Unity frequency and Miller capacitor

Discussion in 'General Electronics Chat' started by anhnha, Jan 9, 2016.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    I am not sure how Cgd is included in the formula below.
    Cgd is a Miller capacitor. How is it converted to parallel with Cgs?

    Unitiy frequency.PNG
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    In the AC analysis of analog circuits, one of the primary assumptions is that there is zero ohms AC impedance between all power supply rails and GND. So from an AC point of view, the drain and source are shorted together in the above drawing. This places Cgs in parallel with Cgd, so they add.

    In a real-world circuit with inadequate power source bypassing, the above assumption is not true and all of the standard circuit equations change, leading to distortion, oscillations, etc.

    ak
     
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  3. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    774
    48
    Thanks, I've got it now.
    BTW, do you design analog IC?
     
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