MSc Degree Examination: Physics (PH 213 – Basic Electronics).pdf

Branch II Physics - PH 213 Basic Electronics Summary

Duration: 3 hours | Maximum Marks: 75

Part A (Answer any 5 questions, each worth 3 marks):

1. Inverting Zero-Crossing Detector: Diagrammed below, this circuit utilizes a comparator and an RC filter to detect the zero-crossing of a sine wave, inverting the output at each transition.

[Diagram of Inverting Zero-Crossing Detector]

2. Varactor Diodes: These are semiconductor diodes whose voltage-current characteristic exhibits a strong dependence on voltage. They act as variable capacitors, with their capacitance varying linearly with applied voltage, making them useful in tuning and signal modulation applications.

3. Ideal Bode Plot Characteristics: An ideal Bode plot shows a -20dB/decade roll-off for both magnitude and phase response beyond the cutoff frequency. It exhibits a flat amplitude response at low frequencies and a sharp phase margin increase as frequency increases.

4. Diode Laser Operation: Diode lasers use semiconductor gain medium (like GaAs or InGaAsP) sandwiched between mirrors to create stimulated emission, resulting in coherent light output at specific wavelengths.

5. Erbium-Doped Fiber Amplifier (EDFA): EDFA plays a crucial role in optical fiber communication by amplifying signals through the stimulated emission of Erbium ions doped into the fiber. It provides high gain, low noise, and long-distance signal transmission capabilities.

Part B (Answer 3 questions, each worth 15 marks):

6. Triangular Wave Generator (10 KHz): A combination of a phase-locked loop (PLL) with a suitable oscillator and frequency divider can generate a triangular wave with the desired frequency.

7. First & Second Order Active Low-Pass Filters:

First Order: Uses an operational amplifier (op-amp) and a resistor-capacitor (RC) network. The RC network acts as a low-pass filter, attenuating higher frequencies while allowing lower frequencies to pass.

Second Order: Combines two first-order filters or uses a single op-amp with multiple feedback paths for increased attenuation and sharper cutoff frequency.

8. Op-Amp Schmitt Trigger: This circuit utilizes an op-amp with hysteresis to trigger when the input voltage exceeds a certain threshold, then switches back when it falls below another threshold. The transfer characteristics show a distinct "hysteresis loop."

Part C (Answer any 3 questions, each worth 5 marks):

9. Signal Distortions in Optical Waveguides: Factors like material dispersion, waveguide curvature, and non-ideal components introduce signal distortions in optical fibers.

10. Avalanche Photodiode (APD) Operation: APD amplifies the photo-generated charge through a process called avalanche multiplication, resulting in high sensitivity even for weak signals.

11. Cathode Ray Tube (CRT) Block Diagram:

Electron Gun: Generates a beam of electrons.
Deflection Yokes: Control the electron beam's path.
Screen: Displayed image is formed on the phosphor-coated screen by the electron beam.

12. Transducers: Convert one form of energy to another.

Active Transducer: Uses an active circuit (e.g., amplifier) for conversion, providing higher sensitivity and output levels.
Passive Transducer: Converts energy without using external power (e.g., resistor as a temperature sensor).

13. 4-Bit Ripple Counter Design: Using JK flip-flops and logic gates, this counter increments by 1 on each rising edge of the clock signal, with carry propagation from bit to bit.

14. BCD-to-Seven Segment Decoder: Converts binary-coded decimal (BCD) inputs into appropriate signals for driving seven-segment displays.

15. Master-Slave JK Flip-Flop: A slave flip-flop receives its clock signal and data from a master flip-flop, enabling asynchronous operation and daisy-chaining.

16. J-K Flip-Flop Conversion from S-R Flip-Flop: A simple logic circuit using AND gates and NOT gates can convert a SR flip-flop to a JK flip-flop by reconfiguring the input signals.

17. Even/Odd Parity Bit Generators: Logic circuits utilizing XOR gates can generate even or odd parity bits based on the number of 1s in the 3-bit binary input.

18. Emission Spectra: The graph shows a broader peak for the white LED, representing its wider wavelength range, compared to the narrower, distinct peak of the 650 nm diode laser.

19. fmax Comparison (4-Bit Counters): The mod-32 ripple counter has a higher fmax due to shorter propagation delays in its logic elements compared to the synchronous J-K counter.

20. Optical Fiber Power Attenuation: Using the given attenuation value and initial power, we can calculate the power level after 4 km using the formula: Power (after distance) = Initial Power exp(-attenuation/km distance).

21. Second Order Low-Pass Filter Design: A suitable RC combination with a cutoff frequency of 1 kHz can be calculated using the RC time constant formula τ = RC. The frequency response curve will show a -20dB/decade roll-off beyond the cutoff.

22. 54/74L91 Register Shift: Logic levels at the input of the register determine whether data shifts in or out. A 1 or 0 shifted into the register will cause it to store or output that bit accordingly, depending on the control signals.