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MOSFET DC Biasing Design

  1. Nov 15, 2005 #1

    I know how to bias a BJT transistor to operate in the active region, so that the amplifier circuit can operate as a small-signal amplifier. Two methods
    (i) Use biasing resistors to form a self-biasing circuit.
    (ii) Use diode biasing, coupled with biasing resistors.

    However, how do we bias the transistor circuit to act as small signal amplifier, if MOSFETs are used instead?
    Design of dc biasing circuit for MOSFET as an amplifier???

    I read from my textbook that we need to bias the MOSFET in the saturation region, which is quite different from the BJT amplifier dc biasing design.
    The relation of MOSFET in saturation region is
    Id = K(Vgs - Vt)^2

    where Id = dc drain current
    K = a property of MOSFET (What exactly is this?)
    Vgs = dc gate-source voltage
    Vt = threshold voltage of MOSFET

    Is there any difference in dc biasing design of n-type enhancement MOSFETs with n-type depletion type MOSFETs? What is the difference?

    After the MOSFET biasing point is fixed (saturated), we input an small ac signal in the input port, to get an amplified output signal in the output port, with the midband gain depending on the amplifier design.
    How is the small signal equivalent circuit model different for enhancement and depletion type MOSFETs?
    My answer is, they are essentially the same, but with one slight difference:
    vgs for enhancement type is positive, while vgs for depletion type is negative, am I correct?

    In the small signal model of MOSFET, the ac drain current
    id = transconductance * vgs ---Linear, voltage-controlled current source
    Why is it this formula, and NOT Id = K(Vgs - Vt)^2?

    transconductance = 2K(Vgs - Vt) --Enhancement and Depletion type MOSFET
  2. jcsd
  3. Feb 8, 2010 #2
    U have to bias the MOSFET to have the full output swings. choose your bias point so that in any case it should not go out of saturation.
    the equation for mosfet in saturation region is
    Id=1/2 U Cox W/L(Vgs - Vt)^2
    where U = mobility, Cox= capacitance and W/L = width and legth of mosfet

    if u put the value of transconductance in current equation which a reverse of resistance in triode mode.....u will get the value of same formula
  4. Feb 8, 2010 #3
    Q1) To bias a MOSFET the voltages from the gate to source and from the gate to drain need to be regulated. The device operates in saturation when the gate to source voltage Vgs is greater then the threshold voltage and when the gate to drain voltage is less then the threshold voltage of the transistor. Both conditions must be satisfied.

    Q2) The only difference between depletion and enhancement MOSFETS is the value of the threshold voltage, at least its the difference you care about in circuit design.

    Q3) Your answer is right

    Q4) You do not use Id = K(Vgs - Vt)^2 because you are using a linear approximation of Id = K(Vgs - Vt)^2. If Vgs is only changing by 1mV then the current Id will change in an almost linear way. The linear approximation allows circuit design to be much simpler by only working with linear equations rather then quadratic equations. Keep in mind that it is an approximation though, the larger the voltage swing at the input of the amplifier the less accurate the approximation is.
  5. Feb 10, 2010 #4
    nMOS ENHANCEMENT mode is when you aply positive voltage at the gate and it form a channel under the gate .....so it attracts the electronwhereas when u apply negative voltage ta the gate , it actually deplete the region under gate bacoz of negative supply.........

    For analog design circuit, u hav to keep the device in saturation mode in case...........coz to set the threshold voltage in triode is very difficult.
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