FET BIASING

DEPLETION-TYPE MOSFETs
The similarities in appearance between the transfer curves of JFETs and depletiontype
MOSFETs permit a similar analysis of each in the dc domain. The primary difference
between the two is the fact that depletion-type MOSFETs permit operating
points with positive values of VGS and levels of ID that exceed IDSS. In fact, for all
the configurations discussed thus far, the analysis is the same if the JFET is replaced
by a depletion-type MOSFET.
The only undefined part of the analysis is how to plot Shockley’s equation for
positive values of VGS. How far into the region of positive values of VGS and values
of ID greater than IDSS does the transfer curve have to extend? For most situations,
this required range will be fairly well defined by the MOSFET parameters and the
resulting bias line of the network. A few examples will reveal the impact of the change
in device on the resulting analysis.

ENHANCEMENT-TYPE MOSFETs
The transfer characteristics of the enhancement-type MOSFET are quite different from
those encountered for the JFET and depletion-type MOSFETs, resulting in a graphical
solution quite different from the preceding sections. First and foremost, recall that
for the n-channel enhancement-type MOSFET, the drain current is zero for levels of
gate-to-source voltage less than the threshold level VGS(Th), as shown in Fig. 6.35. For
levels of VGS greater than VGS(Th), the drain current is defined by


Feedback Biasing Arrangement
A popular biasing arrangement for enhancement-type MOSFETs is provided in Fig.
6.36. The resistor RG brings a suitably large voltage to the gate to drive the MOSFET
“on.” Since IG 0 mA and VRG 0 V, the dc equivalent network appears as shown
in Fig. 6.37.
A direct connection now exists between drain and gate, resulting in

The result is an equation that relates the same two variables as Eq. (6.25), permitting
the plot of each on the same set of axes.
Since Eq. (6.28) is that of a straight line, the same procedure described earlier can
be employed to determine the two points that will define the plot on the graph.