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Miller effect in CMOS inverter!

Discussion in 'Electronic Basics' started by Francesco, Aug 27, 2006.

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  1. Francesco

    Francesco Guest

    Hi, someone can help me to understand this effect?

    I have two inverter in cascade:


    inverter inverter
    vin---|>----vout1----|>----vout

    and I must calculate the propagation time of vout1

    The propagation time is defined in my book as the time to decrease from
    VDD to VDD/2 or rise from 0 to VDD/2.

    I assume the vin rise istantaneusly from 0 to 1 logic, in this situation
    the gate-drain capacitor has a terminal that rise from 0 to VDD and the
    other terminal decrease from VDD to VDD/2.

    For my self it mean that the Miller effect increase the Cgd by a factor
    3/2 instead of 2 as my book say!

    Where I'm wrong?

    Sorry for my english?

    Francesco
     
  2. Miller capacitance effect is normally meant to be the effect of
    multiplication of the feedback capacitance on the input current during
    output swing.

    Since you are assuming that the input voltage on the first stage is
    independent of the input current, there is no miller effect in the
    first stage. You still have to account for the feedback capacitance
    effect on the output rate of change, since it still has to be charged
    through the output impedance after it is reverse charged by the fast
    input swing.

    While that is happening, the second stage miller effect adds current
    load (to that charging the first stage output to supply capacitance
    and second stage input to supply capacitance) on the first output.
    The current through the feedback capacitance also loads the second
    output (in addition to that charging the second output to supply
    capacitance.
     
  3. Francesco

    Francesco Guest

    To better show the situation:

    http://www.cse.ucsc.edu/classes/ee1...04.pdf#search="inverter CMOS "miller effect""

    At page 4 it speak about the miller effect, it explain that there is a
    complete swing in the input voltage and a complete swing in the output
    voltage, and so the multiplication of the Cgd!

    I'm studing an exercise that it ask to calculate the propagation time of
    two inverter in cascade!

    The solution speaks about the Miller effect across the Cgd (gate-drai
    acapacitance) of the first inverter's PMOS and NMOS.

    But I ask, why take into consideration the Miller effect as a 2 factor
    moltiplication of Cgd i the voltage at the drain terminal don't have a
    complete swing? It can be because the swing in the circuit is however
    complete even if we consider the first 50% of the swing?

    If it's true, there's another problem!

    Analysing the second inverter,the exercise solution show,there isn't
    Miller effect because of the second inverter commutation don't come
    before the last stage vout goes under VDD/2!

    This confuse my already very confued ideas about the problem!

    If someone want help me to understand...

    Thanks in advance and still sorry for my very bad english
     
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