Friday, 17 September 2021 07:12

## ELECTRICAL TECHNOLOGY: ELECTRONICS GRADE 12 QUESTIONS - 2018 SEPTEMBER PREPARATORY EXAM PAPERS AND MEMOS

ELECTRICAL TECHNOLOGY: ELECTRONICS
NATIONAL SENIOR CERTIFICATE
SEPTEMBER 2018

INSTRUCTIONS AND INFORMATION

1. This question paper consists of FIVE questions.
2. Sketches and diagrams must be large, neat and fully labelled.
3. Show ALL calculations and round off answers correctly to TWO decimal places. Show the units for ALL answers of calculations.
4. Number the answers correctly according to the numbering system used in this question paper.
5. You may use a non-programmable calculator.
6. A formula sheet is provided at the end of this question paper.
7. Write neatly and legible.

QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY
1.1 Summarise the purpose of the Occupational Health and Safety Act. (2)
1.2 Explain the ‘let-go’ current during an electrical shock.(2)
1.3 State ONE unsafe condition that must be avoided in a workshop. (1)
1.4 Give ONE unsafe act in a school workshop that can cause an accident. (1)
1.5 Describe TWO standard treatments for shock. (2)


QUESTION 2: RLC
2.1 A series RLC circuit consists of an inductor with an inductive reactance of
24 Ω, a resistor with a resistance of 12 Ω and a capacitor with a capacitive reactance of 18 Ω connected across a 30 V supply. Calculate the:
2.1.1 Impedance of the circuit (3)
2.1.2 Current flowing through the circuit (3)
2.1.3 Voltage across each component in the circuit (6)
2.1.4 Prove by calculation that the sum of the three voltages equals the supply voltage. (3)
2.1.5 Circuit phase angle and state whether is it leading or lagging (3)
2.1.6 Draw a fully labelled voltage phasor diagram. (5)
2.2 The phasor diagram in FIGURE 2.2 indicates the current values flowing through the components of a parallel circuit connected across a 240 V/50 Hz supply. Refer to the diagram below and answer the questions that follow. Calculate the:
2.2.1 Total current flow through the circuit (3)
2.2.2 Inductive reactance (3)
2.2.3 Inductance of the coil (3)
2.3 Define the following terms with reference to RLC circuits:
2.3.1 Resonance (2)
2.3.2 Q-factor in a parallel circuit (2)
2.4 A circuit with a resistor of 4 Ω, an inductor with an inductive reactance of 157 Ω and a variable capacitor set to 120 μF are connected in series to a 100 V/50 Hz supply.
Given:
R = 4 Ω
XL = 157 Ω
Cvar = 120 μF
Vs = 100 V
f = 50 Hz
Calculate the:
2.4.1 Value of the capacitance that will result in resonance at 50 Hz (3)
2.4.2 Q-factor of the circuit at resonance (3)


QUESTION 3: SEMICONDUCTORS
3.1 State whether a FET is a voltage or current controlled device. (1)
3.2 Sketch the circuit symbols for the following FETs:
3.2.1 N-channel JFET (3)
3.2.2 N-channel depletion MOSFET (3)
3.3 Explain the main advantage of a FET device over a bipolar junction transistor. (2)
3.4 Describe the difference between the inverting and non-inverting op-amp. (2)
3.5 Draw and label the operating characteristic for UJT by clearly labelling the cut-off, saturation and negative resistance regions. (3)
3.6 Sketch a labelled circuit symbol for an op-amp. (3)
3.7 Give THREE advantages of negative feedback. (3)
3.8 Refer to FIGURE 3.1 and answer the following question. 3.8.1 Identify the circuit diagram above. (1)
3.9 FIGURE 3.2 represents the non-inverting op-amp. 3.9.1 Determine the gain from the information given in FIGURE 3.2. (3)
3.10 Give any THREE modes of operation of a 555 timer IC. (3)
3.11 Sketch the pin layout of a 555 timer IC. (4)
3.12 Calculate the intrinsic stand-off ratio of a UJT if the internal resistance within the device is 2 kΩ for the upper resistance and 5 kΩ for the lower resistance. (3)


QUESTION 4: SWITCHING CIRCUITS
4.1 List THREE types of multivibrators. (3)
4.2 Refer to FIGURE 4.1 and answer the questions which follow. 4.2.1 Identify the circuit diagram above. (1)
4.2.2 Describe the operation of the circuit diagram above. (4)
4.3 State TWO applications of a monostable multivibrator. (2)
4.4 Design the circuit diagram for a Schmitt trigger. (3)
4.5 Determine FOUR applications of a Schmitt trigger. (4)
4.6 Draw a fully labelled circuit diagram of a monostable multivibrator. Also include the input and output waveforms. (5)
4.7 Sketch a fully labelled circuit diagram for a 555 IC astable multivibrator circuit also showing input and output waveforms. (6)
4.8 Refer to FIGURE 4.2 which represents the summing amplifier. Calculate the circuit output voltage. (4) 4.9 Refer to FIGURE 4.3 below to answer the question that follows. 4.9.1 Draw TWO full cycles of the input and output waveforms for this circuit if a sinusoidal waveform was added to its supply. (4)
4.10 FIGURE 4.4 represents a passive RC differentiator. Answer the question that follows. 4.10.1 Sketch the output wave form. (4)
4.11 Explain the operation of an integrator. (3)
4.12 Design a circuit diagram of an op-amp integrator. (4)
4.13 A 555 astable multivibrator circuit has the following components:
Given:
R1 = 47 kΩ
C = 1 𝜇𝐹
R2 = 5 kΩ
Calculate the:
4.13.1 Charging time (3)
4.13.2 Discharging time (2)
4.13.3 Time to complete one cycle (3)
4.13.4 Frequency of oscillation (3)


QUESTION 5: AMPLIFIERS
5.1 What does the term amplifier mean? (2)
5.2 Refer to FIGURE 5.1 and calculate the maximum current and voltage across the transistor. Also draw the circuit load line. (8) 5.3 Draw a neat sketch of an input and output characteristic curve to show a common emitter connected transistor. (4)
5.4 State the biasing condition that must be met for a transistor to operate as an amplifier. (1)
5.5 Explain how Class B biasing is obtained. (2)
5.6 Design a block diagram showing negative feedback. (5)
5.7 Give THREE disadvantages of transformer coupling. (3)
5.8 Name THREE applications of an RC couple amplifier. (3)
5.9 Refer to the circuit below and answer the questions that follow. 5.9.1 Identify the circuit diagram above. (1)
5.9.2 State the chief function of the amplifier above. (1)
5.9.3 Briefly explain the operation of the circuit above. (3)
5.10 Explain the similarities and the differences in circuit components between the Colpitts and Hartley oscillator. (4)
5.11 A Colpitts oscillator operating at 10 kHz uses two capacitors C1 = 600 nF and C2 = 20 μF. Calculate the value of the inductor in the resonant circuit. (5)
5.12 An RC phase shift oscillator uses identical variable capacitors each with a range of 100 pF to 500 Pf. The resistors are each 20 kΩ. Calculate the circuit frequency range of oscillation. (4)
5.13 Explain what comprises the feedback network in the following oscillator circuits:
5.13.1 A Hartley oscillator (1)
5.13.2 A Colpitts oscillator (1)
5.13.3 An RC-phase shift oscillator (1)
5.14 State THREE uses of an RF oscillator. (3)
5.15 Sketch and label the frequency response curve of an RC coupled amplifier. (6)

TOTAL: 200

FORMULA SHEET

 RLC𝑋𝐿=2𝜋𝑓𝐿𝑋𝐶=   1           2𝜋𝑓𝐶𝑍= √𝑅2+(𝑋𝐿−𝑋𝐶)2𝑍= 𝑉𝑇      𝐼𝑇𝑃=𝑉𝐼𝐶𝑜𝑠𝜃 Phase angle:Series circuit: 𝐶𝑜𝑠𝜃= 𝑅                                  𝑍Parallel circuit:𝐶𝑜𝑠𝜃= 𝑉𝑅                                   𝑉𝑍𝑓𝑅=   1          2𝜋√𝐿𝐶 Q factor : 𝑄= 𝑋 𝐿                      𝑅𝑄= 𝑋 𝐶       𝑅𝐵𝑎𝑛𝑑 𝑊𝑖𝑑𝑡ℎ= 𝑓𝑅                       𝑞  SEMICONDUCTOR𝑉𝑜𝑢𝑡= 𝑉𝑖𝑛(−𝑅 𝑓)                 𝑅𝑖𝑛𝑉𝑜𝑢𝑡= 𝑉𝑖𝑛(−𝑅 𝑓)                 𝑅𝑖𝑛 SWITCHING CIRCUIT𝑉𝑜𝑢𝑡= −(𝑉1+ 𝑉2+ 𝑉3…..+𝑉𝑛)𝑉𝑜𝑢𝑡=−{𝑉1×𝑅 𝐹 + 𝑉2×𝑅 𝐹+ 𝑉3× 𝑅 𝐹)                  𝑅𝑖𝑛           𝑅𝑖𝑛            𝑅𝑖𝑛𝑓= 1  =       1,44          𝑇    (𝑅1+2𝑅2)×𝐶𝑡1=0,693(𝑅1+𝑅2)×𝐶𝑡2=0,693×𝑅2×𝐶𝑇 = 𝑡1+ 𝑡2 𝑓= 1  =    1,44    ×𝐶     𝑇    (𝑅1+2𝑅2)𝑡1 =0.693( R1+R2) x C𝑡2 = 0.693 x R2 x CT= 𝑡1+𝑡2 AMPLIFIERS𝑉𝐶𝐸= 𝑉𝐶𝐶𝐼𝐶 𝑚𝑎𝑥= 𝑉𝐶𝐶              𝑅𝐶𝐴𝑉= 𝑉𝑜𝑢𝑡        𝑉𝑖𝑛𝑓𝑅=   1           2𝜋√𝐿𝐶𝐶𝑇=𝐶1× 𝐶2      𝐶1+ 𝐶2𝑓𝑜=    1         2𝜋√6 𝑅𝐶𝑓𝑜=   1          2𝜋√𝐿𝐶