Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech II ELECTRONIC DEVICES AND CIRCUITS Previous Question Paper (November/December - 2017 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit.
Part A
Answer all questions. Each question carries marks as indicated.
For what voltage will the reverse current in p-n junction Germanium diode reach 90% of its saturation value at room temperature?
Answer
The reverse current in a p-n junction diode is given by the equation I = Is(1 - e-V/ηVT). To reach 90% of the saturation current Is, the magnitude of the reverse current must be 0.9Is. Setting the equation to 0.9 = 1 - e-V/VT (assuming η = 1 for Germanium at room temperature), we get e-V/VT = 0.1. Taking the natural logarithm of both sides, -V/VT = ln(0.1), which simplifies to V = VT × ln(10). At room temperature (300K), the thermal voltage VT is approximately 26 mV. Therefore, V = 26 mV × 2.303, resulting in a voltage of approximately 59.88 mV.
Write a short note on Varactor diode.
Answer
A Varactor diode is a special-purpose semiconductor diode that acts as a voltage-controlled capacitor. It is specifically designed to operate in the reverse-biased region, where the depletion region width varies with the applied reverse voltage.
Key characteristics include:
- When the reverse bias voltage increases, the depletion region widens, which decreases the junction capacitance.
- Conversely, when the reverse bias voltage decreases, the depletion region narrows, increasing the junction capacitance.
- The relationship between capacitance (C) and voltage (V) is given by the formula: C = C₀ / (1 + V/Vᵦ)ⁿwhere C₀ is the capacitance at zero bias, Vᵦ is the barrier potential, and n is a constant depending on the doping profile.
- They are primarily used in tuning circuits, such as voltage-controlled oscillators (VCOs), frequency synthesizers, and parametric amplifiers in communication systems.
Derive the ripple factor for full wave rectifier.
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Explain voltage regulation using zener diode.
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Explain how transistor acts as an amplifier.
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Give the Comparisons between CB, CE, CC configurations.
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Define thermal runaway.
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Compare all the three biasing circuits.
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For a p-channel Silicon FET, with effective width ‘a’ = 2×10-4 cm and channel resistivity ρ = 10 Ω. Find the pinch off voltage.
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Draw the circuit diagram of fixed bias arrangement of a JFET.
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Part B
Explain PN diode characteristics in forward bias and reverse bias regions.
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Find the width of the depletion layer in a germanium junction diode which has the following specifications: Area A = 0.001 cm2, σn = 1 mhos/cm, µn = 3800 cm2/sec, µp = 1800 cm2/sec.
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Explain tunnel diode operation with the help of energy band diagrams.
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Explain the static characteristics of SCR.
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A full wave rectifier circuit with C-type capacitor filter is to supply a D.C. Current of 20 mA at 16V. If frequency is 50 Hz ripple allowed is 5%. Calculate: i) Required secondary voltage of the transformer ii) Ratio of I peak/ Imax through diodes and the value of C required.
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With a neat circuit diagram and necessary wave forms explain the operation of half wave rectifier.
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An ac supply of 220V is applied to a half wave rectifier circuit through a transformer with a turns ratio of 10:1. Assume the ideal diode. Find: i) dc output voltage ii) PIV.
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Compare half wave, full wave and bridge rectifier circuits.
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Explain CE configuration with the help of input and output characteristics.
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A transistor is operated at a forward current of 2µA and with the collector open circuited. Calculate the junction voltages VC and VE, the collector to emitter voltage VCE assuming ICO = 2µA, IEO = 1.6µA and αN = 0.98.
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Draw and explain h-parameter model of BJT.
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Qualitatively explain the static V-I characteristics of UJT.
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Explain the need for biasing in electronic circuits. What are the factors affecting the stability factor.
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A transistor with β = 100 is to be used in Common Emitter Configuration with collector to base bias. The collector circuit resistance is RC = 1kΩ and VCC = 10V. Assume VBE = 0. i) Choose RB so that the quiescent collector to emitter voltage is 4V. ii) Find the stability factor.
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Determine the quiescent currents and the collector to emitter voltage for a Ge transistor with β = 50 in the self biasing arrangements. The circuit component values are VCC = 20V, RC = 2kΩ, Re = 0.1 kΩ, R1 = 100 kΩ and R2 = 5 kΩ. Find the stability factor S.
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Explain the terms Bias Stabilization and Bias Compensation.
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Derive the expression for the width of depletion region ‘W’ in the case of p-channel JFET.
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Explain the working of a depletion type MOSFET with a neat construction diagram and its characteristics.
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Draw the circuit of source follower Amplifier and derive the expressions for AI, AV, Ri and Ro.
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