# MOSFET DC Biasing Design

Hi,

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.

Qn1
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

Qn2
Is there any difference in dc biasing design of n-type enhancement MOSFETs with n-type depletion type MOSFETs? What is the difference?

Qn3
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?

Qn4
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

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

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.