Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to draw AC equivalent circuit of CE amplifier?

  1. Feb 28, 2016 #1
    in the attached diagram its seen that while drawing AC equivalent of the common emitter circuit, Re is not taken in, the AC equivalent diagram no.2. why? IMG_20160228_210715.jpg
     
  2. jcsd
  3. Feb 28, 2016 #2
    We simplify assume that CE capacitor ( the capacitor connected across Re resistor) is large enough (very small Xc) so that we can treat him as a "short circuit".
     
  4. Feb 28, 2016 #3

    LvW

    User Avatar

    It is the only purpose of the capacitor CE across RE to ground the emitter node for the operating frequency range.
    Therefore, in the ac equivalent diagram the emitter node is grounded (ac wise).
    In addition, please note that in such an equivalent diagram it is common practice also to replace the transistor by its small-signal equivalent model (h-parameters or y-parameters).
     
  5. Feb 29, 2016 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    The emitter resistor provides feedback that helps stabilise the DC operating point of the transistor. For example if the collector emitter current were to increase for some reason then the emitter resistor causes the emitter voltage to rise. That in turn reduces VBE which reduces the collector emitter current again.

    However a side effect of the emitter resistor is to reduce the gain...

    Gain = RC/RE

    Adding that capacitor doesn't effect the DC biasing but improves the AC gain.

    There is quite a good explanation of that here..

    http://www.electronics-tutorials.ws/amplifier/emitter-resistance.html
     
  6. Mar 1, 2016 #5

    LvW

    User Avatar

    I agree, of course, to CWatter´s explanation.
    However, I don`t agree to his recommendation (link).
    This link is not very serious and the contributions contain many errors and wrong explanations.
    Example: The above link contains the sentence:
    "Now if the supply voltage Vcc increases, the transistors collector current Ic also increases for a given load resistance"
    I think, everybody will agree that this "explanation" shows that the author does not know how a transitor really works.
     
  7. Mar 1, 2016 #6

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    But it does. As VCC increases the base bias voltage increases. That increases the voltage across the emitter resistor and hence the collector current also increases.
     
  8. Mar 1, 2016 #7

    LvW

    User Avatar

    (Typo?)

    I am not sure if this was the idea of the author. The most important reason for RE-feedback is to reduce the influence of tolerances (passive parts and primarily uncertainty of beta).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How to draw AC equivalent circuit of CE amplifier?
  1. Ce amplifier (Replies: 1)

Loading...