GATE EE 2023 Set 1 — Question 44
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Analog & Digital Electronics → Diode Circuits → P-N Junction Diode
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Question
All the elements in the circuit shown in the following figure are ideal. Which of the following statements is/are true?
Correct answer
(B) When switch S is ON, D₁ conducts and both D₂ and D₃ are reverse biased; (C) When switch S is OFF, D₁ is reverse biased and both D₂ and D₃ conduct
Solution
Take the bottom rail as the reference (0 V). Along the top branch: node is the right terminal of the 4 A source (anode of and of ), node is the top of the 20 V source (cathode of and input of the 2 A source), node is the output of the 2 A source (cathode of and anode of ), and the cathode of sits on the top of the 40 V source. Hence V, the cathode of is at V, and bridges node to node . The switch connects node to the rail.Switch ON ():
- has its anode at V and its cathode at V, so it is reverse biased.
- The 4 A must leave node either through (to ) or through (to ). If conducts, V, and then has anode V and cathode V, i.e. reverse biased - a consistent state. (Assuming instead that conducts gives V, which forward biases towards V and contradicts the assumption.)
- KCL at : the 4 A from plus the 2 A from the current source flow to the rail through .
- The 2 A source now has only one path, through , so conducts and V.
- The 4 A at node can leave through (which clamps to V) or through (which would require V). Taking conducting, V, and then has anode V and cathode V, i.e. reverse biased - a consistent state.
- At node the 4 A arriving through splits: 2 A into the current source and 2 A into the 20 V source.
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