BE Computer Engineering (Pokhara University) Electronics Devices and Circuits (PU, ELX 120) Question Paper 2078

(a) Distinguish between intrinsic and extrinsic semiconductors. Explain how n-type and p-type semiconductors are formed by doping, clearly identifying the majority and minority carriers in each case. (7)

(b) Define drift current and diffusion current in a semiconductor. A silicon bar is doped with $5\times10^{16}$ donor atoms/cm³. If the intrinsic carrier concentration $n_i = 1.5\times10^{10}$ /cm³ at room temperature, calculate the equilibrium electron and hole concentrations. Also explain the effect of temperature on the conductivity of an intrinsic semiconductor. (7)

(a) Why is biasing necessary in a BJT amplifier? With a neat circuit diagram, explain the voltage-divider (self) bias configuration and show why it provides better stability against variations in $\beta$ and temperature than fixed bias. (8)

(b) For the voltage-divider bias circuit, $V_{CC}=12\text{ V}$, $R_1=40\text{ k}\Omega$, $R_2=10\text{ k}\Omega$, $R_C=2\text{ k}\Omega$, $R_E=1\text{ k}\Omega$ and $\beta=100$. Determine the Q-point ($I_{CQ}$ and $V_{CEQ}$) and comment on the location of the operating point on the DC load line. (6)

(a) Define an ideal operational amplifier and list its ideal characteristics. Explain the significance of the terms input offset voltage, CMRR and slew rate in a practical op-amp. (6)

(b) With a circuit diagram, derive the expression for the output voltage of an inverting summing amplifier with three inputs. Hence design an op-amp circuit that produces an output $V_o = -(2V_1 + 5V_2 + V_3)$. (6)

(a) State the Barkhausen criterion for sustained oscillations and explain why an oscillator does not require any external input signal. (4)

(b) With a neat circuit diagram, explain the operation of an RC phase-shift oscillator using a BJT. Derive the expression for its frequency of oscillation and state the minimum gain required for the amplifier to sustain oscillations. (6)

Explain the formation of the depletion region in an unbiased PN junction diode. Sketch and explain the V-I characteristic of a silicon diode under forward and reverse bias, indicating the cut-in voltage and reverse breakdown region.

(a) With a circuit diagram and waveforms, explain the working of a full-wave bridge rectifier. (4)

(b) A full-wave rectifier with a capacitor filter feeds a load of $1\text{ k}\Omega$. If the peak rectified voltage is 20 V and the filter capacitor is $470\,\mu\text{F}$ with a supply frequency of 50 Hz, calculate the peak-to-peak ripple voltage and the ripple factor. (4)

Explain the construction and principle of operation of an n-channel JFET. Define pinch-off voltage and draw its drain (output) characteristics, marking the ohmic, saturation and breakdown regions.

With a neat diagram, explain the structure and operation of an enhancement-type n-channel MOSFET. How does its transfer characteristic differ from that of a depletion-type MOSFET? Define the threshold voltage.

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. State one practical application of each.

(a) Draw the circuit of an op-amp integrator and derive the relation between its output and input voltage. (4)

(b) A square wave is applied to the input of an ideal integrator. Sketch the expected output waveform and state one limitation of a practical integrator. (3)

Explain how a Zener diode acts as a voltage regulator with the help of a circuit diagram. A 6.2 V Zener diode with $R_Z \approx 0$ is used to regulate a load from an unregulated 12 V supply through a series resistor of $470\,\Omega$. If the load current is 8 mA, calculate the Zener current and the power dissipated in the Zener diode.

Write short notes on any TWO of the following:

(a) Hartley oscillator

(b) Wien-bridge oscillator

(c) Crystal oscillator and its frequency stability