MOSFET Biasing, Microelectronic circuits, Sedra

Discussion in 'Homework Help' started by alexei_kom, May 7, 2011.

  1. alexei_kom

    Thread Starter New Member

    Nov 22, 2009
    9
    0
    Hello everybody,

    I'm having trouble understanding the following example (from Microelectronic circuits, Sedra):

    k'*(W/L)=1 [mA/V^2]
    Vt=1V
    Vdd=15V

    This is the circuit (with established voltages and currents):

    [​IMG]
    Uploaded with ImageShack.us


    That's what written in the TB:
    "Observe that the DC voltage at the drain (+10V) allows for a positive signal swing of +5V (i.e., up to Vdd) and a negative signal swing of -4V [i.e., down to (Vg-Vt)]."

    I can't figure out how the negative signal swing is -4v? Maybe I'm interpreting this citation wrongly?

    I found that the negative signal swing is -1.9V.

    Here is my solution:

    [​IMG]
    Uploaded with ImageShack.us
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Yes there is something strange about the statement. Suppose the FET were able to fully saturate such that VDS =0V. The maximum possible saturation drain current flow would then be 15V/20k or 0.75mA. In that case VD could fall to a minimum of 7.5V. The maximum possible asymmetrical drain voltage swing would then be +5V to -2.5V [certainly not -4V].

    Perhaps the author is "inadvertently" considering the case where the source resistance is fully AC bypassed (by a capacitor of suitable value). In that case it would certainly be possible for the drain to swing to the -4V level offset - from the static 10V DC value.
     
  3. alexei_kom

    Thread Starter New Member

    Nov 22, 2009
    9
    0
    Thanks for the reply!:)

    I don't think that bypass capacitor was ment here because it is first introduced in later section of this chapter...
     
  4. miguel cool

    New Member

    Mar 15, 2010
    9
    0
    t refears that VD +ΔVD = 10V+5V positive swing

    and VD +ΔVD = 10V-5V negative swing

    that refears to the variation with respect the DC VD
     
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