What is the purpose of Q1 in the circuit shown in Figure E7-3?
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It provides negative feedback to improve regulation
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It provides a constant load for the voltage source
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Correct AnswerIt increases the current-handling capability of the regulator
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It provides D1 with current
What is the purpose of C2 in the circuit shown in Figure E7-3?
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Correct AnswerIt bypasses hum around D1
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It is a brute force filter for the output
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To self-resonate at the hum frequency
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To provide fixed DC bias for Q1
The circuit is a simplified version of a linear series voltage regulator. Q1 is the control element. The circuit is a three-terminal device (one input, one output and one shared ground connection).
If you see closely, C2 is connected in parallel to D1. This will basically act as a filter across D1 and prevent oscillations across D1 and hence prevent the hum.
Remember the term bypass capacitor and that a bypass capacitor will pass above certain wavelengths of AC current while blocking DC current. Bypass capacitors are very common for eliminating undesirable AC current such as noise so it is useful to remember this term.
Hint: C2 is closest to D1 on the schematic, and the correct answer is the only one that references D1.
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What type of circuit is shown in Figure E7-3?
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Switching voltage regulator
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Grounded emitter amplifier
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Correct AnswerLinear voltage regulator
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Emitter follower
What is the purpose of C1 in the circuit shown in Figure E7-3?
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It resonates at the ripple frequency
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It provides fixed bias for Q1
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It decouples the output
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Correct AnswerIt filters the supply voltage
What is the purpose of C3 in the circuit shown in Figure E7-3?
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Correct AnswerIt prevents self-oscillation
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It provides brute force filtering of the output
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It provides fixed bias for Q1
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It clips the peaks of the ripple
What is the purpose of R1 in the circuit shown in Figure E7-3?
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It provides a constant load to the voltage source
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It couples hum to D1
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Correct AnswerIt supplies current to D1
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It bypasses hum around D1
What is the purpose of R2 in the circuit shown in Figure E7-3?
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It provides fixed bias for Q1
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It provides fixed bias for D1
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It decouples hum from D1
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Correct AnswerIt provides a constant minimum load for Q1
What is the purpose of D1 in the circuit shown in Figure E7-3?
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To provide line voltage stabilization
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Correct AnswerTo provide a voltage reference
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Peak clipping
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Hum filtering
Component D1 is a zener diode and when reverse biased as shown and when the voltage equals or exceeds the zener diode specification the zener diode will maintain a constant voltage across it that is based on the zener diode model selected, assuming the source doesn't exceed the current limit of the zener (one purpose of R1). Thus, the zener diode creates a very effective reference voltage.
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