single transistor amplifier - effects of an emitter bypass capacitor on bandwidth

Discussion in 'General Electronics Chat' started by Coollestersmooth, Jan 5, 2015.

  1. Coollestersmooth

    Thread Starter Member

    Dec 18, 2014
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    I just wanted to check i am right in my thinking regarding the effects of an emitter bypass capacitor on the bandwidth of a single transistor amplifier.
    from what i understand the lower bandwidth limit is set by the impedance of the bypass capacitor in parallel with the emitter resistor, however i am uncertain when it comes to the upper bandwidth limit.
    Am i correct in believing that it is due to the impedance of the bypass capacitor being connected in series with the impedance of the base emitter junction?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Voltage Gain of a transistor stage is essentially a ratio of collector impedance divided by emitter impedance. The capacitor allows a lower impedance (more gain) as frequency increases.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The main determinant of upper bandwidth frequency would be the Miller Effect rather than the emitter bypass cap.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    The emitter resistor provides negative feedback and hence reduces the AC gain.
    Thus if you bypass the emitter resistor with a capacitor you are going to restore the gain at higher frequencies. Hence it lowers the gain at low frequencies.
    It determines the low frequency limit, not the high frequency limit.
     
  5. #12

    Expert

    Nov 30, 2010
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    Second time today! It's in the collector circuit that you limit the high frequency response.
     
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