BE Computer Engineering (Pokhara University) Electronics Devices and Circuits (PU, ELX 120) Question Paper 2079

(a) Distinguish between intrinsic and extrinsic semiconductors. Explain how N-type and P-type semiconductors are formed by the process of doping, clearly indicating the role of majority and minority carriers in each case. (7)

(b) Explain the formation of the depletion region in an unbiased PN junction. With suitable energy-band and circuit diagrams, describe the effect of forward bias and reverse bias on the width of the depletion region and the resulting barrier potential. (7)

(a) Draw the circuit diagram of a full-wave bridge rectifier and explain its operation with the help of input and output waveforms for both half-cycles of the input. (6)

(b) A full-wave bridge rectifier is supplied from a transformer with a secondary peak voltage of 24 V (silicon diodes, V_γ = 0.7 V each). Determine the DC output voltage, the ripple factor, and the rectifier efficiency. Explain how the addition of a shunt capacitor filter improves the ripple factor. (6)

(a) Draw the circuit of a voltage-divider (self) bias configuration for an NPN BJT and explain why it provides the most stable operating point compared to fixed bias. (6)

(b) For the voltage-divider bias circuit, V_CC = 12 V, R_1 = 47 kΩ, R_2 = 10 kΩ, R_C = 2.2 kΩ, R_E = 1 kΩ and β = 100. Determine the operating point (I_C and V_CE) and draw the DC load line indicating the Q-point. (8)

(a) Define the following ideal op-amp parameters: input offset voltage, CMRR, slew rate, and gain-bandwidth product. (4)

(b) Draw the circuit of an inverting amplifier using an op-amp and derive the expression for its closed-loop voltage gain assuming an ideal op-amp. (4)

(c) Design an op-amp summing amplifier that produces an output V_o = -(2V_1 + 5V_2). Choose suitable resistor values and show the complete circuit. (4)

Explain the operation of a Zener diode as a voltage regulator. With a circuit diagram, describe how line regulation and load regulation are achieved, and state the conditions under which the Zener diode remains in regulation.

With suitable diagrams, explain the construction and working principle of an N-channel JFET. Define and explain the terms pinch-off voltage (V_P) and drain-source saturation current (I_DSS).

Differentiate between depletion-type and enhancement-type MOSFETs. Sketch the transfer characteristics of each and state one practical application where the enhancement MOSFET is preferred.

Compare the common-emitter, common-base, and common-collector BJT amplifier configurations in terms of voltage gain, current gain, input impedance, output impedance, and phase relationship between input and output.

(a) State the Barkhausen criterion for sustained oscillations. (2)

(b) Draw the circuit of an RC phase-shift oscillator using a BJT and explain how the required 180° phase shift is obtained. Write the expression for its frequency of oscillation. (4)

Draw the circuit of a Wien-bridge oscillator and explain its working. For a Wien-bridge oscillator with R = 10 kΩ and C = 0.01 µF, calculate the frequency of oscillation and state the minimum gain required from the amplifier.

With a neat circuit diagram, explain the operation of an op-amp integrator. Derive the expression for its output voltage and explain why a feedback resistor is often connected in parallel with the feedback capacitor in a practical integrator.

Explain the phenomenon of drift and diffusion currents in a semiconductor. State and explain the Einstein relationship connecting the diffusion constant and the mobility of charge carriers.

With circuit diagrams and transfer characteristics, explain the operation of a positive clipper and a positive clamper circuit. State one application of each.