Arholiadau YsgoloriaethIonawr 2014
Scholarship ExaminationsJanuary 2014
Amser a ganiateir : 2 awrELECTRONEG
Time allowed: 2 hoursELECTRONICS
You may attempt any of the questions. Credit will be given for complete answers.
1. The circuit below has the following truth table:
V / W / X / Y / Z0 / 0 / 0 / 0 / 0
0 / 1 / 0 / 1 / 1
1 / 0 / 1 / 0 / 1
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, E and F.
C / B / A / D / E / F0 / 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.
A 0 / 1B 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.
Parameter / ValueInput Impedance / 20M
Output Impedance / 10
Open loop gain / 105
Gain Bandwidth Product / 2MHz
Slew Rate / 2V.s-1
a) Draw a diagram of the op-amp in an inverting configuration, with a gain of -20. You should choose the resistor values from the E24 series and ensure that the amplifier has an input impedance of at least 50k.
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) 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 following circuit is to be used to bias the base of a transistor amplifier.
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]
Assuming the base current is 50A, what will be the base voltage, VB?
______[3]
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]
The rectifier above is replaced by a full-wave rectifier. Sketch the output voltage for a similar output current to that above.
[4]
8. Complete the diagram of an astable circuit based on a Schmitt inverter.
[3]
If the Schmitt inverter has the following characteristic curve, sketch the voltage at the INPUT of the inverter on the axes below.
[3]
If your resistor is of value 500R, find the capacitor value which gives your circuit an output frequency of 1kHz.
______[3]
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]