MOSFET CS Amplifier (VTC)

Discussion in 'Homework Help' started by jegues, Sep 27, 2011.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    See figure attached for the question and the figures given.

    I'm having a little trouble for part a), I know at point A,

    v_{GS} = V_{t} and the transistor is just turning on, and since v_{DS} > 0 we will enter saturation.

    For point B,

    v_{GS} - V_{t} = v_{DS}

    but how do I determine Rd from this? I haven't been told anything about vGS or iD.

    Hopefully once I get the ball rolling on part a), parts b), c) & d) should fall into place.

    Thanks again!

    EDIT:

    a) Vgs = Vds + Vt, Vds = 0.6V

    i_{D} = \frac{1}{2}k_{n}(v_{GS} - V_{t})^{2}

    i_{D} = \frac{V_{DD} - V_{DS}}{R_{D}}

    i_{D} = 1.25mA, \quad R_{D} = 1.6k \Omega

    b)

    v_{DS} = 0.5V \quad \rightarrow \quad v_{GS} = 1V

    v_{DS} = 2.5V \quad \rightarrow \quad v_{GS} = 3V

    I'm stuck on part c) now.
     
    Last edited: Sep 28, 2011
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    In the first place tell me why you want to find Rd ??
    Rd is given and its equal to Rd = 24kΩ

    As for question a

    The point A is equal to

    V_{ds} = V_{t}

    And point B

    V_{ds} = V_{gs} - V_{t}

    And we also know that:

    V_{ds} = I_{d} * R_{d}

    And

    I_{D} = \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}

    So you need to solve this equation

    V_{dd} - (I_{d} * R_{d}) = V_{gs} - V_{t} = V_{dd} - \frac{1}{2}K_{n}(V_{GS} - V_{T})^{2}*Rd = (V_{gs} - V_{t})
     
    Last edited: Sep 28, 2011
  3. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Sorry I had posted the wrong question by mistake, the correct one is up there now.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    I also made the mistake
     
  5. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    It's okay, the correct question is up there now so hopefully things are more clear.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    As for C

    Simply find Id for Vgs = Vdd next find Vd

    rds = Vd/Id
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    And can you tell me.
    How can you get Vgs --> 3V for Vds = 2.5V for Vdd = 2.5V ?
     
  8. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    I must have done that incorrectly.

    How do I get the value of Vgs for Vds = 2.5V

    Is part a) okay?
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    The part a) looks okay except the last part Vgs=3V for Vds = 2.5V.
    To get 2.5V as a Vds the drain currant must be equal to zero.
    Then there will be no voltage drop across Rd resistor and Vds = Vdd.
     
  10. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    So as a result, Vgs = Vt = 0.5V, correct?
     
  11. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Yes, for Vgs = Vt----> Id=0A

    By the way what "VTC" means ?
     
  12. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    For part C),

    I solved the 2 equations for id for Vds and found that Vds = 0.077V, id = 1.51mA and rd = 50.85 ohms.

    Is this correct?
     
  13. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Voltage Transfer Characteristic
     
  14. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    Here's my work up to Part C), I'm unsure if the Vds->Id->rd I got for part C) is correct or not.
     
  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Can you telly me what value for Kn you use in your calculations?
     
  16. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    735
    43
    k_{n} = k_{n}^{'}(\frac{W}{L})

    Where the values of kn' and W/L are given in my attempt.
     
  17. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    So Kn is 10m right ?
     
Loading...