ECE3161 Mock Exam Questions


This mock exam is for you to familiarise yourself with this hybrid mode of exam and
the submission procedure. The question types and styles may be different in the
final assessment.

This mock exam has TWO questions. Answer all questions.

Read the following instructions carefully.

Instructions:

• Combine all your answers into a single PDF file for submission.
• For handwritten answers, scan/photograph your answers and combine into your
word processing software. More info here.
• Clearly label each answer with the question number.
• For LTspice plots, ensure the waveforms are clearly visible. Choose appropriate
line colour and thickness.
• Name and save your LTspice files (.asc) as instructed in the questions.
• Copy your exam script (.pdf) and LTspice (.asc) files into a folder. Rename the
folder as per your student ID.
• Compress the folder (.zip). For detailed instructions, click here: Windows | Mac.
• Upload your ZIP folder into the Mock Assessment Dropbox on Moodle.

Question 1: 30 points

A common-base amplifier is shown in Figure 1.

a) Find the Q-point (IC, VC) of the circuit if R1 = 4 k, R2 = 1 k, RC = 1 k, and VCC
= 5 V. Assume that the AC voltage source vin has a source impedance of 100.
4 points



Figure 1

b) Using LTspice, investigate the small-signal gain, input and output impedance of
the circuit. Compare your simulation results with the theoretical values. State any
assumptions made in your calculations. Save your .asc file as [Student
ID_Q1b.asc]. 8 points

c) Modify your circuit in 2(b) to include a current mirror topology to bias the circuit
instead of using resistors R1 and R2. Maintain the same Q-point. Use VEE = -5 V
if necessary. Simulate your newly designed circuit using LTspice and compare your
results. Save your .asc file as [Student ID_Q1c.asc]. 8 points

d) Search the web for LM555 data sheet. Identify at least two different sub-circuits in
the schematic diagram for LM555 (current mirrors, comparator, differential
amplifiers, etc.). Clearly label each section. 4 points

e) A 555 timer can be used to generate accurate oscillations. Select appropriate
values of resistors (For example Ra and Rb) and capacitor C to design a 10 kHz
oscillator using the timer. 6 points







Question 2: 30 points

Figure 2 shows a 2nd order low pass filter.



Figure 2

a) Draw a switched capacitor implementation for this filter. 6 points

b) If R1 = R2 = 3 k, and the switching frequency fc is 1 MHz, what is the required
value of the switching capacitor? 8 points


c) What is the cut-off frequency and the Q-factor of the switched-capacitor filter in
2(b) if C1 = 0.05 F and C2 = 0.025 F? 8 points

d) Sketch the magnitude and phase plot for the filter. 4 points

e) Comment on the stability of this filter design. 4 points

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