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  1. #1
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    simple question about NMOS when Vg=0

    Hi all,

    Please see the following circuit from the book Sedra and Smith. Here the transistor is in saturation even when the gate voltage Vg = grounded. I want to know , when the gate has been grounded there is no positive charges on the gate. Hence no inversion takes place below the gate and no channel is formed. If there is no channel at all then how is that there is a current flow and the transistor is in saturation. Please excuse me if the question is very simple.

    thanx in advance
    chethan

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  2. #2
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    Re: simple question about NMOS when Vg=0

    It's not JUST the gate voltage, what matters is the gate-source voltage (gate voltage - source voltage).

    In the picture, the gate voltage is grounded. You have TWO supplies in the picture, a positive 2.5V and a negative 2.5V.

    ==> Let us say that the source voltage in the picture is -1.0V (I'm just making up numbers here to go along with my example). The gate voltage is 0V. So your gate-source voltage is (0V) - (-1V) = 1V. If 1V is greater than the threshold voltage, you could indeed get current to flow in the transistor.


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