GATE EC 2016 Set 1 — Question 24
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Analog Circuits → BJT → LED Principle & Operation
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Question
The following signal of peak voltage 8 V is applied to the non-inverting terminal of an ideal opamp. The transistor has V, ; V, V and V. The number of times the LED glows is ________
Correct answer
2.9 to 3.1
Solution
1.The voltage at the inverting terminal () is determined by the voltage divider connected to V:
2.The op-amp acts as a comparator. Its output goes high when , which means .
3.When the op-amp output is high, it provides sufficient base current to turn the transistor ON, allowing current to flow through the LED, causing it to glow.
4.The input signal is a sinusoid with a peak voltage of .
5.The LED glows whenever exceeds the threshold. In each cycle of the sine wave, there is one continuous interval during the positive half-cycle where .
6.Observing the provided waveform for , there are 3 complete cycles where the peak reaches .
7.Therefore, the LED glows 3 times.
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