Arholiadau Ysgoloriaeth Ionawr 2016
Scholarship Examinations January 2016
Amser a ganiateir : 2 awr ELECTRONEG
Time allowed: 2 hours ELECTRONICS
You may attempt any of the questions. Credit will be given for complete answers.
1. The circuit below has the following truth table:
0 / 0 / 1 / 1 / 1
0 / 1 / 1 / 0 / 0
1 / 0 / 0 / 1 / 0
1 / 1 / 0 / 0 / 0
Write a Boolean expression for the logic function that corresponds to the output of each component, A, B and C:
Component A: ______
Component B: ______
Component C: ______[3]
What single component would give the same behaviour as this circuit?
______[1]
2. A logic diagram is shown below.
a) Complete the truth table for signals D and E.
C / B / A / D / E0 / 0 / 0
0 / 0 / 1
0 / 1 / 0
0 / 1 / 1
1 / 0 / 0
1 / 0 / 1
1 / 1 / 0
1 / 1 / 1
[2]
b) Redraw the circuit replacing each circuit with its NAND gate equivalent.
[3]
c) Cross out any redundant pairs of NAND gates in your diagram. [2]
3. Simplify the following Boolean expressions, showing all appropriate working.
B 0
1
a) = ______[1]
b) = ______[1]
c) By means of a Karnaugh map or the rules of Boolean algebra, simplify the following expression as much as possible.
BA 00 / 01 / 11 / 10DC 00
01
11
10
______
______
______
______
______
[4]
Apply DeMorgan’s theorem to the following expression and simplify the result.
______
______
______
[3]
4. An extract from the data sheet for an op-amp is shown below.
Input Impedance / 20MW
Output Impedance / 10W
Open loop gain / 105
Gain Bandwidth Product / 1MHz
Slew Rate / 10V.ms-1
a) Draw a diagram of the op-amp in a non-inverting configuration, with a gain of 40. You should choose the resistor values from the E24 series and ensure that the amplifier has an input impedance of at least 10kW.
The op-amp is powered from a ±15V supply and saturation occurs at ±14V.
[4]
b) How long will the amplifier take to change its output from -10V to +10V, on the application of a large step input?
______[1]
c) If a 500 kHz sine wave with amplitude 250mV is applied to the input of this circuit determine whether the output is limited by the slew rate or the gain-bandwidth of the amplifier. [Hint: the maximum slew rate of a sinusoidal wave of amplitude A and frequency f is 2pfA].
______
______
______
______
______[6]
d) The following waveform is applied to the input of your amplifier. Sketch the output waveform on the axes provided.
[3]
5. Use the information in the circuit diagram to find the values of the quantities listed below.
I1 ______
______[2]
I2 ______
______[2]
V1 ______
______[2]
R1 ______
______[2]
6. The output voltage Vout of the following circuit is going to be measured using a voltmeter.
Find and draw the Thévénin equivalent circuit for this potential divider.
Open circuit voltage, VOC = ______
Short circuit current, ISC = ______
Equivalent circuit resistance, RO = ______
Thévénin equivalent circuit
[5]
If the measured voltage is 5V, what is the equivalent resistance of the voltmeter?
______
______
______
______
______[5]
State, with a reason, whether the meter is more likely to be a moving-coil voltmeter, or a digital voltmeter.
______[2]
7. Complete the following diagram of a half-wave rectified power supply.
[3]
On the axes below sketch the output voltage, VOUT. The dotted line shows the input voltage.
[2]
Sketch below how the output regulation of your design can be improved by the addition of a Zener diode.
[4]
8. Design an oscillator, using a 555 timer, to produce a rectangular wave with a pulse repetition frequency of 50 kHz and a mark-space ratio (duty cycle) of 80%. Show how you arrived at your component values.
______
______
______
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______
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______
[8]
Explain how you could add a D-type flip flop to your circuit to produce a 50% duty cycle signal at the same frequency.
______
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______
[8]
9. Draw a circuit diagram of a “simple” radio receiver.
[4]
Draw a block diagram of a superheterodyne radio receiver.
[5]
Choose three of the blocks in your diagram above and explain their function.
i)
______
______[2]
ii)
______
______[2]
iii)
______
______[2]
State two advantages of the superheterodyne receiver over the simple receiver.
______
______[2]