Electronic device and circuits (utu question paper)

ELECTRONIC DEVICE AND CIRCUIT
SEM-III, 2011-12
UTU PREVIOUS YEAR QUESTION PAPERS

time : 3hr] 

Total marks :100

SECTION A

Q1:- Attempt any four of the following :

1. Distinguish between
   (i) intrinsic semiconductor and extrinsic semiconductor
   (ii) energy level and energy band
2. A silicon p-n junction employs the following doping levels : N_A = 1 x 10⁶ cm^-3 and  N_D = 5 x 10¹⁵ cm^-3 . Determine the hole and electron concentration on the two sides.
    [ For silicon, n_i  = 1.08 x 10¹⁰   cm^-3]
3. Due to manufacturing error, the p-side of a pn junction has not been doped. If    N_D = 3 x 10¹⁶ cm^-3, calculate the built in potential at T = 300 K.
4. In the junction shown in fig 1, the depletion region has a width of b on the n side and a on the p side. Sketch the electric field as a function of x.

   

   FIG.1

5. Find the resistivity of intrinsic Si at 300 K.
   
   ( μ_n = 1350 cm² /V –s and μ_p = 480 cm² /v –s ) and (n₁  = 1.5 x 10¹⁰   cm^-3).
6. Discuss the role of Ebers Moll and Hybrid- pi model used for BJT.

SECTION B

Q2:- Attempt any two of the following :

1. Consider the circuit of fig .2 Calculate I_x for V_x = 1V using
   (a) An exponential model with I_s = 10^-16 A
   (b)  A constant-voltage model with   V_D,ON = 800 mV.
   

   

   FIG .2

2. The cross section area of a diode operating in the forward bias region is increased by a factor of 10.
   (i) Determine the change in  I_D if  V_D is maintained constant.
   (ii) Determine the change in   V_D if  I_D is maintained constant I_D ≈ I_s exp (V_D/V_T).
3. In integrated circuits, the follower is typically realized as shown in Fig.3. Determine the voltage gain if the current source is ideal and V_A =∞.

   

   FIG.3

SECTION C

Q3:- Attempt any two of the following :

1. Plot the current flowing R1 and D1 as a function of Vin for the circuits af Fig. 4. Assume constant-voltage diode model.

   

   FIG.4(a)

   

   FIG.4(b)

2. Draw a Darlington Emitter Follower and mention its application. Explain why the input impedance is higher than that of a single stage emitter follower.

Why an amplifier with negative feedback has more bandwidth (3dB frequencies) than the amplifiers without feedback? Let an amplifier without feedback provides an output signal of 15V with 10% second harmonic distortions when the input signal is 15 mV. If 1.5 % of the output is feedback to the input in a negative series-shunt amplifier, what is the output voltage?

SECTION D

Q4:- Attempt any two of the following :

1. In the circuit of Fig.5, it is observe that the collector current of Q₁ and Q₂ are equal if V_BE1 – V_BE2 = 20 mV. Determine the ratio of transistor cross section areas if the other device parameters are identical.  

   

   FIG.5

   
   
   

   

   FIG.5

   
   

Draw the circuit of class A and B amplifier. Explain their working. Also derive their expression for efficiency.

  3.  Discuss the frequency range limitation seen in various sine wave oscillators. Sketch the circuit of Wein-bridge Oscillator. Obtain the condition of oscillations.

SECTION E

Q5:- Attempt any two of the following :

1. Biasing techniques used in MOS circuits.
2. Hartely and Clapp Crystal Oscillators
3. Tunnel diodes and theri applications.

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GOOD LUCK!!!