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 top of the – divider and the top of the LED resistor are both connected to V.The waveform starts at V; its dash-dot reference lines are at V, V and V. It has three positive half-cycles: the first and third are single humps; the second has two humps whose dip between them reaches only about V (between the V and V lines). The third negative half-cycle similarly has two troughs with a rise between them only to about V.
The number of times the LED glows is ________
Correct answer
2.9 to 3.1
Solution
The inverting input sits at V. The ideal op-amp is a comparator: its output is V when V (transistor on, LED glows) and V otherwise (transistor off).When on, mA, enough to saturate the transistor, so the LED does glow.Count the separate intervals with V:
1.First positive half-cycle — one interval.
2.Second positive half-cycle — the dip between the two peaks only reaches about 3 V, which is still above 2 V, so this is one continuous interval.
3.Third positive half-cycle — one interval.
Negative half-cycles (including the double trough) never exceed 2 V.Answer: 3Continue learning with Success Tracker
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