EECE 310 Homework 6, Fall 2011

American University of Beirut DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING EECE 310 – Electronics Fall 2011 – 2012 Due Wednesday November 16, 2011 at 9:00 am Homework 6 Problem 1. [25 points] a) [5 points] The mobility of electrons in the channel of an NMOS device is 1000 cm2/V.sec. Find the transconductance parameter k’n for this MOSFET if its oxide thickness is 7 nm. The permittivity of the oxide (SiO2) is  ox =3.9 0 = 3.453×10–11 F/m. b) [10 points] The threshold voltage Vt0 for this NMOS transistor is 0.55 V. Find the required dimensions of the gate of the NMOS transistor (W and L) to get a drain current of 1 mA when the MOSFET is biased at (VGS = 3 V, VDS = 3 V). Minimize the area of the gate, but keep the minimum dimension (L or W) at 0.32 m. c) [10 points] The drain current of this NMOS transistor is ID1 at (VGS = 3 V, VDS = 3 V). When VDS becomes 2.5 V while keeping other voltages constant, the current decreases by 3% to 0.97 ID1. Find the values of VA and VA’ for this transistor. Problem 2. [35 points] The circuit shown below is a voltage-controlled attenuator. Assume VGG = 2.5 V, R = 4.7 k, and for the MOSFET: Vt = 0.55 V, k’(W/L) = 0.4 mA/V2. a) [10 points] For what range of output voltages does the MOSFET behave as a (voltage- controlled) resistor? Assume that the square term is negligible when it is less than 3% of the linear term in the MOSFET current equation. b) [15 points] Find the value of VOV = VGS – Vt for the MOSFET and the value of its conductance gDS and resistance rDS when vO satisfies the condition of part (a). c) [10 points] When the circuit satisfies the condition of part (a), plot the attenuation factor (vo/vs) as a function of VGG, for VGG in the range 0 V – 5 V. Problem 3. [40 points] Two identical enhancement MOSFETs with k’n(W/L) = 450 A/V2, Vt = 0.55 V, and VA= 9 V are connected as shown below. The MOSFET drain current is 145 A. Assume that VDD = 12 V. a) [10 points] Find the value of VGS for the lower MOSFET. In what region is this MOSFET operating? b) [10 points] Find the voltage at the gate of the upper MOSFET. In what region is this MOSFET operating? c) [10 points] Find the resistance R1 when R2 = 120 k. d) [10 points] Find the power dissipated by the supply VDD.