EC-304 Electronic Devices - Dec 2024 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
   aDraw and explain the working of varactor diode.
   How to answer

   • Definition and symbol of varactor diode
   • Working principle based on transition capacitance under reverse bias ($C_T \propto 1/V^n$)
   • Depletion layer functioning as variable dielectric/width
   • Schematic diagram and capacitance-voltage characteristic curve
   • Applications (tuning circuits, FM modulators)
2. 7 Marks
   bExplain the energy bands and equilibrium condition is semiconductors.
   How to answer

   • Energy band diagram showing conduction band, valence band, and bandgap ($E_g$)
   • Concept of Fermi level ($E_F$) in intrinsic vs n-type and p-type semiconductors
   • Equilibrium conditions: balance between generation and recombination
   • Mass-action law ($n \cdot p = n_i^2$) and constant Fermi level at thermal equilibrium
3. 7 Marks
   aDiscuss voltage regulator. Explain series voltage regulator circuit in detail.
   How to answer

   • Definition and purpose of voltage regulation (line and load regulation)
   • Circuit diagram of transistor series voltage regulator using a pass transistor and reference Zener diode
   • Working operation for variations in input voltage $V_{in}$ and load current $I_L$
   • Mathematical expressions for output voltage: $V_o = V_Z - V_{BE}$
4. 7 Marks
   bCompare the ideal and practical diode with their advantages and disadvantages.
   How to answer

   • Comparison table of key parameters: forward drop ($0\text{ V}$ vs $0.7\text{ V}$), internal resistance, reverse leakage current
   • V-I characteristic curve for both ideal and practical diodes
   • Advantages and disadvantages of ideal model (simplified analysis vs inaccuracy in real circuits)
   • Practical diode trade-offs (real performance, power dissipation)
5. 7 Marks
   aDescribe PNP transistor in common base configuration.
   How to answer

   • Circuit schematic of PNP transistor in common base (CB) mode with proper biasing ($V_{EB}$, $V_{CB}$)
   • Current relationship: $I_E = I_B + I_C$ and $I_C = \alpha I_E + I_{CBO}$
   • Input characteristics ($I_E$ vs $V_{EB}$ for constant $V_{CB}$)
   • Output characteristics ($I_C$ vs $V_{CB}$ for constant $I_E$) showing active, saturation, and cut-off regions
6. 7 Marks
   bFor a CE configuration based NPN transistor, calculate $\beta$ and $R_B$ if, $V_{CC} = 9\text{V}, V_{BB} = 4\text{V}, I_C = 5\text{ mA}, V_{CE} = 5\text{V}, \beta = 50$ and $V_{BE} = 0.7\text{ V}$.
   How to answer

   • Note that $\beta = 50$ is already given; compute base current: $I_B = I_C / \beta = 5\text{ mA} / 50 = 0.1\text{ mA}$
   • Apply KVL to base-emitter loop: $V_{BB} - I_B R_B - V_{BE} = 0$
   • Calculate $R_B = (V_{BB} - V_{BE}) / I_B = (4 - 0.7) / (0.1\text{ mA}) = 33\text{ k}\Omega$
   • State final calculated values clearly
7. 7 Marks
   aExplain the fixed and self bias circuit of transistor.
   How to answer

   • Fixed bias circuit diagram, KVL equations, and stability limitation ($S = 1 + \beta$)
   • Self bias (voltage divider bias / emitter bias) circuit diagram
   • Analysis of self bias circuit: Thevenin equivalent voltage and resistance
   • Explanation of thermal stabilization mechanism via emitter resistor $R_E$
8. 7 Marks
   bWrite short note on : i) DC and AC load line ii) Transistor as a switch
   How to answer

   • Part (i): Definition and equations of DC load line and AC load line on output characteristics; slope comparison ($-1/R_{dc}$ vs $-1/R_{ac}$); significance of Q-point
   • Part (ii): Transistor switching states (cut-off as OFF switch, saturation as ON switch); switching conditions and practical applications
9. 7 Marks
   aFind the h-parameters for CE configuration from the characteristic curve.
   How to answer

   • Define the 4 h-parameters for CE mode: $h_{ie}$, $h_{re}$, $h_{fe}$, $h_{oe}$
   • Formulate CE hybrid equations: $v_{be} = h_{ie} i_b + h_{re} v_{ce}$ and $i_c = h_{fe} i_b + h_{oe} v_{ce}$
   • Graphical derivation of $h_{ie} = \left.\frac{\Delta V_{BE}}{\Delta I_B}\right|_{V_{CE}}$ and $h_{re} = \left.\frac{\Delta V_{BE}}{\Delta V_{CE}}\right|_{I_B}$ from input curves
   • Graphical derivation of $h_{fe} = \left.\frac{\Delta I_C}{\Delta I_B}\right|_{V_{CE}}$ and $h_{oe} = \left.\frac{\Delta I_C}{\Delta V_{CE}}\right|_{I_B}$ from output curves
10. 7 Marks
    bExplain the class B power amplifier circuit in detail.
    How to answer

    • Definition of Class B operation (conduction angle = $180^\circ$)
    • Circuit schematic of Class B push-pull amplifier (complementary or transformer-coupled)
    • Operation of each transistor during alternating half-cycles
    • Derivation of maximum theoretical collector efficiency $\eta = \frac{\pi}{4} \approx 78.5\%$
    • Discussion of crossover distortion and remedy
11. 7 Marks
    aDescribe the Darlington amplifier circuit with suitable diagram and applications.
    How to answer

    • Darlington pair circuit diagram with two cascaded BJTs
    • Current gain expression: $\beta_{total} \approx \beta_1 \beta_2$
    • Key characteristics: very high input impedance, high current gain, low output impedance
    • Applications: buffer amplifiers, power output stages, motor driver circuits
12. 7 Marks
    bExplain Boot-Strapping technique and current mirror circuit in brief.
    How to answer

    • Bootstrapping concept: positive feedback to dramatically increase effective input resistance in emitter follower circuits
    • Circuit schematic of bootstrapped emitter follower
    • Current mirror principle: matched transistors providing constant mirror current $I_{out} \approx I_{ref}$
    • Circuit schematic of BJT current mirror and expression for reference current
13. 7 Marks
    aExplain Thyristor, its principle of operation and applications in details.
    How to answer

    • Definition, four-layer (PNPN) structure and symbol of SCR/thyristor
    • Two-transistor model (coupled PNP-NPN analogy) and regenerative turn-on mechanism
    • V-I characteristics showing forward blocking, forward conduction, reverse blocking, holding/latching currents
    • Applications: power switching, motor speed control, phase controlled rectifiers
14. 7 Marks
    bDraw the equivalent circuit of a MOSFET operating in enhancement mode.
    How to answer

    • Schematic of small-signal AC equivalent model of enhancement MOSFET
    • Identification of components: $g_m v_{gs}$ voltage-controlled current source, channel resistance $r_o$ (or $r_{ds}$)
    • Gate capacitances $C_{gs}$, $C_{gd}$, and $C_{ds}$ for high-frequency model
    • Brief explanation of parameters and terminal behaviors
15. 14 Marks
    Write short note on: i) Pinch off voltage ii) Tunnel diode iii) Clipper and clamper iv) Class A amplifier
    How to answer

    • i) Pinch-off voltage ($V_P$): definition in JFET, depletion layer pinching channel, saturation current $I_{DSS}$
    • ii) Tunnel diode: highly doped p-n junction, quantum tunneling, negative resistance region, V-I curve
    • iii) Clipper and clamper: waveform shaping; clippers remove portions of signal, clampers shift DC baseline using diode, capacitor, resistor
    • iv) Class A amplifier: transistor conducts for full $360^\circ$ cycle, maximum theoretical efficiency ($25\%$ direct-coupled, $50\%$ transformer-coupled)