A silicon-based 5.1 V Zener diode requires a minimum operating current I of 5.0 mA to maintain its Zener voltage Vz - AQA - A-Level Physics - Question 4 - 2019 - Paper 8
Question 4
A silicon-based 5.1 V Zener diode requires a minimum operating current I of 5.0 mA to maintain its Zener voltage Vz.
Draw on Figure 11 the general I−V characteristi... show full transcript
Worked Solution & Example Answer:A silicon-based 5.1 V Zener diode requires a minimum operating current I of 5.0 mA to maintain its Zener voltage Vz - AQA - A-Level Physics - Question 4 - 2019 - Paper 8
Step 1
Draw on Figure 11 the general I−V characteristic for this diode.
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Answer
The I−V characteristic for a Zener diode shows a stable voltage across the diode as current increases past its Zener voltage. The graph typically has a knee developing around 0.7 V and does not exceed 1.5 V at 30 mA, stabilizing at approximately 5.1 V with a near vertical drop.
Step 2
State the function of the Zener diode in this circuit.
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Answer
The Zener diode provides a reference voltage for the non-inverting input of the operational amplifier, ensuring that the output remains stable at the Zener voltage of 5.1 V.
Step 3
Deduce whether a 100 Ω, 0.13 W resistor is suitable for R in Figure 12.
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Answer
To determine if the resistor is suitable, we calculate the power rating:
P=I2R
Substituting values:
P=(0.005)2imes100=0.025extW
Since 0.025 W is less than the rated 0.13 W, the resistor is suitable.
Step 4
Explain how this affects the light intensity at which W changes.
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Answer
With the Zener voltage reduced at the non-inverting input, the voltage across the Zener diode is now smaller, resulting in a lower output from the operational amplifier. As a result, the brightness of the light at W will change at a lower light intensity than before.
Step 5
Write the Boolean algebra expression for the output Q in terms of W, X and Y based on the logic gates shown in Figure 13.
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The output Q can be expressed as:
Q=(X+Y)imesW
Step 6
State one property that makes the MOSFET suitable for interfacing with logic gates.
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Answer
The MOSFET has a large input impedance, making it suitable for interfacing with logic gates without loading the previous stage.
