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lec3------high frequency response

lec3------high frequency response - The model can be...

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The model can be further simplified if we ignore shown in Figure . db C
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Symbol Name Origin  Value C gs Gate-Source capacitance Physical (gate oxide) C gd Gate-Drain capacitance (small) Physical (drain diffusion overlap under gate oxide) L ov  is  the  overlap  length  between drain & gate C sb Source-Substrate Capacitance Depletion capacitance between the source and the substrate C sb0 is the source-substrate when V SB = 0 V o is the junction built-in potential (0.6V to 0.8V) C db Drain-Substrate Capacitance Depletion capacitance between the drain and the substrate C sb0 is the drain-substrate capacitance when V DB = 0 2 3 ox WLC ov ox WL C 0 1 sb sb SB o C C V V = + 0 1 db db DB o C C V V = +
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Normalized high-frequency response of the amplifier
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Let us calculate the MOSFET unity-gain frequency f T . f T is considered a figure of merit for the high- frequency operation of the MOSFET as an amplifier. f T is defined as the frequency at which the short- circuit current-gain of the common-source amplifier becomes unity.
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Short- circuit current gain I o /I i . Frequency at which the short circuit current gain of CS becomes Unity Input and the output between two devices are the same level. gs gd gd gs V sC sC V I - = - = 1 I gs gd gs m o V sC V g I - =
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) ( ) ( 1 ) 1 ( gd gs i gs gd gs T C gs m o C C s I V C C s X V g I + = + = - - - - gs gd gs m o V sC V g I - = I At frequencies of interest C gd is small Capacitive reactance is very small so sC gd V gs can be neglected.
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