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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering - Stage 3
(EELXE_8_Y3)
Spring 2008
Analogue and Digital Electronics
(Time: 3 Hours)
Instructions Answer FIVE (5) questions, at least two (2) from each section. All questions carry equal marks.
Requirements: 6-decade Log/Lin Graph Paper)
Examiners: Mr. P. Collins Dr. B.V. Donovan Prof. W.G. Hurley Dr. S. Foley
Section A
Q1.
Assume that for the transistors in this question Kp = 20μA/V^2 , Vto = +1V for the n-channel MOSFETs, Vto = -1V for the p-channel MOSFETs, lambda = 0.
a) Write expressions for the current in an N-channel MOSFET - one for the linear region of operation and one for saturated operation. Write similar expressions for a P-channel device. What terminal voltage conditions determine the operating region for both devices? [2 Marks]
b) Calculate the currents flowing in M1 and M2 in Figure Q1a? [4 Marks]
W=20u L=1u M
W=10u L=1u M
V V1 2V 5V
V 2V
Fig Q1a.
c) What is the voltage at node OUT in Figure Q1b? [ 4 Marks ]
W=20u L=1u M
R
W=10u 20k L=1u M
V
V1 2V
5V
V
2V
Out
Fig Q1b.
d) What is the voltage at node OUT in fig Q1c? [ 10 Marks ]
W=20u L=1u M
W=10u L=1u M
V
V1 2V
5V
V
2V
Out
W=10u L=10u M
Fig Q1c.
Q3.
Assume that for the transistors in this question Kp = 20μA/V^2 , Vto = +1V for the n-channel MOSFETs, Vto = -1V for the p-channel MOSFETs , lambda = 0.02.
For the CMOS amplifier shown in Figure Q3 assume that the poles are given by 1 18 1 2 1 2 18 20
M OUT OUT M
P
gm R R C P gm C
The zero is given by 18 (^21)
Z gm^ M
π C
= − if R 1 = 0.
a) Draw a simplified small signal model for the CMOS amplifier shown in Figure Q3. Explain the origins of the components in the model. [4 Marks]
b) With R 1 = 0Ω calculate the poles and the zero, and then draw a Bode plot for the amplifier. Use 6 decade log-linear paper with a frequency scale of 100Hz to 100MHz. Include on your answer script a written description of all turning points on the Bode plot along with the slopes of any lines. [8 Marks]
c) Repeat this exercise for R 1 = 1 /gmM18. [4 Marks]
d) From the Bode plots, what is the phase margin for this amplifier when used in a unity gain configuration? Calculate this for R 1 = 0Ω and for R 1 = 1 /gmM18. Which value of phase margin is better and why? [4 Marks]
80uA
W=10uL=1u MbreakPM
W=20uL=1u MbreakN M
W=20u MbreakNL=1u M
5V
W=10uL=1u MbreakPM
2.5V
2.5V
W=20uL=1u MbreakPM
W=20uL=1u MbreakN M
W=20uL=1u MbreakNM
W=20uL=1u MbreakN M
C 0.5pF 0.5pFC
R
Fig Q
Q5. Sketch a simplified Block diagram of either a Boost or Inverting (Buck/Boost) voltage converter and give a brief description of its operation, paying particular attention to the voltage waveforms at the output side of the Inductor. [5 Marks]
Referring to the simplified diagram of the Buck converter below (Fig Q5a), draw exact diagram of the (steady state) current flowing through the Inductor for a 5 Ohm load [5 Marks]
At what load does this converter become discontinuous and what would be the possible consequences? [4 Marks] Estimate the efficiency of this converter, for the conditions shown, if the components have the following specifications: RDS (on) of the SW1 = 10 mΩ, R (^) inductor = 20mΩ, Vdiode = 400 mV (1A) Ignore switching losses. Show all calculations clearly. [6 Marks]
Ohms Raw DC
PWM Control
25V 5V
Sw 1 L = 15 uH
D C
F =40 kHz
Fig Q 5a
Q6. Write a brief note, itemising the advantages of using F-to-V/V-to-F converters, as a medium for reliable data acquisition. [4 Marks]
Identify at least two environments where the technique is advantageous, [2 x 2 Marks] And show how appropriate ‘cabling’ would ensure reliability in both cases. [2 x 2 Marks]
The circuit below (Fig Q6a) is a simple voltage-to-(Pulse Repetition Rate) converter. Describe its principle of operation with appropriate Oscillograms/waveforms assuming that the switching thresholds for the CMOS gates is 0.5Vcc.
[5 Marks] Estimate values for the Three resistors ( Re, Rs and R ) to satisfy the Input/output relationship shown , for a pulse length of about 70us. [3 Marks]
I Vin
Re
V =^ V (^) in
+v cc
z
Cf (0.1uF)
R 10R
Vcc
R
s (^) s
I Charge = I/ 0.01uF
Fig Q6a
(5V)
Fout = 5 kPulses/s
