QUESTION 2: SWITCHING CIRCUITS 2.1 State ONE distinct difference between the astable multivibrator and the bistable multivibrator with reference to their inputs. 2.2 Refer to FIGURE 2.2 and answer the questions that follow. 2.2.1 Identify the multivibrator in FIGURE 2.2. 2.2.2 Describe the operation of the multivibrator in FIGURE 2.2 from a point where the capacitor has just discharged and ends. 2.2.3 Explain how the frequency of the multivibrator in FIGURE 2.2 can be increased. 2.3 FIGURE 2.3 below shows a monostable multivibrator using a 555 IC. 2.3.1 State the function of capacitor C2. 2.3.2 FIGURE 2.3.2 below shows the trigger input, capacitor charge and output signals of the multivibrator in FIGURE 2.3. The variable resistor (VR1) is set to 5 kΩ. On the ANSWER SHEET for QUESTION 2.3.2, draw the charging voltage (Vc) and corresponding output if variable resistor (VR1) is changed to 10 kΩ. 2.3.3 State whether LED 1 or LED 2 will be ON after the trigger switch is pressed. Motivate your answer. 2.4 FIGURE 2.4 below shows the circuit diagram of a non-inverting Schmitt trigger. Answer the questions that follow. 2.4.1 Indicate whether this is an open-loop mode op-amp circuit or a closed-loop mode op-amp circuit. Motivate your answer. 2.4.2 Draw the output waveform of the Schmitt trigger on the ANSWER SHEET for QUESTION 2.4.2. 2.4.3 State how the trigger voltage levels of the Schmitt trigger can be adjusted without changing the supply voltage. 2.5 Refer to FIGURE 2.5 below and answer the questions that follow. 2.5.1 State the function of capacitor C1. 2.5.2 Calculate the value of the output voltage. 2.5.3 Explain what makes it possible for this amplifier to amplify both positive and negative voltages. 2.5.4 Explain how the output voltage will be affected if switch S1 is open. 2.6 Refer to FIGURE 2.6 below and answer the questions that follow. 2.6.1 Identify the op-amp circuit in FIGURE 2.6. 2.6.2 Draw the output signal of the op-amp on the ANSWER SHEET for QUESTION 2.6.2. 2.6.3 Explain why this op-amp circuit is driven into saturation during its operation. 2.7 Explain the operation of the passive RC differentiator in FIGURE 2.7 below. 2.8 FIGURE 2.8 below shows an op-amp integrator with input and output signals. Answer the questions that follow. 2.8.1 Explain why the op-amp integrator is able to produce a steadily falling output voltage when a square wave is applied to it. 2.8.2 On the ANSWER SHEET for QUESTION 2.8.2, draw the output waveform of the op-amp when the RC time constant is shortened. 2.8.3 On the ANSWER SHEET for QUESTION 2.8.3, draw the output waveform of the op-amp when the RC time constant is lengthened from the given state.

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QUESTION 2: SWITCHING CIRCUITS 2.1 State ONE distinct difference between the astable multivibrator and the bistable multivibrator with reference to their inputs - NSC Electrical Technology Digital - Question 2 - 2020 - Paper 1

Question 2

QUESTION 2: SWITCHING CIRCUITS 2.1 State ONE distinct difference between the astable multivibrator and the bistable multivibrator with reference to their inputs. 2... show full transcript

Worked Solution & Example Answer:QUESTION 2: SWITCHING CIRCUITS 2.1 State ONE distinct difference between the astable multivibrator and the bistable multivibrator with reference to their inputs - NSC Electrical Technology Digital - Question 2 - 2020 - Paper 1

Step 1

2.2.1 Identify the multivibrator in FIGURE 2.2.

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The multivibrator in FIGURE 2.2 is identified as an astable multivibrator.

Step 2

2.2.2 Describe the operation of the multivibrator in FIGURE 2.2 from a point where the capacitor has just discharged and ends.

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At the moment the capacitor discharges, the output of the astable multivibrator switches to a high state. This occurs because the voltage across the capacitor transitions from 0V to a positive voltage, triggering the output to remain high until the capacitor recharges. Once the capacitor voltage reaches a certain threshold, it discharges again, causing the output to toggle back to low, and the cycle continues. This creates a continuous oscillation between high and low states.

Step 3

2.2.3 Explain how the frequency of the multivibrator in FIGURE 2.2 can be increased.

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The frequency of the multivibrator can be increased by decreasing the values of the resistors or capacitors in the timing circuit. Specifically, reducing the resistance or capacitance will reduce the time constant, allowing the output to change states more rapidly.

Step 4

2.3.1 State the function of capacitor C2.

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Capacitor C2 serves to filter out high-frequency noise and stabilize the operation of the 555 timer by providing a smooth voltage transition.

Step 5

2.3.2 Draw the charging voltage (Vc) and corresponding output if variable resistor (VR1) is changed to 10 kΩ.

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On the answer sheet, sketch the Vc waveform showing a slower charging curve due to the increased resistance, which results in a longer time for the capacitor to charge compared to when VR1 is set to 5 kΩ. The corresponding output should reflect the delayed response, indicating the high state after a longer duration.

Step 6

2.3.3 State whether LED 1 or LED 2 will be ON after the trigger switch is pressed. Motivate your answer.

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LED 2 will be ON after the trigger switch is pressed. This is because pressing the trigger input causes the output of the 555 timer to go high, forwarding biasing LED 2 while backward biasing LED 1.

Step 7

2.4.1 Indicate whether this is an open-loop mode op-amp circuit or a closed-loop mode op-amp circuit. Motivate your answer.

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This is a closed-loop mode op-amp circuit. It includes feedback from the output to the non-inverting input, which stabilizes the output and improves the linearity of the response.

Step 8

2.4.2 Draw the output waveform of the Schmitt trigger.

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On the answer sheet, draw a waveform that features hysteresis, showing the output toggling between high and low states, with clear thresholds for rising and falling inputs.

Step 9

2.4.3 State how the trigger voltage levels of the Schmitt trigger can be adjusted without changing the supply voltage.

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The trigger voltage levels can be adjusted by changing the values of the resistors in the divider network that determines the input levels at which the output transitions occur.

Step 10

2.5.1 State the function of capacitor C1.

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Capacitor C1 is used to couple AC signals while blocking DC signals, ensuring only the desired AC variations influence the output.

Step 11

2.5.2 Calculate the value of the output voltage.

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To calculate the output voltage, apply the formula for an inverting amplifier: V_out = - (R_f/R_in) * V_in, where the resistor values and the input voltage are substituted into the equation.

Step 12

2.5.3 Explain what makes it possible for this amplifier to amplify both positive and negative voltages.

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The amplifier is designed to have a bipolar power supply (positive and negative voltages), allowing it to respond to both positive and negative input signals and amplify them accordingly.

Step 13

2.5.4 Explain how the output voltage will be affected if switch S1 is open.

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If switch S1 is open, the circuit will no longer have a feedback path or input signal, resulting in the output voltage decreasing towards zero or entering a floating state.

Step 14

2.6.1 Identify the op-amp circuit in FIGURE 2.6.

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The op-amp circuit in FIGURE 2.6 is identified as an inverting comparator.

Step 15

2.6.2 Draw the output signal of the op-amp.

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On the answer sheet, sketch the output signal that switches between full positive and negative saturation states as the input crosses the reference voltage.

Step 16

2.6.3 Explain why this op-amp circuit is driven into saturation during its operation.

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The op-amp circuit is driven into saturation because there is no feedback loop to limit the gain of the op-amp, resulting in the output quickly reaching its maximum or minimum voltage levels based on the input changes.

Step 17

2.7 Explain the operation of the passive RC differentiator in FIGURE 2.7.

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The passive RC differentiator responds to changing input signals by generating an output that reflects the rate of change of the input. When a square wave is applied, the circuit's capacitor charges and discharges, creating sharp spikes in the output that correspond to the transitions of the input square wave.

Step 18

2.8.1 Explain why the op-amp integrator is able to produce a steadily falling output voltage when a square wave is applied to it.

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The op-amp integrator produces a steadily falling output voltage because the square wave input causes the capacitor to charge and discharge in such a way that the average output voltage steadily decreases over time, following the integral of the input waveform.

Step 19

2.8.2 Draw the output waveform of the op-amp when the RC time constant is shortened.

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On the answer sheet, draw an output waveform that reflects a quicker response with less voltage change over time due to the shortened RC time constant.

Step 20

2.8.3 Draw the output waveform of the op-amp when the RC time constant is lengthened from the given state.

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On the answer sheet, sketch a waveform that shows a more gradual output response, indicating a slower decline based on the increased RC time constant.

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