General Notes:
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Time allowed is 90 minutes
Closed book exam
Assume any reasonable assumptions
Pencil answers will not considered for re-evaluation
Please write your Name and I.D on each page
You can use the back pages for calculations and/or answers
Wrong answers in multiple choice will be given “–ve 1/3 mark”
Total mark is 60
The exam consists of five questions in eight pages including this page
Question 1: Choose the correct answer only :
1. Zener diodes are normally used as:
a. D.C Amplifiers
c. current regulators
[10 Marks]
b. A.C Amplifiers
d. voltage regulators
2. The voltage drop across a silicon diode when conducting is about:
a. 0.2V
b. 0.3V
c. 0.7V
d. 1.3V
3. In a forward biased pn junction, the electrons:
a. flow from p side to n side
b. flow from n side to p side
c. remain in the n region
d. remain in the p region
4. A varactor diode acts like a variable:
a. resistance
b. voltage regulator
c. capacitance
d. inductance
5. For asymmetrical pn junction (one side is highly doped than the other side) the depletion
layer will be:
a. mainly in the highly doped side
b. mainly in the lightly doped side
c. equal in both sides
d. zero
6.
The block marked 'Rectifier' in the diagram is to:
a. turn the AC voltage from the transformer into a fluctuating DC voltage
b. rectify any waveform errors introduced by the transformer
c. turn the sinewave f the rectifier into a square wave
d. remove any AC components from the output of the transformer
7. The block marked 'Regulator' in the diagram could consist of:
a. four silicon power diodes in a regulator configuration
b. two silicon power diodes and a centre-tapped transformer
c. a zener diode with series resistance
d. a single silicon power diode connected as a half-wave rectifier
8. The following unit in a DC power supply performs a rectifying operation:
a. an electrolytic capacitor
b. fuse
c. a center-tap transformer
d. a full-wave diode bridge
9. The following unit in a DC power supply performs a smoothing operation:
a. an electrolytic capacitor
b. a fuse
c. a center-tap transformer
d. a full-wave diode bridge
10. The output voltage of a DC power supply decreases when current is drawn from it because:
a. drawing output current causes the input mains voltage to decrease
b. drawing output current causes the input mains frequency to decrease
c. all power supplies have some internal resistance
d. some power is reflected back into the mains.
Answer Table:
Question 1
Answer
2
3
4
5
6
7
8
9
10
Question 2:
[10 Marks]
a) Assuming ideal diodes calculate I and V in the circuit shown.
b) If both diodes have been replaced by zener diodes with Vz=3V, and rz=0
Calculate the new values of I and V.
Question 3:
[10 Marks]
a) [4 Marks] PSPICE circuit simulation has been carried out for the following limiter (clipping)
circuits to study its transfer characteristics (Vo versus Vi). Please refer each transfer characteristic to
its generating circuit.
+5v
-4V
D1
R1
-6V
Vin
Output Waveform "A"
Vo1
1k
-8V
+5v
D2
-10V
-10V
R2
-6V
-8V
Vin
-4V
8V
6V
4V
2V
0V
-2V
Vin
10V
10V
Vo2
Output Waveform "B"
1k
5V
1k
Vo3
Vin
0V
R3
D3
-5V
-5v
1k
-10V
-10V
-6V
-4V
-2V
0V
2V
4V
6V
8V
4V
6V
8V
10V
10V
D4
R4
-8V
Vin
Vo4
Vin
Output Waveform "C"
5V
-5v
0V
Solution:
Circuit
Vo1
Waveform
-5V
Vo2
Vo3
Vo4
-10V
-10V
-8V
-6V
-4V
-2V
0V
2V
10V
Vin
10V
Output Waveform "D"
8V
6V
4V
-10V
-8V
-6V
-4V
-2V
0V
Vin
2V
4V
6V
8V
10V
b) [6 Marks] If we connect first and third circuits at input and output, the following circuit will be
obtained. Find the new transfer characteristics (Vo5 versus Vi). Assume that the diode start
conduction at 0.6V and it will fully conduct at 0.8V with a 1 m.A forward current. Label all
+5v
important current/voltage values in your sketch.
D5
R5
1k
1k
Vo5
Vin
R6
D6
-5v
Question 4:
For the diode rectifier circuit shown, assume VDD=5V
And R=100Ω:
a) Write the load line equation
b) Draw the load line with the diode C/Cs
c) Find the operating point (ID and VD) graphically
-15
d) Given Is=10 A, and n=1. Analyze the
circuit iteratively to find the operating point
(ID and VD)
e) Assume piece wise linear diode model
with VDO=0.7V and rd=5Ω. Analyze the
circuit to find the operating point (ID and VD)
[15 Marks]
Question 5:
[15 Marks]
It is required to design a zener shunt regulator to provide regulated voltage of about 10V. The
available 10V zener diode is specified to have a 10V drop at a test current of 25 mA. At this current
its rz is 7 Ω. The raw supply available has a nominal value of 20V but can vary by as much as
±40%. The regulator is required to supply a load current of 0 to 15 mA. Design for a minimum
zener current of 5 mA.
a) Find Vzo
b) Calculate the required value of R (maximum R)
c) Find the line regulation
d) Find the load regulation
e) What is the maximum current that the zener in your design should be able to conduct? What
is the zener power dissipation under this condition?.