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(ELEC304)Midterm_2004.pdf
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The Hong Kong University of Science and Technology
Department of Electrical and Electronic Engineering

ELEC304 Midterm Exam 15th April 2004
Name:
Student Number:

Time: 15:00-16:30 (1.5-hours closed-book examination) Do ALL problems in the answer book provided
Q1. The figure shown below is a single-ended differential amplifier in unity-gain configuration. The negative input of the amplifier is connected to the amplifier output.
VDD


(a) What is the type of feedback? (5 Marks)
v
o
(b) Find the closed-loop voltage gain . (15 Marks)
v
in
(c) Find the closed-loop output resistance Rocl. (10 Marks) Notes: (1) You have to include all evaluation steps.

(2) You have to include all ro in your expressions.
Q2. There are many amplifiers in an IC system and the amplifiers generally share the same supply-voltage line. As a result, noise from an amplifier may pass to another amplifier through VDD. The following figure of a common-source amplifier with feedback illustrates this idea. There is a signal source vdd, which represents the noise signal from other circuits.

Fig. 2.1
Power-supply rejection ratio (PSRR) is a specification to measure how good an amplifier can amplify an input signal and reject the supply noise. It is defined as
A
d
PSRR =
A
vdd where Ad is the input-to-output voltage gain (i.e. vo/vin) and Avdd is the supply-to-output voltage gain (vo/vdd). A higher PSRR shows a better amplifier performance.
According to the principle of superposition, both Ad and Avdd can be found individually by disabling the other source (i.e. Disable vdd to find vo/vin, and disable vin to find vo/vdd). Fig. 2.2 and Fig. 2.3 are the configurations to perform the analysis. It is noted that the circuits are in AC domain for AC analysis.

Fig. 2.2 Fig. 2.3
(a) From Fig. 2.2, find Ad = vo/vin (Note: It is a shunt-shunt feedback). (b) From Fig. 2.3, find Avdd = vo/vdd (Note: It is a series-shunt feedback) (c) Find PSRR. Notes: (1) You have to include all evaluation steps. (2) You have to include all ro in your expressions. (20 Marks) (15 Marks) (5 Marks)
Written By: Prof. Alex K. N. Leung p. 2

Q3. The circuit of a self-VTH biasing is shown below.

(a)
What is the function of M5? (5 Marks)

(b)
It is known that the circuit has startup problem. Design and draw a startup circuit for this self-VTH bias circuit in your answer book. State the startup mechanism. (25 Marks)



- END OF MIDTERM EXAM PAPER -ELEC304 Midterm Exam (Solution)

Q1. (a) Series-shunt feedback
(b) Open-loop parameters:
Rf = 0, RLf = , and f = 1
A'= gm1 .(ro2 // ro4 )

Ro '=ro2 // ro4
Closed-loop parameters:
vA' g .(r // r )

om1 o2 o4
A == =
cl
vin 1+ A' f 1+ gm1 .(ro2 // ro4 )
For g .(r // r ) >> 1, Acl = 1V/V.

m1 o2 o4
ro2 // ro4
(c) Rocl =
1+ gm1 .(ro2 // ro4 )
For g .(r // r ) >> 1, Rocl = 1/gm1.

m1 o2 o4
Q2. (a) Since it is a shunt-shunt feedback, the voltage source vin is change