GATE EC 2016 Set 3 — Question 49
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Analog Circuits → Diode Circuits → Zener Diode Applications
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Question
The I-V characteristics of the zener diodes D1 and D2 are shown in Figure I. These diodes are used in the circuit given in Figure II. If the supply voltage is varied from 0 to 100 V, then breakdown occurs in
Correct answer
(A) D1 only
Solution
In Figure II, D1 and D2 are in series with the same orientation (both cathodes towards the top), and the source polarity drives current downward through them, so both diodes are reverse-biased.From Figure I: D1 breaks down at 80 V and D2 at 70 V; in the pre-breakdown region D1 has the smaller reverse saturation current (its curve lies closer to the -axis).Being in series, both diodes carry the same current. Before any breakdown, that current cannot exceed D1's smaller saturation current. D2 needs only a small reverse voltage to carry a current below its own saturation level, so nearly the entire supply voltage appears across D1.As the supply rises, the voltage across D1 reaches 80 V first, so D1 breaks down. After that, D1 holds about 80 V, leaving at most V for D2, which is well below D2's 70 V breakdown voltage. D2 therefore never breaks down.
- (B) and (C) would require about 70 V across D2, which is impossible.
- (D) is wrong because D1 does reach 80 V.
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