A little question about bypass capacitor in CE amplifier.

Discussion in 'General Electronics Chat' started by mmosT, Mar 25, 2012.

  1. mmosT

    Thread Starter New Member

    Jul 3, 2010
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    Hi I all, I just learned the working of a common emmitter amplifier, biased with voltage divider bias. Now, from what I learnt, I understand that, we feed input signal to base, then inside the transistor amplification process takes place ( I know how does it theoratically... I dont konw how EXACTLY the current flows during amplification.. And I also seems not to understand how the electrons from base crosses the reverse biased BC junction.. ).
    Well, after the amplifiaction process, we get our output signal at collector. All fine upto here.
    Now I know that the resistance at the emitter is for the biasing purpose, and the capacitor which is connected in parallel with it (i.e. bypass capacitor) is used to provide low impedance for the ac signal.
    and I exactly dont understand this thing at all. I read several posts, several references but I am just not getting that..

    I have googled one sample circuit just for reference and it is located at here >
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/npnce2.gif

    Let me short my question,
    How can the AC current flow from that capacitor which is grounded when all the amplification process is taking place in BJT and we are even getting output at collector.

    I am very damn confused about the current flow in the circuit as well..

    Please help me guys, my final exam of Solid State Devices And Circuits is only one month away!! thanks in advance.
    Also, if I am wrong correct me please.. thank you guys..
     
  2. #12

    Expert

    Nov 30, 2010
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    I believe you can see that the DC bias current is caused by the voltage divider on the base. The slightly less voltage at the emitter is limited by the emitter resistor. That's all well and good for the DC bias, but the emitter resistor also limits the current when an AC signal comes along and meets the base. Adding a capacitor across the emitter resistor is just a trick to get the AC signal to flow more freely without messing up the DC bias.
     
  3. mmosT

    Thread Starter New Member

    Jul 3, 2010
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    Thanks for that quick response!! I think I should clarify my question even more..
    Let's start from source, I apply the AC current with some voltage to it. Bjt is already biased in active region, so through input capacitor and series resistor AC current gets into the base. Here, amplification takes place, then, through bjt, we get AC output at collector after output capacitor.

    In this process, when and how the AC signal passed through that bypass capacitor?
     
  4. MrChips

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    Oct 2, 2009
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    The emitter resistor RE is very important for temperature stability. It applies negative feedback to the amplifier.

    If the emitter current increases because of increasing temperature, the voltage across the emitter increases. This reduces the base-emitter voltage which has the effect of reducing the base current and hence the emitter current.

    By including RE, the gain of the amplifier is reduced.

    If we bybass RE with a capacitor, the DC bias and DC gain is not affected but we reclaim the AC gain that was lost because of the negative feedback.

    Try measuring the AC gain without and with the capacitor and you will see a huge difference in gain.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Note that the voltage gain of a bipolar CE transistor is basically equal to the collector resistance Rc, divided by the equivalent emitter resistance.

    If the external emitter resistance, Re, has a capacitor of sufficient size across it, then the AC gain is determined by the internal intrinsic emitter resistance, re, only and equals Rc / re, since any AC emitter current is bypassed by the capacitor.

    If the emitter resistor Re is not bypassed, then the AC gain is Rc / (re +Re).

    You can thus see that bypassing the emitter resistor increases the AC gain.
     
  6. mmosT

    Thread Starter New Member

    Jul 3, 2010
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    Ok, thanks for reply guys, I understand that the rc increases the gain. But my question still remains unanswered, which was, when and how the AC signal passed through that bypass capacitor
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    In simple terms - because AC is reversing its polarity and the DC is not. It is just a natural function of the capacitor. You can read about capacitors and other things electronic in our AAC E-book , the links are at the top of every forum page.
     
  8. MrChips

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    AC does not pass through the Re bypass capacitor.
    The capacitor holds Ve at a constant voltage and hence eliminates the negative feedback.
     
  9. Ron H

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    Signal current does indeed pass through the Re bypass capacitor. Virtually ALL the AC collector current flows through that capacitor (assuming mid-band frequencies).
     
    Last edited: Mar 27, 2012
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  10. MrChips

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    But there is no AC current. It has been eliminated, reduced to near zero.
     
  11. Ron H

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    Come again?:confused::confused:
    Are you saying that AC current does not exist globally, or only that it does not exist in the capacitor in question?
     
  12. MrChips

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    I am referring to the voltage and current on a reservoir capacitor.
    If the voltage is constant on the capacitor, there is no AC current.
     
  13. Ron H

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    That's like saying current can't flow in a short circuit. This capacitor is basically a short circuit to AC (|Xc|=1/(ω*C)).
    EDIT: The DC voltage on the Re bypass capacitor will always have some miniscule amount of ripple if a signal voltage is present on the base.

    EDIT: Perhaps this can explain it better than I am apparently able to do.
     
    Last edited: Mar 27, 2012
  14. MrChips

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    There is always some ripple voltage and hence there is a ripple current associated with the discharge and recharge of the capacitor.

    We call it a bypass capacitor, or an "AC ground" because it is a low impedance path for high frequency signals. In reality, the AC signal has been smoothed out. There are no electrons oscillating back and forth in the capacitor (assuming ESR is zero).
     
  15. Ron H

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    MrChips, do you agree that signal current flows in the collector (and emitter) of a CE amplifier? Where does that current flow, if not through the Re bypass capacitor?
     
  16. MrChips

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    Current flows in Rc and Re.

    If you short an AC supply with a capacitor, the capacitor will blow up because of the high current. And yet if you insert a rectifier diode in series, as in a half-wave rectifier circuit, the capacitor is safe, because the AC current is reduced to zero.

    btw, I give up.
     
  17. Ron H

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    You need to give up, because you are WRONG.
     
  18. MrChips

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    Ok, just to prove my point.

    How does one measure current?
    Place a low value resistor in series with the source and measure the voltage across the resistor.

    I challenge you to put a 1-ohm resistor in series with the bypass capacitor and measure the voltage across the resistor. Tell me how much AC current was bypassed by the bypass capacitor.

    Over and out!
     
  19. Ron H

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    That's an experiment you should try. You will be very surprised.:cool:
     
  20. crutschow

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    Those two paragraphs are contradictory. In the first you say there is current and in the second you say there isn't. Certainly if there are AC signals across the capacitor there will also be AC current and oscillating electrons (ESR does increase the capacitor impedance but even if it's zero the cap still has capacitive reactance).

    Any time an AC signal passes through or is bypassed by a capacitor there is current. A bypass capacitor across a resistor carries current proportional to the voltage across the capacitor and its impedance at the frequency of the AC signal. At high frequencies the impedance, and consequently the voltage, can be very low, but it is still finite.

    Your understanding of capacitor operation seems to be a little hazy. Suggest you take another shot at that. ;)
     
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