Mumbai University > Electronics ana telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 10M

Years: Dec 14

Biasing of Enhancement MOSFET:

Figure 2.11 shows the drain-to-gate bias circuit for enhancement mode MOSFET. Here, the gate bias voltage is $V_GS = [ \frac{R_1}{(R_(1 )+ R_f )}]V_DS.$ This circuit offers the d.c. stabilization through the feedback resistor R_f. However, the input resistance is reduced because of Miller effect.

Also, the voltage divider biasing technique given for JFET can be used for the enhancement MOSFET. Here, the d.c. stability is accomplished by the d.c. feedback through $R_S$.

But the self-bias technique given for JFET cannot be used for establishing an operating point for the enhancement MOSFET. Figure 2.12 shows an N-channel enhancement mode MOSFET common source circuit with source resistor. The gate voltage is

$V_G=V_GS = [\frac{ R_1}{(R_(1 )+ R_2 )}]V_DS$

And the gate to source voltage is $V_GS= V_DD - V_G$

Assuming that $V_GS \gt V_TN$ and the MOSFET is biased in the saturation region, the drain current is

$I_D= K_N (V_GS-V_TN )^2$

Here the threshold voltage$ V_TN$ and the conduction parameter $K_N$ are functions of temperature.

The drain to source voltage is

$V_DS $= $V_DD-I_D R_D$

If $V_DS \gt V_DS_{(sat)}$ = $V_GS - V_TN$ then the MOSFET is biased in the saturation region. $V_DS \lt V_DS_{(sat)} $= $V_GS$ - $V_TN$ then the MOSFET is biased in the non-saturation region, and the drain current is given by,

$I_D=K_N[2(V_GS-V_TN)V_DS-D_DS^2]$

The transfer characteristics of the enhancement type MOSFET are completely different from that of a JFET.

The most important difference between the two is that for the n-channel enhancement type MOSFET, drain current $I_D$ = 0 for $V_GS \lt V_GS_{(th)}$as shown in Fig. 2.13.

For $V_GS \lt V_GS_{(th)}$ the relation between $I_D and V_GS$ is defined as follows:

$I_D= K_N (V_GS-V_TN )^2$ ….. (1)

As $V_GS$ is sometimes denoted by $V_T.$

Plotting the transfer characteristics:

The first point plotted is point "A" where $V_GS$ = $V_(GS(th))$ and $I_D$ = 0.

The second point which can be easily plotted is point "B" which corresponds toV_GS = $V_(GS(on))$ and $I_D$ = $I_(D(on))$.

To plot the remaining points we will have to obtain the value of conduction parameter "k" as follows:

$ I_D=k[V_GS-V_GS(Th)]^2$

$I_D(ON)=k[V_GS(ON)-V_GS(Th)]^2$

k =$\frac{I_D(ON)}{ [V_GS(ON)-V_GS(Th)]^2} = \frac{I_D(ON)}{ [V_GS(ON)-V_T]^2}$------ (2)

Once the value of "k" is obtained from above Equation (2), we can substitute different values of V_GS in Equation and obtain the corresponding values of I_D, to plot the remaining points of characteristics.

Biasing of Depletion MOSFET:

Both the self-bias technique and voltage divider bias circuit given for JFET can be used to establish an operating point for the depletion mode MOSFET.