# RTU Previous Exam Papers BE CS 3rd Semester

# Electronics Devices and Circuits January 2013

**UNIT – I**

(a) Explain the formation of potential barrier in a PN junction diode. Derive an expression for contact potential. Also, describe qualitatively, how the width of this layer changes with the applied voltage ?

(b) Draw the neat circuit of a voltage Doubler ? Explain its operation with its output waveform for the voltage across the two capacitors.

(c) For a particular semiconductor material » = 1.5xlO^{24}/ m n_{v} = 1.3×1(rVffT and E_{G} = 1.43 eV at 300/t.

(i) Determine the intrinsic carrier concentration of the semiconductor at T=300k.

(ii) Determine the effective masses M_{e} and M_{n} of electron and holes respectively.

**OR**

(a) What do you mean by energy density function ? Estimate the position of Fermi level in conductor having N number of free electron per CC. Deduce all steps necessary for deriving the threshold.

(b) Prove that the magnitude of maximum electric field E_{m} at a graded (step) junction with N_{A} » Np is given by E_{m} = where W is the thickness of depletion layer.

**UNIT – II**

2. (a) Explain different components of current flowing through the structure of a NPN transistor. How the emitter’s injection efficiency and base transport factor influences the amplification factor ?

(b) For the circuit shown in fig. 2.1, assuming h_{FE} = 100 and Vfjfr = 0.8F , determine,

(i) if the silicon transistor is in cutoff, saturation or active region.

(ii) Find Vq.

(iii) Find the minimum value for base resistor for which the transistor operates in active region.

2. (a) Show by means of a circuit diagram, how a BJT can be used as a doide. Calculate the maximum and minimum collector current in the circuit as given in fig. 2.2.

(b) What do you understand by Thermal resistance of BJT ? Explain its significance. Toensure thermal stability of a BJT, it. is necessary that Justify the above statement.

**UNIT – III**

(a)Explain the transfer characteristics of a N-channel and P-channel MOSFET along with their output characteristics of N-channel enhancement MOSFET.

(b) Analyze the circuit given in fig. 3.1, to determine the voltage gain.

(c) Calculate Z_{in}, Z_{out} and Ay for the circuit given in fig. 3.2.

**OR**

(a) Explain the meaning of Pinch off voltage in a JFET. How does the current flow in a JFET after Pinch-off point ? Also, explain the terms related to JFET.

(b) In a CE amplifier, shown in fig. 3,3, employing emitter feedback, find :

(i) Ram

(li) K_{ac}

(lii) A,

(iv) and

(v) G_{p}

Take p = 100. How will these values change, if emitter bypass capacitor is removed ?

**UNIT – IV**

4. (a) Discuss the biasing problem in darlington pair. How it is solved ? Explain boots trapping principle and how effectively it is used in darlington pair.

(b) A negative feedback amplifier in voltage series configuration feeds 10% of the output back to the input. Voltage gain of the complifier without feedback is 100. Input and output resistances are 10 M2 and IkQ respectively. Find % reduction in voltage gain, input resistance and output resistance with feedback.

(a) The circuit of a given fig. 4.1, has the following parameters :

R = 40 k£l ^{h}fe = ^{50} , R, = 10k ,

(b) Explain the working of an emitter follower and show how it performs the function of impedance transformation.

**UNIT – V**

5. (a) Write short note on : (!) Hartley oscillator

(ii) Crystal oscillator

(c) A Colpitt’s oscillator is designed with Q = 100 pF and

C_{2} =7500 pF. The inductance is variable. Determine the range of inductance values, if the frequency of oscillation is varied between 950 kHz and 2050 kH_{t}3.

**OR**

(a) Show that the gate width of a collector-coupled monostable multi-vibrator is 0.69 RC.

(b) Calculate the component values of monostable multi-vibrator developing an output pulse of 500 |i.y duration. Assume.