EC-304 Electronic Devices - Jun 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the intrinsic and extrinsic semiconductor in details.
   How to answer

   • Definition of intrinsic semiconductor (pure form, electron-hole pairs generation).
   • Definition of extrinsic semiconductor and doping process.
   • Types of extrinsic semiconductors: n-type (pentavalent impurity) and p-type (trivalent impurity).
   • Carrier concentrations and Fermi level positions in both types.
2. 7 Marks
   bDescribe the energy band structure of insulators, semiconductors and conductors.
   How to answer

   • Concept of valence band, conduction band, and forbidden energy gap ($E_g$).
   • Conductors: overlapping valence and conduction bands ($E_g \approx 0\text{ eV}$).
   • Semiconductors: small energy gap ($E_g \approx 1\text{ eV}$).
   • Insulators: large energy gap ($E_g > 5\text{ eV}$).
   • Energy band diagrams for all three materials.
3. 7 Marks
   aExplain the V-I characteristics of zener diodes along with its applications.
   How to answer

   • Basic operating principle of a Zener diode (heavily doped PN junction).
   • V-I characteristic curve showing forward bias and reverse breakdown (Zener breakdown voltage $V_Z$).
   • Key parameters: knee current, maximum current rating, breakdown voltage.
   • Major applications, prominently as a shunt voltage regulator.
4. 7 Marks
   bDetermine the temperature coefficient of a zener diode having voltage of $5\text{ V}$ at $25^\circ\text{C}$, drops to $4.8\text{ V}$ at temperature of $100^\circ\text{C}$.
   How to answer

   • State given values: $V_{Z1} = 5\text{ V}$ at $T_1 = 25^\circ\text{C}$, $V_{Z2} = 4.8\text{ V}$ at $T_2 = 100^\circ\text{C}$.
   • Calculate $\Delta V_Z = 4.8 - 5 = -0.2\text{ V}$ and $\Delta T = 100 - 25 = 75^\circ\text{C}$.
   • Apply formula: $TC = \frac{\Delta V_Z}{V_{Z1} \cdot \Delta T} \times 100\% / ^\circ\text{C}$ or in $\text{mV}/^\circ\text{C}$.
   • Compute: $TC = \frac{-0.2}{5 \times 75} \times 100 = -0.0533\% / ^\circ\text{C}$ (or $-2.67\text{ mV}/^\circ\text{C}$).
   • State negative temperature coefficient nature.
5. 7 Marks
   aDraw and explain the clipper and clamper circuits.
   How to answer

   • Definition and purpose of a clipper circuit (removes portions of an input signal).
   • Schematic of a positive or negative diode clipper with input/output waveforms.
   • Definition and purpose of a clamper circuit (shifts DC level without changing waveform shape).
   • Schematic of a positive or negative clamper (using diode, capacitor, resistor) with input/output waveforms.
6. 7 Marks
   bExplain full wave and bridge rectifier with suitable diagram.
   How to answer

   • Working principle of center-tapped full-wave rectifier with circuit diagram.
   • Working principle of bridge rectifier (four diodes) during positive and negative half-cycles with circuit diagram.
   • Input and output waveforms for full-wave rectification.
   • Comparison of key performance parameters: PIV, efficiency, and ripple factor.
7. 7 Marks
   aDefine photo transistor. Explain the various configuration of BJT.
   How to answer

   • Definition and working principle of a photo transistor (light-sensitive BJT).
   • BJT configurations overview: Common Emitter (CE), Common Base (CB), and Common Collector (CC).
   • Circuit diagrams for CB, CE, and CC configurations.
   • Key comparative features: current gain, voltage gain, input/output impedance.
8. 7 Marks
   bExplain any one biasing technique of BJT with suitable diagram.
   How to answer

   • Need for transistor biasing and Q-point stability.
   • Selection of a biasing technique (typically Voltage Divider / Self-bias).
   • Circuit diagram of the chosen biasing configuration.
   • Circuit analysis for base voltage, collector current, and collector-emitter voltage ($V_{CE}$).
   • Expression for stability factor showing superior thermal stability.
9. 7 Marks
   aDescribe the class A and class B amplifiers.
   How to answer

   • Class A amplifier: operating principle, Q-point in active region center, $360^\circ$ conduction angle, maximum theoretical efficiency (25% or 50%).
   • Class B amplifier: operating principle, Q-point at cutoff, $180^\circ$ conduction angle, maximum theoretical efficiency (78.5%).
   • Mention push-pull configuration and crossover distortion in Class B.
   • Waveforms of collector current for both classes.
10. 7 Marks
    bExplain the coupling method in multistage amplifier.
    How to answer

    • Purpose of cascading amplifier stages and role of coupling networks.
    • RC coupling: circuit diagram, operation, frequency response, advantages, and applications.
    • Transformer coupling: impedance matching, high efficiency, limited bandwidth.
    • Direct coupling: low-frequency response, DC amplification, drift problem.
    • Brief comparison table of the coupling methods.
11. 7 Marks
    aGive the analysis of transistor amplifier using h-parameter.
    How to answer

    • Define two-port hybrid model equations and equivalent circuit.
    • Derive Current Gain: $A_i = \frac{-h_f}{1 + h_o R_L}$.
    • Derive Input Impedance: $Z_i = h_i + h_r A_i R_L = h_i - \frac{h_f h_r R_L}{1 + h_o R_L}$.
    • Derive Voltage Gain: $A_v = \frac{A_i R_L}{Z_i}$.
    • Derive Output Admittance: $Y_o = h_o - \frac{h_f h_r}{h_i + R_s}$.
12. 7 Marks
    bWrite short note on : i) Current mirror circuit ii) Push-pull amplifier
    How to answer

    • i) Current Mirror: Concept of copying current through an active device, basic two-transistor BJT circuit, reference current equation $I_{ref} \approx I_C$, applications in IC biasing.
    • ii) Push-pull Amplifier: Operating principle using complementary pair or center-tapped transformer, cancellation of even harmonics, elimination of DC core saturation, crossover distortion problem and solution.
13. 7 Marks
    aExplain the construction and working of JFET.
    How to answer

    • Basic construction of n-channel and p-channel JFET (channel, source, drain, gate).
    • Working principle: variation of depletion region width with reverse gate-to-source voltage $V_{GS}$.
    • Pinch-off mechanism and pinch-off voltage $V_P$.
    • Shockley's equation for drain current: $I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_P}\right)^2$.
14. 7 Marks
    bDescribe the principle of operation and characteristics of UJT in details.
    How to answer

    • UJT construction and equivalent circuit (two base resistors $R_{B1}, R_{B2}$ and a diode).
    • Intrinsic standoff ratio $\eta = \frac{R_{B1}}{R_{B1} + R_{B2}}$ and peak voltage $V_P = \eta V_{BB} + V_D$.
    • Working principle and triggering mechanism.
    • Static emitter characteristic curve: cutoff, negative resistance, and saturation regions.
    • Mention common applications (relaxation oscillator, SCR triggering).
15. 14 Marks
    Write short note on: i) Schottky diode ii) E-M model iii) MOSFET iv) Darlington amplifier
    How to answer

    • i) Schottky diode: Metal-semiconductor junction, majority carrier device, fast switching speed, low forward voltage drop.
    • ii) Ebers-Moll (E-M) model: Non-linear large-signal BJT model, coupled diode equations representing forward and reverse active modes.
    • iii) MOSFET: Metal Oxide Semiconductor FET, enhancement and depletion types, operation, high input impedance.
    • iv) Darlington amplifier: Cascaded emitter follower pair configuration, very high input impedance and extremely high current gain ($h_{fe} \approx \beta_1 \beta_2$).