MOSFET as Amplifier and as Switch
The basis of using the MOSFET as an amplifier is that:
 In saturation the MOSFET acts as a voltage controlled current source
 Changes in vGS → Changes in iD → Changes in output voltage across a load
 We need to get around the nonlinear relation to obtain linear amplification
 Technique of dc Biasing:
 Make MOSFET operate at a certain VGS and ID, (Operating Point)
 Superimpose the signal to be amplified vgs on the dc bias VGS.
 Keeping the signal small → changes in iD almost proportional to vgs
Large signal operation (Overall Picture)
Common Source Circuit (CS)
Equation imposed by the load (RD):
𝑖𝐷 =
𝑉𝐷𝐷 −𝑣𝑂
𝑅𝐷
On the characteristics of the MOSFET (iD -vs- vDS curves),
Draw the load line (solution to the above equation)
𝑣𝐺𝑆 = 𝑣𝐼
𝑣𝐷𝑆 = 𝑣𝑂
The intersection between the load line and the characteristics gives the operating points for
every 𝑣𝐺𝑆 = 𝑣𝐼 .
 Determine vO for various values of vI
 Voltage Transfer Characteristics (VTC)
Can be done graphically or analytically
Graphically: Point by Point we can draw the (VTC)
looking at the intersection points
corresponding to each value of vI.
Analytically: Write the equation of iD in MOSFET
Write equation of iD across RD,
Equate the two to obtain a relation
between vO and vI
Voltage Transfer Characteristics VTC
Point X vI = 0 → MOSFET is cut off → iD = 0 → vO = VDD
Point A vI = Vt → MOSFET is cut off → iD = 0 → vO = VDD
Between Point A and Point B: MOSFET is saturated
→ iD increases with increasing vI
→ 𝑣𝑂 = 𝑉𝐷𝐷 − 𝑅𝐷 𝑖𝐷 decreases as vI is increasing
After Point B: MOSFET is triode
Curves are very crowded →
further decrease of vO slow (VTC between B and C)
At Point C: vI = VDD → vO = VOC (very small )
MOSFET can be used as a switch; we use the two extreme regions of the VTC:
1. 𝑣𝐼 Low (𝑣𝐼 ≤ 𝑉𝑡 )
2. 𝑣𝐼 High (𝑣𝐼 ~𝑉𝐷𝐷 )
𝑣𝑂 High (𝑣𝑂 ~𝑉𝐷𝐷 )
𝑣𝑂 Low (𝑣𝑂 ~𝑉𝑂𝐶 )
Linear Amplifier
To operate MOSFET as a linear amplifier use saturation region of
the VTC:
1- MOSFET is biased at a point located in the middle of the
curve: (Point Q)
Q ≡ Operating Point or Quiescent Point
2- The signal vi to be amplified is superimposed on the dc
voltage VIQ.
 Keeping vi small
restrict operation to an almost
linear segment of the VTC
 Output signal vo is proportional to vi
 Amplifier is nearly linear
𝑣𝑜 = 𝐴𝑣 ∙ 𝑣𝑖
 𝐴𝑣 ≡ voltage gain at Q
𝐴𝑣 =
𝑑𝑣𝑂
𝑑𝑣𝐼 𝑣 =𝑉
𝐼
𝐼𝑄
is the slope of VTC at Q
Selection of Operating Point
 Bias Point Q1 does not leave sufficient room for
positive signal swing ( too close to VDD)
 Bias Point Q2 too close to boundary of the triode
region: does not leave sufficient room for negative
signal swing
 Operating Point Q ( below) give best selection