought + side to emitter of q2 but after rethinking, think it's the other way around

    what is it?

    Thank you
     
  12. Ylli

    Active Member

    Nov 13, 2015
    700
    201
    + on the resistor side.
     
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