BJT Common Emitter Amplifier

Discussion in 'Homework Help' started by The_Cook, Oct 20, 2014.

  1. The_Cook

    Thread Starter Member

    May 29, 2014
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    upload_2014-10-20_22-18-20.png
    The values are :
    • Vs = 10 mV
    • Rb = 100 Kohms
    • Rc = 2 Kohms
    • Rl = 4 kohms
    • gm = 170 mA/V
    • r(pi) = 580 ohms
    • Rs = 50 ohms
    For this circuit I have to draw (a) draw a small signal amplifier equivalent circuit and (b) find the mid-band voltage gain for given Vs

    I would appreciate if someone helps me get started with the equivalent circuit.

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That circuit will not amplify AC as it is not biased. The output will basically be a half-wave rectified signal. Here's an example of a transistor biased to amplify an AC signal.
     
  3. The_Cook

    Thread Starter Member

    May 29, 2014
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    Okay but can a small signal equivalent circuit be drawn for it? I am kind of lost on where to begin
     
  4. b1u3sf4n09

    Member

    May 23, 2014
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    Last edited: Oct 21, 2014
  5. The_Cook

    Thread Starter Member

    May 29, 2014
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    upload_2014-10-21_0-6-46.png
    This is what I came up with, could any tell me if it is right or if I need to fix anything?
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Everything looks ok except Rs resistor, it should be connected in series with Vs
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    However, as Zapper pointed out, the hybrid PI model cannot be used to model the original schematic because the hybrid pi model assumes that the biasing has already been done.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The problem is that you will have to use a value of zero for the model gain (gm) since the transistor is biased off. :rolleyes:
     
  9. The_Cook

    Thread Starter Member

    May 29, 2014
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    Yeah I understand that the Hybrid Pi model cannot be used. But if we assumed that it is biased how would you go about solving the problem? I don't know if my professor made a mistake when he wrote it or not?
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well, simply solve this small-signal circuit as a ordinary DC circuit.
     
  11. The_Cook

    Thread Starter Member

    May 29, 2014
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    upload_2014-10-21_22-3-55.png
    Hey guys could you check my work real quick, appreciate it!
     
  12. b1u3sf4n09

    Member

    May 23, 2014
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    With the given information, vbe = 0, therefore gain = 0. Not only is the amplifier not on, but Vs is a DC voltage, which will have some issues bypassing the coupling capacitor. Is there a given frequency for Vs?
     
  13. The_Cook

    Thread Starter Member

    May 29, 2014
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    Nope just the voltage
     
  14. crutschow

    Expert

    Mar 14, 2008
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    He made a mistake if he intended the amplifier to have any gain above zero. :rolleyes:
     
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