Gate capacitance

Discussion in 'General Electronics Chat' started by Jaguarjoe, Nov 24, 2010.

  1. Jaguarjoe

    Thread Starter Active Member

    Apr 7, 2010
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    I read somewhere that the gate capacitance between the gate and the drain is more critical than the capacitance between the gate and the source. Why is that so, or isn't it? How can you affect one without the other, there's only one value for gate capacitance (or charge)
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The gate charge is the charge required to charge both the gate-drain capacitance and the gate-source capacitance.
     
  3. Jaguarjoe

    Thread Starter Active Member

    Apr 7, 2010
    770
    90
    I found it. Cdg is important because it is the Miller capacitance. It is feedback from the drain to the gate, like C-B in a BJT and some vacuum tubes had/have. I think that's one reason for screen grids.
     
    Last edited: Nov 24, 2010
  4. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Cdg can also contribute to gate failure from overvoltage due to transient voltage on the drain (if the gate-source shunt resistance is too high). Depending on fet technology, Cdg can be quite non-linear, and a significant contributor to Coss (which may be a power loss mechanism in certain switchmode topologies).
     
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