BPHET-143 (DIGITAL AND ANALOG CIRCUITS AND INSTRUMENTATION)

(Valid from 1st January, 2023 to 31 st December, 2023)

PART A

1. a) The intrinsic carrier concentration of a semiconductor is \(2.4 \times 10^{18} \mathrm{~m}^{-3}\). It is doped with a trivalent impurity with dopant atom density of \(3.4 \times 10^{19} \mathrm{~m}^{-3}\). The electron and hole mobilities are \(0.5 \mathrm{~m}^2 \mathrm{~V}^{-1} \mathrm{~s}^{-1}\) and \(0.3 \mathrm{~m}^2 \mathrm{~V}^{-1} \mathrm{~s}^{-1}\) respectively. Calculate its conductivity before and after the doping.

b) Explain the statement “Bipolar Junction Transistor is a current controlled device while Field Effect Transistor is a voltage controlled device”.

2. a) Discuss the three methods of biasing a BJT and compare their relative advantages and disadvantages.

b) Explain the two types of breakdown mechanisms observed in a zener diode.

c) \(h\)-parameters of a single stage CE amplifier are given as \(h_i=2 \mathrm{k} \Omega, h_r=5 \times 10^{-4}\), \(h_f=75\) and \(h_o=20 \mu \mathrm{AV}^{-1}\). Calculate \(A_i, A_v, Z_{\text {in }}\) and \(Z_{\text {out }}\) with \(r_L=5 \mathrm{k} \Omega\) and \(r_S=100 \Omega\).

3. a) Convert 0001010001001001 (BCD) into its binary equivalent and then divide it by \(1001_2\). Express the result in octal equivalent.

b) Draw the circuit to obtain a 2-input NOR gate using diodes, transistor and resistors.

4. a) Simplify the following Boolean expression and draw a logic gate circuit to implement the MSP.
\[
Y=(A+\bar{B})(B+\bar{C}) \overline{(A}+C)
\]

b) With the help of a suitable diagram, explain the subtraction of binary equivalent of decimal number 7 from binary equivalent of decimal number 12 using a 2’s complement binary adder-subtractor.

c) With the help of an appropriate diagram of a 4-bit binary adder explain the addition of 1110 and 0101.

PART B

5. a) Why is a class \(B\) amplifier more efficient than class \(A\) amplifier?

b) What will be the effect on the output of a push-pull amplifier if unmatched transistors are used?

c) An amplifier has maximum voltage gain of 1000 . Express it in dB scale. Calculate the gain at the cut-off frequencies on \(\mathrm{dB}\) scale.

6. a) Why are \(L C\) oscillators preferred over \(R C\) oscillators to generate high frequencies?

b) Why is a 2-stage amplifier required in Wien bridge oscillator in contrast to a single stage amplifier in the phase shift oscillator?

c) Draw the circuit of a Colpitts oscillator. Calculate its frequency of oscillation if value of both the capacitors is \(1 \mathrm{nF}\) and inductor is \(10 \mathrm{mH}\).

7. a) Explain the working of full wave bridge rectifier with the help of an appropriate diagram.

b) Design and draw a circuit of series pass voltage regulator to provide \(5 \mathrm{~V}\) output voltage and \(200 \mathrm{~mA}\) maximum load current using a silicon \(n-p-n\) transistor with \(\beta=100\). Assume the input unregulated voltage to be \(10 \mathrm{~V}\) and minimum zener diode current to be \(8 \mathrm{~mA}\). Specify the voltage and power rating of zener diode as well as value and power rating of the resistor used.

8. a) Compare the characteristics of IC \(741 \mathrm{C}\) with an ideal op amp.

b) Design an op amp comparator to provide \(+V_{S A T}\) output for input signal less than \(+1 \mathrm{~V}\) and \(-V_{S A T}\) output for input signal more than \(+1 \mathrm{~V}\).

c) Draw the output waveform indicating proper scales on the time and voltage axes for the given input waveform to the circuit shown in Fig. 1.

9. a) Draw the schematic diagram of cathode ray tube showing its major components and explain the functions of these components.

b) Design an astable multivibrator using IC 555 timer to generate a rectangular wave of \(75 \%\) duty cycle at \(20 \mathrm{kHz}\) frequency.