## ELECTRONIC DEVICES AND CIRCUITS JNTU previous years question papers

**Time: 3hours Max.Marks:75**

**Answer any FIVE questions**

**All questions carry equal marks**

1.a) Define the following terms for a PN diode

i) Dynamic resistance ii) Load line

iii) Difference capacitance iv) Reverse saturation current

b) A reverse bias voltage of 90V is applied to a Germanium diode through a resistance R. The reverse saturation current of the diode is 50 μA at an operating temperature of 250C. Compute the diode current and voltage for

i) R = 10 MΩ ii) R = 100 KΩ [15]

2.a) List out the merits and demerits of Bridge type Full Wave rectifiers over centre tapped type Full Wave rectifiers.

b) The secondary voltages of a centre tapped transformer are given as 60V-0V-60V the total resistance of secondary coil and forward diode resistance of each section of transformer secondary is 62 Ω. Compute the following for a load resistance of 1 KΩ.

i) Average load current

ii) Percentage load regulation

iii) Rectification efficiency

iv) Ripple factor for 240 V/50Hz supply to primary of transformer.

c) What is bleeder resistance in L – section filters? [15]

3.a) Describe the functioning of a BJT in common base configuration.

b) Determine the collector current of a BJT with both of its junctions reverse biased. Assume ICO = 5μA, IEO = 3.58 μA, αN = 0.98 and any other parameter values as required.

c) How do you identify the region of operation of a BJT to be saturation region from the values of various circuit currents? [15]

4.a) Describe the significance of operating point, DC and AC load lines to ensure active region operation of a BJT in CE amplifier application.

b) Calculate the Q – point for the DC biased circuit shown below. [15]

5.a) With the help of a hybrid equivalent circuit of a BJT amplifier, derive expressions for voltage gain and current gain when the source and load resistances of finite values are connected.

b) List out the typical values of h – parameters in the three BJT configurations (CE, CB and CC).

c) Describe how hie and hfe can be determine from BJT characteristics. [15]

6.a) Differentiate between enhancement and depletion modes of a MOSFET with the help of its characteristics and construction.

b) Determine the pinch off voltage for an N – channel silicon. JFET if the thickness of its gate region is given as 3.2×10-4 cm and the donor density in n-type region is 1.2 ×105/cm3.

c) Establish a relation between the three JFET parameters, μ, rd and gm. [15]

7.a) Describe how a FET can be used as a voltage variable Resistance (VVR).

b) With the help of circuit diagram and its equivalent circuit of a source follower, derive an expression for the voltage gain possible. [15]

8. Describe the following briefly:

a) Principle of operation of a photodiode.

b) Energy band structure and V – I characteristics of a tunnel diode. [15]