JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRONIC DEVICES AND CIRCUITS Previous Question Paper (March - 2021 - R15)
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Main
Draw the V-I characteristics of diode and explain its temperature dependency.
Answer
The V-I characteristics of a p-n junction diode consist of a forward bias region where current increases exponentially with voltage after the knee voltage, and a reverse bias region where a very small saturation current flows. The forward characteristic is governed by the diode equation: I = I₀(exp(V/ηVₜ) - 1).
Temperature dependency is primarily driven by two factors:
- Reverse Saturation Current (I₀): I₀ approximately doubles for every 10°C rise in temperature. This occurs because the number of thermally generated minority carriers increases significantly with temperature.
- Forward Voltage (Vf): For a constant forward current, the forward voltage required decreases by approximately 2 mV/°C. This is because the thermal voltage Vₜ = kT/q increases with temperature, requiring less external voltage to achieve the same diffusion current.
Neglecting the effect of carrier generation and recombination in the space-charge region, find an expression for the diffusion capacitance of an ideal p-n junction diode in which the doping concentrations of the p and n sides are comparable to each other.
Answer
Diffusion capacitance (C_d) is defined as the rate of change of injected minority carrier charge with respect to the diode voltage. For an ideal p-n junction, the total injected charge Q is the sum of holes injected into the n-region and electrons injected into the p-region.
The current I is given by:
The diffusion capacitance is related to the mean lifetime of carriers (τ) and the current (I) by the relation:
Where τ = τₚ + τₙ. Given the doping concentrations are comparable, both sides contribute significantly to the total charge storage. Substituting the expression for current:
Since I ≈ I₀ exp(V/Vₜ) for forward bias, the expression simplifies to:
This shows that diffusion capacitance is directly proportional to the forward current and the carrier lifetime, representing the charge storage effect due to the finite time required for minority carriers to recombine.
Explain the principle of operation of varactor diode and also write its applications.
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How Zener diode is used for voltage regulation? Draw the circuit diagram.
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Derive an expression for the ripple in a ∏-section filter when used with a halfwave rectifier.
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The output of a full wave rectifier is fed from a 40-0-40V transformer. The load current is 0.1A. Two 40μF capacitances are available. The circuit resistance exclusive of the load is 50Ω. Calculate value of inductance for a two stage L-section filter. The inductances are to be equal. The ripple factor is to be 0.0001.
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What are the different transistor current components for a forward biased emitter junction and a reverse biased collector junction and derive the equations for emitter efficiency, transport factor and large signal current gain.
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Draw the common base configuration circuit, input characteristics, output characteristics and identify different regions of operation.
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Show that the exact expression for hfe in terms of the CB hybrid parameters is:
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Given a single stage transistor amplifier with the h parameters hic=1.1K, hfc= -51, hrc=1, hoc= 25μA/V, calculate AI, AV, Avs, Ri and Ro for the CC transistor configuration, with Rs = RL = 10K.
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Determine the stability factor for the fixed bias circuit and compare its merits and demerits with other biasing circuits.
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A transistor with β=100 is to be used in the collector-emitter feedback bias configuration with Rc=1.5K, Re=2K, Rb=370K, VCC=12V, VBE=0.6V and IC0=0. Calculate the collector to emitter voltage VCE and also find the stability factor?
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How a thermistor and sensistor are used for transistor bias compensation?
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What is meant by thermal instability in a transistor? How to achieve stability?
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Draw the V-I characteristics of enhancement MOSFET and explain it.
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Consider the common drain amplifier and assume the FET is properly biased and the FET has the following parameters: gm=2mA/V and rd=100K. Using the small signal low frequency model derive an expression for the gain of the amplifier.
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