Guide for Analog and Digital Electronics
Principle of Operation:
When a positive Drain (D)-Source (S) voltage(VDS) is applied with Gate(G) shorted with the
Source (S) terminal (VGS=0), the electrons in the N-Channel are attracted to the Drain (D)
terminal and due to the flow of electrons, Drain Current (ID) is established. The value ID depends
on the applied VDS and the resistance of the N-Channel. There is uniform voltage drop across the
channel and the two P-N junctions are reversed biased. This results in increase of width of the
depletion regions. The depletion regions are wider near the drain region.
ID increases linearly with the increase of VDS till saturation effect sets in. The value of VDS where
the saturation effect sets in is referred to as Pinch-Off (VP) voltage. When VDS reaches VP, the
value of ID remain same with the further increase of VDS.
The Gate–Source voltage (VGS) is to control the value ID. When a negative voltage is applied
between Gate and Source terminals, there is an increase of width of the depletion layers and as a
result the value of Drain Current (ID) decreases. As the value VGS is made further negative, at a
certain value of negative VGS, the Drain Current become zero. This voltage is referred as GateSource pinch-Off voltage.
The relation between the Drain Current, ID for a given value of VGS is given by
ID IDSS 1 GS
Where IDSS is the Drain to Source Current when Gate is shorted with the Source.
VP is the Pinch –Off voltage.
The drain resistance (rd) in the saturation region is given by
Where r0 is the resistance at VGS=0 and VP is the Pinch –Off voltage.