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For the given input voltage waveform Vᵢₙ(t), the output voltage waveform V₀(t), across the capacitor is correctly depicted by [Figure: Vᵢₙ is a pulse train: +5 V from 0 to 5 μs, 0 from 5 to 10 μs, +5 V again from 10 to 15 μs. It is applied through 1 kΩ to a 10 nF capacitor, and V₀ is taken across the capacitor. Options: (a) rises to about 2 V by 5 μs, sags slightly until 10 μs, then rises towards 3 V by 15 μs; (b) straight-line rise to 2 V and straight-line fall to 0, repeated; (c) rises to 2 V and stays flat; (d) rises to 2 V and decays back to 0 by 10 μs, then repeats.]

Asked in JEE Main 6th Sept 1st Shift 2020 · Output waveforms

Figure: Output waveforms
Answer: (1) Rises to about 2 V, sags, then climbs towards 3 V

Step-by-step solution

τ=RC=10³×10×10⁻⁹=10 μs

First pulse (0 to 5 μs): V=5(1-e^-0.5)=1.97 V≈2 V

Gap (5 to 10 μs): V=1.97e^-0.5=1.19 V

Second pulse: V=5-(5-1.19)e^-0.5=2.69 V, close to 3 V

Only (a) shows the build-up over successive pulses. (Its sag during the gap is drawn smaller than the true drop.)

Why the other options are wrong

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