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n-channel MOSFET common source amplifier

Discussion in 'Electronics Homework Help' started by studentLab, Apr 23, 2014.

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  1. studentLab

    studentLab

    11
    1
    Apr 21, 2014
    Hello and good day for all, I need advice in finding Vout and ac voltage gain in n-channel MOSFET common source amplifier.

    [​IMG]


    Here i get :

    Since the source is grounded, the gate-to-source voltage is
    VGS = (R2 / (R1 + R2)) x Vdd

    VGS = (33k / (51k + 33k)) x 15

    VGS = 5.893V


    ID = k [ VGS – VGS(th) ]2

    Since: VGS = 7.5V, VGS(th) = 5V, ID = 5mA

    Therefore : ID = k [ VGS – VGS(th) ]2

    k = ID / [ VGS – VGS(th) ]2

    k = 5mA / [7.5V - 5V]2

    k = 0.8mAV-2


    The Drain Current ID at VGS = 5.89V is ID = 0.8mAV-2 [ 5.89V – 5V ]2

    ID = 0.8mAV-2 [ 5.89V – 5V ]2

    ID = 0.634mA


    The Drain Voltage is VD = VDS = 15V – [0.634mA x 3.3k]

    VD = VDS = 15V – [0.634mA x 3.3k]

    VDs = 12.90V


    The Drain to source resistor is rDS = VDS / ID

    rDS = 12.90V / 0.634mA

    rDS = 20.35k


    Rin = R1||R2

    Rin = 51k x 33k / 51k + 33k

    Rin = 1683k / 84k

    Rin = 20.04k




    ro = VM / ID

    ro = 50V / 0.634mA

    ro = 78.9k


    Vout = −gmVgs(ro || RD) ----------------------> It is VGS same with Vgs?

    Vout = -6mS Vgs ( 78.9k x 3.3k / 78.9 + 3.3k )

    Vout = -6mS Vgs ( 3.17k )

    Vout = -19.02 Vgs

    [​IMG]
     
  2. studentLab

    studentLab

    11
    1
    Apr 21, 2014
    Vgs = ( Rin / Rin + 0 ) x Vin

    Vgs = ( 20.04k / 20.04k + 0 ) x 45mV

    Vgs = 45mV


    Vin = Vgs

    Vin = 45mV


    Vout = −gmVgs(ro || RD)

    Vout = -6mS x 45mV ( 78.9k x 3.3k / 78.9 + 3.3k )

    Vout = -6mS x 45mV ( 3.17k )

    Vout = -0.856V


    Ac output gain = Av = Vout / Vin

    Av = -0.856V / 45mV

    Av = -19.02

    Do i doing right here?
     
  3. Ratch

    Ratch

    1,079
    330
    Mar 10, 2013
    For calculating K you are using a formula for the triode region of the MOSFET. Besides, you left out the Vds term. I think you want to use the formula for the saturated region which is Id = K*(Vgs-Vt)^2 , which gives K=0.006272 .

    Ratch
     
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