Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 10M

Year: Dec 2015

The different biasing circuit used for E-MOSFET are as follows :

1. Feedback Biasing Circuit
2. Voltage Divider Biasing Circuit

1) Feedback Biasing Circuit

i. A popular biasing arrangement for enhancement type MOSFET is shown in Figure 1.

ii. The resistor $R_G$ brings a suitably large voltage to the gate of MOSFET so as drive it in ‘ON’. Since drain to gate resistance $R_G$ returns a part of output signal back to input, hence the circuit is called feedback biasing.

iii. Since $I_G$ = 0mA and $VR_G$ = 0V, the dc equivalent shown in Figure 2.

Figure 2 : DC quivalent of network

iv. So that direct connection exists between drain and gate, resulting in

$V_D$ = $V_G$

VDS = VGS …………………………..(1)

v. For the output circuit,

VDS = VDD - $I_DR_D$

Substituting equation (1) in above equation,

VGS = VDD - $I_DR_D$……………………….(2)

vi. This equation is a straight line, to describe two points that will define graph resulting in operating point.

Put $I_D$= 0mA in equation (2)

VGS = VDD

Substitute VGS = 0 V in equation (2)

$I_D$ = $\frac{VDD}{R_D}$……………………….(3)

Hence plot becomes,

Figure 3 : Graph for determining Q-point