EECE 310 Exam 2, Fall 2004

EECE 310 – Quiz 2 – Fall 2004/2005 – Page 1 of 10 American University of Beirut P1 /16 Department of Electrical and Computer Engineering P2 /12 EECE 310 – Electronics I P3 /20 Fall 2004 – 2005 P4 /10 Quiz 2 – December 22, 2004 P5 /12 Closed Book – 90 minutes P6 /10 P7 /20 Grade /100 NAME:___________________________ ID Number: ______________________ I have neither given nor received aid on this exam SIGNATURE Problem 1 [16 points] The drain current ID of an enhancement n-channel MOSFET is measured at different values of VGS, VDS, and VBS. The results are shown in the table below. For this transistor, find the values of Vt0, k’(W/L), λ, and γ. Assume that 2|φf| = 0.6 V. VGS (V) VDS (V) VBS (V) ID (µA) 2 2 0 101.25 3 3 0 340.20 2 3 0 105.00 2 3 -2 63.69 EECE 310 – Quiz 2 – Fall 2004/2005 – Page 2 of 10 Problem 2 [12 points] For the MOSFET shown in the circuit below, find VGS, VDS, and ID. In what region is the transistor operating? The MOSFET parameters are Vt = 1 V and k’(W/L) = 0.1 mA/V2. 15 K 1.5 M 3.3 K 12 V EECE 310 – Quiz 2 – Fall 2004/2005 – Page 3 of 10 Problem 3 [20 points] a) The MOSFET shown in the circuit below is to be biased in the saturation region at VDS = 5 V, ID = 0.5 mA. Find the required values of RD and RS, and calculate the values of the DC voltages VG, VS, and VD. The MOSFET parameters are Vt = 0.8 V, k’(W/L) = 0.06 mA/V2, and λ = 0.03 V-1. RD RS 12 V RG RG CS EECE 310 – Quiz 2 – Fall 2004/2005 – Page 4 of 10 b) The circuit is used as an amplifier with input at the gate, and output taken at the drain. What is the voltage gain of this amplifier at midband? c) What is the maximum sinusoidal voltage swing at the drain (with input signal applied at the gate) for the MOSFET to remain in saturation? Express the swing as a DC value, a maximum value, and a minimum value. EECE 310 – Quiz 2 – Fall 2004/2005 – Page 5 of 10 Problem 4. [10 points] a) A MOSFET is biased at VDS = 10 V, VGS = 5 V, and the resulting drain current is ID = 1 mA. Show the small-signal T-model of this MOSFET at the bias point (with all component values). The MOSFET parameters are Vt = 1 V, and λ = 0.01 V-1. b) How would you modify the T-model if body-effect is present with a signal voltage vbs appearing between body and source? Show the resulting circuit. EECE 310 – Quiz 2 – Fall 2004/2005 – Page 6 of 10 Problem 5. [12 points] a) Three identical amplifier stages are cascaded. For each of the amplifier stages, the input resistance is 100 KΩ, the output resistance is 20 KΩ, and the open-circuit voltage gain is 30. Show the model of the resulting (single) voltage amplifier, with values of Ri, Ro, and Avo. b) Show the equivalent model, with component values, for a (single) transconductance amplifier. EECE 310 – Quiz 2 – Fall 2004/2005 – Page 7 of 10 Problem 6. [10 points] Find the input resistance, voltage gain (vout/vsig), and output resistance of the MOSFET amplifier shown. The MOSFET small-signal parameters are gm and ro. vsig RS VDD RG RG vOUT Rsig EECE 310 – Quiz 2 – Fall 2004/2005 – Page 8 of 10 Problem 7. [20 points] a) Find the midband gain vout/vsig of the amplifier shown in the circuit below. The MOSFET is biased such that its small-signal transconductance is gm = 1 mA/V. Express the gain in dB. VDD CS vOUT 50 KΩ vsig 100 KΩ 100 KΩ 10 KΩ 5 KΩ b) What is the upper 3 dB frequency (in Hz) of this amplifier if Cgs = Cgd = 1 pF? Use the Miller approximation. What is the bandwidth of this amplifier if its lower 3 dB frequency is 100 Hz? EECE 310 – Quiz 2 – Fall 2004/2005 – Page 9 of 10 c) Show the magnitude Bode plot for the amplifier transfer function Vout/Vsig. Use a range of frequencies from 1 Hz to 1 MHz. EECE 310 – Quiz 2 – Fall 2004/2005 – Page 10 of 10 d) Find the value of the bypass capacitor CS to get a lower 3 dB frequency of 100 Hz. Hint: Use the T-model to find Vout(s)/Vsig(s) at low frequency, then use the definition of the lower 3 dB frequency to solve 2 ) ( ) ( gain midband j V j V L sig L out = ω ω .