EC-304 Electronic Devices - Jun 2023 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 operation of Full Wave Rectifier with necessary graphs.
   How to answer

   • Centre-tapped / bridge full-wave rectifier circuit diagram
   • Conduction in positive and negative half cycles
   • Input vs output waveforms and pulsating DC nature
   • PIV, average and RMS values
2. 7 Marks
   bDerive the expression for transition capacitance of a diode.
   How to answer

   • Depletion width variation with reverse bias
   • Charge stored in depletion region
   • Differentiation $C_T = dQ/dV$
   • Final expression $C_T = C_0/(1+V_R/V_0)^n$
3. 7 Marks
   aDerive the expression for Ripple factor for Full Wave Rectifier with L-section filter.
   How to answer

   • Full wave rectifier with L-section (choke-input) filter circuit
   • Fourier analysis of rectified output
   • Attenuation of AC components by L-C section
   • Ripple factor expression $\Gamma \approx 0.47/(\omega^2LC)$ form
4. 7 Marks
   bExplain the working of p-n diode in forward and reverse bias conditions.
   How to answer

   • PN junction structure and bias connections
   • Forward bias: barrier lowering, diffusion current
   • Reverse bias: barrier increase, drift / saturation current
   • V-I characteristics
5. 7 Marks
   aDerive the operating point using AC and DC load lines.
   How to answer

   • DC load line from DC equivalent circuit
   • AC load line with ac load resistance
   • Intersection gives Q-point / operating point
   • Significance for undistorted amplification
6. 7 Marks
   bExplain about Punch through and Base width modulation.
   How to answer

   • Definition of base width modulation (Early effect)
   • Change in effective base width with collector voltage
   • Punch-through condition when depletion regions meet
   • Effects on transistor characteristics and breakdown
7. 8 Marks
   aDraw and explain the CE characteristics of a transistor.
   How to answer

   • CE input characteristics $I_B$ vs $V_{BE}$
   • CE output characteristics $I_C$ vs $V_{CE}$
   • Active, saturation and cut-off regions
   • Current gain $\beta$ from characteristics
8. 6 Marks
   bThe reverse leakage current of the transistor when in CB configuration is $0.3\, \mu A$ while it is $16\, \mu A$ when the same transistor is connected in CE configuration. Determine $\alpha$, $\beta$ and $\gamma$.
   How to answer

   • Relation between $I_{CBO}$ and $I_{CEO}$
   • Use $I_{CEO} = (1+\beta)I_{CBO}$ to find $\beta$
   • Find $\alpha = \beta/(1+\beta)$
   • Find $\gamma = 1/(1-\alpha)$
9. 7 Marks
   aWith the help of neat diagram explain the voltage divider biasing method for FET.
   How to answer

   • Voltage divider bias circuit for FET/JFET
   • Gate voltage from divider and source self-bias
   • Q-point stability against device variation
   • Design equations for $V_G$, $V_{GS}$, $I_D$
10. 7 Marks
    bExplain the construction and characteristics of Tunnel diode.
    How to answer

    • Heavily doped p-n structure and thin depletion layer
    • Tunneling mechanism and negative resistance
    • V-I characteristics with peak and valley points
    • Applications
11. 7 Marks
    aWhy we call FET as a Voltage Controlled Device?
    How to answer

    • Gate controls depletion width / channel conductivity
    • Almost zero gate current, high input impedance
    • Drain current controlled by $V_{GS}$ not $I_G$
    • Voltage-controlled current source model
12. 7 Marks
    bDraw the circuit diagram of Varactor and explain its operation along with its characteristics.
    How to answer

    • Varactor reverse-biased diode circuit diagram
    • Voltage-dependent depletion capacitance operation
    • Capacitance-voltage characteristics
    • Tuning applications
13. 7 Marks
    aDraw and Explain BJT small signal model, compare the performance of CE, CB, CC amplifier.
    How to answer

    • Hybrid-pi / h-parameter small signal model
    • CE, CB, CC circuit configurations
    • Comparison table of $A_v$, $A_i$, $R_i$, $R_o$
    • Suitability for different applications
14. 7 Marks
    bGiven $I_E = 2.5mA, h_{fc} = 140, h_{oc} = 20\, \mu s$ and $h_{ob} = 0.5\, \mu s$. Determine the common-emitter hybrid equivalent circuit.
    How to answer

    • Convert CB h-parameters to CE hybrid parameters
    • Calculate $h_{ie}$, $h_{fe}$, $h_{oe}$, $h_{re}$
    • Draw CE hybrid equivalent circuit
    • Label component values
15. 7 Marks
    aExplain the working of MOSFET amplifier and discuss the gain and frequency response characteristics.
    How to answer

    • MOSFET amplifier bias and small-signal operation
    • Voltage gain expression
    • Low, mid and high frequency response
    • Cut-off frequencies and bandwidth
16. 7 Marks
    bAn n-channel JFET has $I_{DSS} = 10mA$ and $V_P = -2V$. Determine the drain source resistance $r_{ds}$ for

    1. i)$V_{GS} = 0V$
    2. ii)$V_{GS} = -0.5V$
    How to answer

    • Formula $r_{ds} = \Delta V_{DS}/\Delta I_D$ or $r_d = V_P^2/(2I_{DSS})$ at origin
    • Calculate $I_D$ for given $V_{GS}$
    • Evaluate $r_{ds}$ for $V_{GS}=0V$
    • Evaluate $r_{ds}$ for $V_{GS}=-0.5V$