Electrical Technology Grade 12 NSC November 2020 QP (Eng)

Downloaded from hlayiso.com GRAAD 12 SENIOR CERTIFICATE/ NATIONAL SENIOR CERTIFICATE GRADE 12 ELECTRICAL TECHNOLOGY: ELECTRONICS NOVEMBER 2020 MARKS: 200 TIME: 3 hours This question paper consists of 18 pages, a 1-page formula sheet and 5 answer sheets. Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 2 DBE/November 2020 SC/NSC INSTRUCTIONS AND INFORMATION 1. This question paper consists of FIVE questions. 2. Answer ALL the questions. 3. Answer QUESTIONS 2.1.1, 2.2.4, 4.3.2, 4.4.2, 4.6.2, 4.8.2 and 4.8.3 on the attached ANSWER SHEETS. 4. Write your CENTRE NUMBER and EXAMINATION NUMBER on every ANSWER SHEET and staple them to your ANSWER BOOK, whether you have not used them or not. 5. Sketches and diagrams must be large, neat and FULLY LABELLED. 6. Show ALL calculations and round off answers correctly to TWO decimal places. 7. Number the answers correctly according to the numbering system used in this question paper. 8. You may use a non-programmable calculator. 9 Calculations must include: 9.1. Formulae and manipulations where needed 9.2 Correct replacement of values 9.3 Correct answer and relevant units where applicable 10. A formula sheet is attached at the end of this question paper. 11. Write neatly and legibly. Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 3 DBE/November 2020 SC/NSC QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 1.1 Define health and safety equipment. (2) 1.2 Name ONE human right in the workplace. (1) 1.3 Name TWO incidents that should be reported to inspectors at the workplace. (2) 1.4 State THREE types of victimisation by an employer that are forbidden. (3) 1.5 Describe how the master switch in a workshop contributes to safety. (2) [10] QUESTION 2: RLC CIRCUITS 2.1 Two AC voltages, VR and VX, each have maximum values of VR = 20 V and VX = 30 V respectively. 2.1.1 Draw the phasor diagram (NOT to scale) on the ANSWER SHEET for QUESTION 2.1.1 if VX lags VR by 35°. (2) 2.1.2 Explain whether the voltages represent an RL circuit or an RC circuit. (2) 2.2 The RLC series circuit in FIGURE 2.2 below consists of a resistor of 15 Ω, an inductance of 20 mH and a capacitive reactance of 25 Ω. The components are all connected in series across a 150 V/60 Hz AC supply. Answer the questions that follow. R = 15 Ω L = 20 mH XC = 25 Ω VR = 98 V VL = 49 V VC IT = 6,5 A 150V/60Hz FIGURE 2.2: RLC SERIES CIRCUIT Given: R = 15 Ω L = 20 mH XC = 25 Ω VT = 150 V f = 60 Hz IT = 6,5 A Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 4 DBE/November 2020 SC/NSC 2.2.1 Calculate the inductive reactance of the inductor. (3) 2.2.2 Calculate the voltage drop across the capacitor. (3) 2.2.3 Indicate whether the supply voltage is lagging or leading. Motivate your answer. (2) 2.2.4 Draw the phasor diagram of the circuit on the ANSWER SHEET for QUESTION 2.2.4. (5) 2.3 Refer to FIGURE 2.3 below and answer the questions that follow. IC IR VT IL FIGURE 2.3: PHASOR DIAGRAM 2.3.1 Identify the phasor diagram in FIGURE 2.3. (2) 2.3.2 State why the supply voltage is used as the reference in the phasor diagram. (1) 2.3.3 Explain the relationship between the total current and the impedance in a parallel resonant circuit. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 5 DBE/November 2020 SC/NSC 2.4 Refer to FIGURE 2.4 below and answer the questions that follow. R = 20 Ω L = 300 mH C = 150 µF VS FIGURE 2.4: RLC SERIES CIRCUIT Given: R = 20 Ω L = 300 mH C = 150 µF 2.4.1 Calculate the resonant frequency. (3) 2.4.2 Calculate the quality factor of the circuit. (3) 2.4.3 Determine the impedance of the circuit at resonance. Motivate your answer. (2) 2.4.4 Calculate the value of the capacitance required for the circuit in FIGURE 2.4 to resonate at 2 kHz. (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 6 DBE/November 2020 SC/NSC 2.5 FIGURE 2.5 below shows the Q-factor characteristic curve of an RLC circuit, NOT to scale. Q1, Q2 and Q3 indicate how a change in the L/C ratio affects the Q-factor of a resonant circuit. Answer the questions that follow. Q1 Q1 0,707 Imax Q2 Q2 0,707 Imax Q3 Q3 0,707 Imax 25 30 35 40 45 f (kHz) FIGURE 2.5: Q-FACTOR CHARACTERISTIC CURVE 2.5.1 Identify the curve with the highest selectivity. (1) 2.5.2 Calculate the resonant frequency for response curve Q 1. (3) 2.5.3 Calculate the quality factor for Q1. (3) [40] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 7 DBE/November 2020 SC/NSC QUESTION 3: SEMICONDUCTOR DEVICES 3.1 FIGURE 3.1 below shows a cross-section of the construction of an enhancement mode MOSFET. Answer the questions that follow. A B C Silicon-dioxide insulation Enhanced channel P-substrate FIGURE 3.1: ENHANCEMENT MODE MOSFET 3.1.1 Identify A, B and C. (3) 3.1.2 Indicate whether the enhanced channel consists of P-type or N-type material. (1) 3.1.3 State what will happen to the current between terminal A and terminal C if the voltage on terminal B is 0 V. (1) 3.1.4 FIGURE 3.1.4 below shows the various FET symbols. Identify the components represented by symbols A, C and E. D D D G G G S S S (A) (B) (C) D D D G G G S S S (D) (E) (F) FIGURE 3.1.4: FET SYMBOLS (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 8 DBE/November 2020 SC/NSC 3.2 Explain the term negative resistance with reference to the UJT. (2) 3.3 Refer to FIGURE 3.3 below and answer the questions that follow. + VCC R1 RC Output C Input T1 C1 B T2 E R2 RE CE 0V FIGURE 3.3: DARLINGTON PAIR 3.3.1 Explain why the Darlington pair is preferred to a single transistor for this circuit. (2) 3.3.2 Explain why the Darlington pair needs a minimum of 1,2 V to 1,4 V across its base-emitter terminals to operate. (3) 3.4 Refer to FIGURE 3.4 below and answer the questions that follow. V+ Non-inverting input Output Inverting input V- FIGURE 3.4: OP-AMP 3.4.1 Name TWO characteristics of an ideal op-amp. (2) 3.4.2 Explain what makes the op-amp ideal to amplify alternating voltages. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 9 DBE/November 2020 SC/NSC 3.5 FIGURE 3.5 below shows the circuit diagram of an inverting op amp. RF RIN = 1,8 kΩ +9 V X VOUT = 8 V VIN = 0,4 V t t -9 V 0V FIGURE 3.5: INVERTING OP AMP 3.5.1 Explain why point X on the diagram is also known as the virtual ground. (2) 3.5.2 Calculate the value of feedback resistor RF. (3) 3.6 FIGURE 3.6 below shows the internal circuit diagram of a 555 IC. Answer the questions that follow. + VCC Control Supply Discharge Threshold voltage 8 7 6 5 5 kΩ Comparator 1 Q S Flip 5 kΩ Flop Comparator 2 R Q T1 5 kΩ Output Driver 1 2 3 4 Ground Trigger Output Reset FIGURE 3.6: 555 IC 3.6.1 State ONE industrial application where the 555 IC is used as a timing device. (1) 3.6.2 Indicate whether transistor T1 acts as a switch or as an amplifier in the circuit. (1) 3.6.3 Explain the function of the two comparators in FIGURE 3.6. (4) [30] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 10 DBE/November 2020 SC/NSC QUESTION 4: SWITCHING CIRCUITS 4.1 State ONE distinct difference between the astable multivibrator and the bistable multivibrator with reference to their inputs. (2) 4.2 Refer to FIGURE 4.2 below and answer the questions that follow. RF +12 V A - B + -12 V R1 C1 VOUT VC R2 t FIGURE 4.2: MULTIVIBRATOR 4.2.1 Identify the multivibrator in FIGURE 4.2. (1) 4.2.2 Describe the operation of the multivibrator in FIGURE 4.2 from a point where the capacitor has just discharged and point A is low. (6) 4.2.3 Explain how the frequency of the multivibrator in FIGURE 4.2 can be increased. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 11 DBE/November 2020 SC/NSC 4.3 FIGURE 4.3 below shows a monostable multivibrator using a 555 IC. +9V R1 1 kΩ R3 330 Ω R2 VR1 LED 1 10 kΩ 10 kΩ 8 4 7 Output 3 6 555 2 1 5 R4 Trigger C1 330 Ω C2 10 µF LED 2 10 nF FIGURE 4.3: MONOSTABLE MULTIVIBRATOR 4.3.1 State the function of capacitor C2. (2) 4.3.2 FIGURE 4.3.2 below shows the trigger input, capacitor charge and output signals of the multivibrator in FIGURE 4.3. The variable resistor (VR1) is set to 5 kΩ. On the ANSWER SHEET for QUESTION 4.3.2, draw the charging voltage VC1 and corresponding output if variable resistor (VR1) is changed to 10 kΩ. Trigger input ⅓ VCC 0V t ⅔ VCC VC1 when VR1 is set to 0V 5 kΩ t High Output Low t FIGURE 4.3.2: SIGNALS (4) 4.3.3 State whether LED 1 or LED 2 will be ON after the trigger switch is pressed. Motivate your answer. (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 12 DBE/November 2020 SC/NSC 4.4 FIGURE 4.4 below shows the circuit diagram of a non-inverting Schmitt trigger. Answer the questions that follow. +9 V VOUT -9 V VIN VIN R1 RF t FIGURE 4.4: NON-INVERTING SCHMITT TRIGGER 4.4.1 Indicate whether this is an open-loop mode op-amp circuit or a closed-loop mode op-amp circuit. Motivate your answer. (2) 4.4.2 Draw the output waveform of the Schmitt trigger on the ANSWER SHEET for QUESTION 4.4.2. (4) 4.4.3 State how the trigger voltage levels of the Schmitt trigger can be adjusted without changing the supply voltage. (2) 4.5 Refer to FIGURE 4.5 below and answer the questions that follow. V1 0,5 V t RF 10 kΩ C1 R1 2 kΩ R4 S1 +12 V V2 0,2 V - t C2 R2 500 Ω + R5 VOUT -12 V FIGURE 4.5: SUMMING AMPLIFIER 4.5.1 State the function of capacitor C1. (1) 4.5.2 Calculate the value of the output voltage. (3) 4.5.3 Explain what makes it possible for this amplifier to amplify both positive and negative voltages. (3) 4.5.4 Explain how the output voltage will be affected if switch S 1 is open. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 13 DBE/November 2020 SC/NSC 4.6 Refer to FIGURE 4.6 below and answer the questions that follow. +V R1 +V VIN VIN VOUT VREF -V R2 0V FIGURE 4.6: OP-AMP CIRCUIT 4.6.1 Identify the op-amp circuit in FIGURE 4.6. (2) 4.6.2 Draw the output signal of the op amp on the ANSWER SHEET for QUESTION 4.6.2. (4) 4.6.3 Explain why this op-amp circuit is driven into saturation during its operation. (2) 4.7 Explain the operation of the passive RC differentiator in FIGURE 4.7 below. C Output Input +9 V +9 V R 0 0 t t -9 V FIGURE 4.7: PASSIVE RC DIFFERENTIATOR (5) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 14 DBE/November 2020 SC/NSC 4.8 FIGURE 4.8 below shows an op-amp integrator with input and output signals. Answer the questions that follow. +V CF Input RIN +V 0V VIN t - 0V VOUT + Output -V -VSAT 0V FIGURE 4.8: OP-AMP INTEGRATOR 4.8.1 Explain why the op-amp integrator is able to produce a steadily falling output voltage when a square wave is applied to the input. (6) 4.8.2 On the ANSWER SHEET for QUESTION 4.8.2, draw the output waveform of the op-amp when the RC time constant is shortened from the given state. (2) 4.8.3 On the ANSWER SHEET for QUESTION 4.8.3, draw the output waveform of the op-amp when the RC time constant is lengthened from the given state. (2) [60] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 15 DBE/November 2020 SC/NSC QUESTION 5: AMPLIFIERS 5.1 Define the following terms with reference to amplifiers: 5.1.1 Feedback (2) 5.1.2 Distortion (2) 5.2 Refer to FIGURE 5.2 below and answer the questions that follow. + 9 VCC R1 RC 470 Ω C2 10 kΩ 10 µF C1 1 µF T1 β = 120 VCE VIN 2,38 V 2 mV (AC) CE R2 RE 100 µF 1,5 kΩ 1 kΩ 0V FIGURE 5.2: AMPLIFIER CIRCUIT DIAGRAM 5.2.1 Identify the amplifier in FIGURE 5.2. (1) 5.2.2 Describe the biasing method for Class A amplification. (4) 5.2.3 Determine the voltage drop across RC. (3) 5.2.4 Calculate the voltage gain of the circuit in decibels. (3) 5.2.5 Explain how an increase in VIN will affect the output voltage (VCE) of the amplifier in FIGURE 5.2. (5) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 16 DBE/November 2020 SC/NSC 5.3 FIGURE 5.3 below shows the input characteristic curve of a common emitter amplifier. Identify A, B and C. A Q Base Current B C FIGURE 5.3: CHARACTERISTIC CURVE (3) 5.4 Refer to FIGURE 5.4 below and answer the questions that follow. +VCC RB1 T1 T2 Q1 Output signal Q2 RB2 RE 0V FIGURE 5.4: AMPLIFIER CIRCUIT DIAGRAM 5.4.1 Identify the amplifier circuit in FIGURE 5.4. (1) 5.4.2 Identify the type of transistor used in the circuit. (1) 5.4.3 Explain why the circuit in FIGURE 5.4 will not consume power when the input signal is zero. (3) 5.4.4 Discuss the principle of operation of the amplifier in FIGURE 5.4. (5) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 17 DBE/November 2020 SC/NSC 5.5 Refer to the LC resonant circuit of a radio-frequency amplifier and its input characteristics in FIGURE 5.5 below and answer the questions that follow. + Vcc C2 C1 L1 L2 Output IC (mA) IB (µA) C Collector Current T1 Δ IC VCE Audio input RE CE Q θC < 180° Δ VCE 0V Output voltage FIGURE 5.5: LC RESONANT CIRCUIT AND INPUT CHARACTERISTICS 5.5.1 Name the class of amplification used by the circuit in FIGURE 5.5. (1) 5.5.2 Describe a radio-frequency amplifier. (3) 5.5.3 Indicate how the resonating frequency of the circuit in FIGURE 5.5 can be varied. (1) 5.5.4 Name the filter which blocks all frequencies, except those between certain limits. (1) 5.5.5 Explain energy transfer in the tank circuit which allows the RF amplifier to execute its function. (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology: Electronics 18 DBE/November 2020 SC/NSC 5.6 Refer to FIGURE 5.6 below and answer the questions that follow. + Vcc L3 R1 C1 COUT C2 Q1 L1 C3 VOUT L2 R2 RE CE 0V FIGURE 5.6: OSCILLATOR CIRCUIT DIAGRAM 5.6.1 Identify the oscillator in FIGURE 5.6. (1) 5.6.2 Discuss how oscillation is achieved in this circuit. (6) 5.6.3 Describe how feedback is obtained in the circuit. (4) 5.6.4 Explain the function of inductor L3. (3) 5.7 Explain why a field-effect transistor (FET) is preferred over a bipolar junction transistor (BJT) in oscillator circuits. (2) 5.8 State TWO similarities between the Colpitts oscillator and the Hartley oscillator. (2) [60] TOTAL: 200 Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC FORMULA SHEET RLC CIRCUITS SEMICONDUCTOR DEVICES P  V  I  cos θ V R  Gain A V  OUT   F  XL  2 fL VIN  RIN  1  R  XC  VOUT  VIN    F  2 fC  R IN  1 f f fr  OR fr  2 1  R  2 LC 2 VOUT  VIN  1  F  f  R IN  BW  r OR BW  f2  f1 Q SWITCHING CIRCUITS Series  R R R  VOUT    V1 F  V2 F  ...VN F  VR  I R  R1 R2 RN  VL  I XL VOUT VOUT Gain A V   VC  I XC VIN V1  V2  ...VN  VT VOUT   (V1  V2  ...VN ) IT  OR IT  IR  IC  IL Z AMPLIFIERS VC Z  R 2  X L  X C  IC  2 RC VT  VR  VL  VC  2 2 OR VT  I Z VCC  VCE  ICRC R VR Cos θ  OR Cos θ  VB  VBE  VRE Z VT XL XC VL VC 1 L VOUT Q     AV  R R VT VT R C VIN Parallel I A I  OUT VT  VR  VL  VC IIN P IR  VT A P  OUT A P  A V  AI R PIN OR V IC  T A = β1 x β 2 OR A V  A V1  A V2  A V3 XC VT IL  PIN  I2  ZIN POUT  I2  Z OUT XL AND IT  IR  IL  IC  2 2 GAIN IN DECIBELS VT I Z A I  20log10 OUT IT IIN I V Cos θ  R A V  20log10 OUT IT VIN XL XC IL IC 1 L POUT Q     A P  10log10 R R IT IT R C PIN Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC CENTRE NUMBER: EXAMINATION NUMBER: ANSWER SHEET QUESTION 2: RLC CIRCUITS 2.1.1 VR = 20 V Transfer mark to answer book MOD FIGURE 2.1.1 (2) 2.2.4 Transfer mark to answer book MOD FIGURE 2.2.4 (5) Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC CENTRE NUMBER: EXAMINATION NUMBER: ANSWER SHEET QUESTION 4: SWITCHING CIRCUITS 4.3.2 Trigger input ⅓ VCC 0V t ⅔ VCC VC1 when VR1 is set to 5 kΩ t High Output Low t ⅔ VCC VC1 when VR1 is set to 10 kΩ t High Transfer mark to Output when answer book VR1 is set to 10 kΩ Low t MOD FIGURE 4.3.2 (4) Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC CENTRE NUMBER: EXAMINATION NUMBER: ANSWER SHEET 4.4.2 VIN Upper trigger level 0 t Lower trigger level +V 0 t Transfer mark to answer book -V MOD FIGURE 4.4.2 (4) Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC CENTRE NUMBER: EXAMINATION NUMBER: ANSWER SHEET 4.6.2 VIN VREF 0V +VSAT 0V t Transfer mark to answer book -VSAT MOD FIGURE 4.6.2 (4) Copyright reserved

Downloaded from hlayiso.com Electrical Technology: Electronics DBE/November 2020 SC/NSC CENTRE NUMBER: EXAMINATION NUMBER: ANSWER SHEET 4.8.2 +V Input 0V t 0V RC time constant is Transfer mark to shorter answer book Output -VSAT MOD FIGURE 4.8.2 (2) 4.8.3 +V Input 0V t 0V RC time constant is Transfer mark to longer answer book Output -VSAT MOD FIGURE 4.8.3 (2) Copyright reserved