common emitter BJT amplifier question

Discussion in 'Homework Help' started by xmadmaxx, Apr 22, 2007.

  1. xmadmaxx

    Thread Starter New Member

    Apr 22, 2007
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    Hi, I'm new here so I hope you answer quickly as I have to hand this up tomorrow. I'm planning to visit this site regulary beacuse it's got almost all I need for uni...

    I have a problem with this question: If you vary Vcc (in common-emitter BJT amplifier using voltage-divider bias circuit) how does that affect input and output signals and hence voltage gain Av?

    Thanks in advance
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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  3. xmadmaxx

    Thread Starter New Member

    Apr 22, 2007
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    ok, sorry about that, didn't see it and I wanted to be as quick as possible...


    basically I figured out that varying Vcc doesnt change Vi at all, but it is proportional to Vo. And beacuse Av=Vo/Vi, Av is also proportional to Vcc. Is that right?

    And now I have read somewhere on the site that DC bias has to be sufficient enough so decreasing Vcc below some value I guess will cause the Vo to clip?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    Good work!
     
  5. xmadmaxx

    Thread Starter New Member

    Apr 22, 2007
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    thanks! I was hoping thats right...

    And I got one more query thats very similar to the other one...

    If now, input signal Vi is slowly increased (Vcc stays the same), I know the Vo will get clipped on top and bottom (on the graph), but I don't know which side will get clipped first, top or bottom?

    I'm guessing that top Vo will get clipped first and then soon after the bottom part will... I guess its beacuse at the top of Vo Vi is down the bottom and hence Ib is down the bottom (close to zero) so it's gotta be cut off,

    and when Vi is on top, Vo is on bottom, and beacuse Ib is high and Ic also that means its saturation...

    hehe, hope you undestood what I meant...
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That depens on the bias.
    If it is too low it will clip at the bottom, if too high it will clip on the high side.
     
  8. xmadmaxx

    Thread Starter New Member

    Apr 22, 2007
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    And what does it mean low bias, Q-point closer to y-axis(saturation mode)?

    So basically, If I know that it clipped at the bottom first this means transistor was working in saturation mode?
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If it clips at the bottom, than it means that the DC component of the signal is too low, and because it cannot go lower than ground it clips at the bottom.

    Common emitter inverts the signal, so if you clip at the bottom, the bias is a little higher than it should be. The right bias will clip high signal at both sides simetrically.
     
  10. xmadmaxx

    Thread Starter New Member

    Apr 22, 2007
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    I dont quite understand this, if increasing Vi, clips Vo at the bottom first, for the common emitter,the DC voltage bias at the input is a little higher than it should be?
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    Remember: The common emitter amp is an inverting amp. The higher the i/p, the lower the o/p.
     
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