JNTU
JNTU B.Tech III PULSE AND DIGITAL CIRCUITS Previous Question Paper (October - 2020 - R13)
Answer any five questions. All questions carry equal marks.
Main
Discuss the response of a low-pass RC circuit for step, ramp and sinusoidal inputs with help of neat diagrams.
Answer
A low-pass RC circuit acts as an integrator. Its response to different inputs is as follows:
1. Step Input: When a step voltage V is applied, the output rises exponentially towards V with a time constant τ = RC. The output is given by vo(t) = V(1 - e-t/τ).
2. Ramp Input: For an input vi(t) = kt, the output is vo(t) = k(t - τ + τe-t/τ). For t >> τ, the output follows the ramp with a time delay of τ.
3. Sinusoidal Input: The circuit acts as a filter that attenuates high frequencies. The output is a sine wave of the same frequency but with a phase lag φ = -tan-1(ωRC) and an amplitude scaled by 1/sqrt(1 + (ωRC)2).
A symmetrical square wave of ±5V at a frequency of 5KHz is applied to a high-pass RC circuit with a cut off frequency of 20KHz. Calculate the steady state output voltage and draw the steady state input/output voltage waveform.
Answer
Given: f = 5 kHz, T = 1/f = 200 μs. The half-period T/2 = 100 μs. Cut-off frequency fc = 20 kHz. The time constant τ = 1 / (2πfc) = 1 / (2π * 20,000) ≈ 7.96 μs.
Since τ << T/2 (7.96 μs << 100 μs), the circuit is in the 'differentiator' region. The output voltage vo(t) decays exponentially: vo(t) = Vinitial * e-t/τ. At the transition of the square wave from +5V to -5V, the output jumps by 10V. The steady-state peak voltage V' is given by V' = V * (1 - e-T/2τ) / (1 + e-T/2τ). Substituting values: V' = 5 * (1 - e-100/7.96) / (1 + e-100/7.96) ≈ 5 * (1 - 0.000003) / (1 + 0.000003) ≈ 4.9999 V.
The output waveform consists of positive and negative spikes of approximately ±5V, which decay exponentially towards 0V within each half-cycle.
Discuss the response of RC high-pass circuit to different types of input voltages along with input output waveforms.
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A 10V step input is given to a 22KΩ resistor in series with a 300pF capacitor. Calculate the rise time of the capacitor voltage, the time required for the capacitor to charge to 63.2% of its maximum change and the time for the capacitor to become completely charged.
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State and prove clamping circuit theorem and explain how sine wave can be converted into a square wave using Zener diodes.
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Design a diode clamper to restore a DC level +3V to an input signal of peak to peak value of 10V. Assume drop across the diode is 0.6V.
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Draw the neat circuit of emitter-coupled transistor clipper and explain its working with help of its waveforms.
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Design a diode clamper circuit to clamp the positive peaks of the input signal at zero level. The frequency of the input signal is 500Hz.
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Draw and explain four diode sampling gate.
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Describe the design considerations of a transistor switch.
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Explain various transistor switching times.
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Explain the working of bidirectional sampling gate with circuit diagram.
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Explain the working of a transistor bi-stable multivibrator with help of neat diagram and draw the input/output wave form for the same.
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Design a collector coupled astable multivibrator for the following specifications: F=10 KHz, Vcc=9V, ic(max) = 2mA and hfe = 20. Explain similarities and differences between Miller sweep circuit and boot strap sweep circuit.
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Explain the working of monostable relaxation circuit as a frequency divider with neat circuit diagram and necessary waveforms.
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Explain the concept frequency division in sweep circuits with necessary waveforms.
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