MOSFET: Solved Example Problems for Practice

Examples for Practice

Ex. 4.5.11 : For the transistor shown in the Fig. 4.5.15 the parameters are V_T =1V, k = 0.5 mA/V2. Determine V_GS, I_D and V_DS.

[Ans. : V_GS = 1.732 V, V_DS = 4.464 V, I_D = 0.268 mA]

Ex. 4.5.12 : Calculate V_GS, V_DS and I_D for the Fig. 4.5.16, the MOSFET parameters are :

 [Ans. : V_GS = 1.552 V, V_DS = 5.344 V, I_D = 0.776 mA]

b. Constant Current Source Biasing Circuit

• Another way of biasing MOSFET to stabilize the Q-point is to use constant current source instead of source resistance, as shown in the Fig. 4.5.17 (a). The constant current source is independent of the MOSFET parameters, thereby stabilizing the Q-point.

• The Fig. 4.5.17 (b) shows the d.c. equivalent circuit for common source circuit biased with a constant current source.

Ex. 4.5.13 The parameters of the MOSFET in the circuit shown in Fig. 4.5.17 (a) are VT = 0.8 V, k' = 80 µA/v2 and W/L = 3. Design the circuit such that the quiescent values are ID = 500 pA and VD = 5 V. Assume VDD = + 10V and Vss = - 10 V.

Sol. : The d.c. equivalent circuit is shown in Fig. 4.5.17 (b). Since vi = 0, the gate is at ground potential and there is no gate current through RG- Assuming the MOSFET is biased in the saturation region, we have

The voltage at the source terminal is VS = - VGS = - 2.84 V.

The drain current can also be written as,

The drain-to-source voltage is

V_DS = V_D - V_S = 5 -(-2.84) = 7.84 V

Validity of assumption :

Since V_DS = 784 V > V_DS(sat) = = V_GS - V_T = 2.84 - 0.8 = 2.04 V the MOSFET, is biased in the saturation region, as initially assumed.

Review Questions

1. Write a note on biasing of MOSFET.

2. Draw and explain the working of common-source biasing circuit for MOSFET.

3. Write a note on constant current source biasing of MOSFET.