ANALOG & DIGITAL ELECTRONICS Jan 2024

Analog & Digital Electronics

Diploma Electrical Engineering 2nd Year 3rd Semester

Paper Solve

2024

 JANUARY 2024

ANALOG & DIGITAL ELECTRONICS

Time Allowed: 2.5 Hours Full Marks: 60 

Answer to Question No. 1 of Group A must be written in the main answer script. In Question No. 1, out of 2 marks for each MCQ, I mark is allotted for right answer and I mark is allotted for correct explanation of the answer. Answer any Five (05) Questions from Group-B.

GROUP-A

1. Choose the correct answer from the given alternatives and explain your answer (any ten): 2 × 10 = 20

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i] The NOR Gate is OR gate followed by

Options:

(a) AND

(b) NAND gate

(c) NOT gate

(d) none of the above

Answer: (c) NOT gate

Explanation: A NOR gate is the combination of an OR gate followed by a NOT gate. The OR gate provides the sum of the inputs, and the NOT gate inverts this output.

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ii] The universal gate is

Options:

(a) NAND gate

(b) OR gate

(c) AND gate

(d) none of the above

Answer: (a) NAND gate

Explanation: NAND and NOR gates are considered universal gates because they can be used to implement any Boolean function without needing any other type of gate.

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iii] Operating point of transistor represents

Options:

(a) Values of $I_c$ and $V_{CE}$ when signal is applied

(b) The magnitude of the signal

(c) Zero signal values of $I_c$ and $V_{CE}$

(d) none of the above

Answer: (c) Zero signal values of $I_c$ and $V_{CE}$

Explanation: The operating point (or Q-point) of a transistor is defined by the DC values of $I_c$ (collector current) and $V_{CE}$ (collector-emitter voltage) with no AC signal applied.

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iv] In a transistor, $I_c = 100$ mA and $I_e = 100.2$ mA. The value of $\beta$ is

Options:

(a) 500

(b) 100

(c) 200

(d) 0.99

Answer: (b) 100

Explanation: $\beta$ (current gain) is given by:

β=IcIb\beta = \frac{I_c}{I_b}

Where $I_b = I_e - I_c = 100.2 - 100 = 0.2$ mA.

β=1000.2=100\beta = \frac{100}{0.2} = 100

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v] Which type of gate can be used to add two bits

Options:

(a) EX-OR

(b) EX-NOR

(c) AND

(d) NOR

Answer: (a) EX-OR

Explanation: The EX-OR gate is used in binary addition to determine the sum bit. For two input bits, it outputs 1 if the inputs are different.

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vi] The output of sequential circuit depends on

Options:

(a) present input

(b) past output

(c) both present input and past output

(d) past input

Answer: (c) both present input and past output

Explanation: Sequential circuits depend on both the current input and the history of past inputs (stored as the output state), unlike combinational circuits that depend only on current inputs.

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vii] The input impedance of transistor should be

Options:

(a) high

(b) low

(c) medium

(d) none of the above

Answer: (a) high

Explanation: A high input impedance ensures minimal loading on the previous stage of a circuit, improving signal transfer and efficiency.

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viii] A multiplexer is also known as

Options:

(a) a data accumulator

(b) a data restorer

(c) a data selector

(d) a data distributor

Answer: (c) a data selector

Explanation: A multiplexer (MUX) selects one input from several inputs and forwards it to the output based on control signals, hence called a data selector.

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ix] JFET is a device

Options:

(a) voltage controlled

(b) current controlled

(c) both

(d) none of the above

Answer: (a) voltage controlled

Explanation: A Junction Field Effect Transistor (JFET) is a voltage-controlled device because the input voltage at the gate controls the current flowing through the channel.

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x] Which part of the transistor is heavily doped

Options:

(a) Emitter

(b) Base

(c) Collector

(d) All are equally doped

Answer: (a) Emitter

Explanation: The emitter is heavily doped to supply a large number of charge carriers (electrons or holes), which ensures efficient current conduction.

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xi] Ripple factor of full-wave rectifier is

Options:

(a) 1.216

(b) 0.48

(c) 1.414

(d) none of the above

Answer: (b) 0.48

Explanation: The ripple factor is a measure of the AC components in the rectified output. For a full-wave rectifier, the ripple factor is 0.48.

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xii] PIV for a diode in a Bridge Rectifier is

Options:

(a) $V$

(b) $2V$

(c) $V/2$

(d) $V/4$

Answer: (a) $V$

Explanation: In a bridge rectifier, the Peak Inverse Voltage (PIV) for each diode is equal to the maximum input voltage $V$.

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xiii] In which Bias does the Zener diode operate

Options:

(a) Forward

(b) Reverse

(c) Both

(d) None of the above

Answer: (b) Reverse

Explanation: Zener diodes are designed to operate in reverse bias and maintain a constant voltage across the load when the reverse breakdown voltage is reached.

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xiv] For a half-wave or full-wave rectifier, the Peak Inverse Voltage of the rectifier is always

Options:

(a) Greater than the input voltage

(b) Smaller than the input voltage

(c) Equal to the input voltage

(d) none of the above

Answer: (c) Equal to the input voltage

Explanation: For rectifiers, the Peak Inverse Voltage (PIV) is the maximum voltage a diode can withstand in reverse bias, which equals the peak input voltage.

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xv] How is JK flip-flop made to toggle

Options:

(a) $J=0, K=0$

(b) $J=1, K=0$

(c) $J=0, K=1$

(d) $J=1, K=1$

Answer: (d) $J=1, K=1$

Explanation: In a JK flip-flop, when both $J=1$ and $K=1$, the output toggles between high and low with each clock pulse.

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GROUP-B 

2) Describe the theory of positive and negative feedback. Write down the types of feedback amplifier. (3+3+2=8) 

Answer:

Theory of Positive and Negative Feedback

Feedback in electronic circuits involves feeding a portion of the output signal back to the input. Depending on how this feedback is applied, it can either stabilize or destabilize the circuit. There are two types of feedback: positive and negative.

Positive Feedback

1. Amplification: Positive feedback adds the feedback signal to the input signal, enhancing the input signal.

2. Oscillation: If the feedback is too strong, it can lead to oscillations, making the circuit an oscillator.

3. Regeneration: Used in applications like oscillators, where the goal is to generate a continuous waveform.

Negative Feedback

1. Stabilization: Negative feedback subtracts the feedback signal from the input signal, reducing the overall gain.

2. Reduced Distortion: This leads to a more stable and linear operation, minimizing distortion.

3. Improved Bandwidth: It increases the bandwidth of the amplifier.

4. Noise Reduction: Reduces noise and improves the signal-to-noise ratio.

Types of Feedback Amplifiers

1. Voltage Feedback Amplifier (VFA):

   • Input: Voltage

   • Feedback Signal: Voltage

   • Application: Widely used in operational amplifiers (Op-Amps).

2. Current Feedback Amplifier (CFA):

   • Input: Current

   • Feedback Signal: Current

   • Application: High-speed applications requiring fast response times.

3. Series-Shunt Feedback (Voltage Series Feedback):

   • Connection: Series at input and shunt at output.

   • Benefit: Improved input and output impedance.

4. Series-Series Feedback (Current Series Feedback):

   • Connection: Series at both input and output.

   • Benefit: Increases input impedance and decreases output impedance.

5. Shunt-Shunt Feedback (Voltage Shunt Feedback):

   • Connection: Shunt at both input and output.

   • Benefit: Decreases input impedance and output impedance.

6. Shunt-Series Feedback (Current Shunt Feedback):

• Connection: Shunt at input and series at output.

• Benefit: Decreases input impedance and increases output impedance.

3) Write short notes on (a) Colpitts oscillator (b) Wien Bridge oscillator (4+4=8)

Answer:

(a) Colpitts Oscillator:

The Colpitts oscillator is a type of electronic oscillator that generates sine wave oscillations. It is widely used in RF (radio frequency) applications and consists of an active device (like a transistor or an op-amp) and a feedback network made up of inductors and capacitors. The key feature of the Colpitts oscillator is its LC network, which determines the frequency of oscillation.

• Feedback Network: The Colpitts oscillator uses a capacitive divider along with an inductance in the feedback loop. The capacitors (C1 and C2) and the inductor (L) are arranged in a specific configuration that provides the necessary phase shift and feedback for sustained oscillations.

• Frequency of Oscillation: The frequency $f$ of the Colpitts oscillator is given by the formula:

  $$f = \frac{1}{2 \pi \sqrt{L \cdot C_{\text{eq}}}}$$

  where $C_{\text{eq}}$ is the equivalent capacitance of the two capacitors in the feedback network.

• Condition for Oscillation: The oscillator will start oscillating when the phase shift around the loop is 360 degrees (or 0 degrees) and the gain around the loop is equal to or greater than one. This is achieved by designing the active component (transistor or op-amp) to provide the necessary amplification.

• Applications: Colpitts oscillators are used in high-frequency applications such as RF signal generation, radio transmitters, and as local oscillators in communication systems.

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(b) Wien Bridge Oscillator:

The Wien Bridge oscillator is a type of electronic oscillator that generates a sine wave, known for its low distortion and stable frequency. It uses a bridge circuit made up of resistors and capacitors, along with an amplifier, to create oscillations.

• Bridge Circuit: The Wien Bridge consists of a series RC network and a parallel RC network, forming a bridge. The resistor and capacitor values are selected to set the frequency of oscillation. The network ensures that when the phase shift across the bridge is zero, the conditions for sustained oscillations are met.

• Frequency of Oscillation: The frequency $f$ of the Wien Bridge oscillator is given by the formula:

  $$f = \frac{1}{2 \pi R \sqrt{C_1 C_2}}$$

  where $R$ is the resistance in the bridge circuit, and $C_1$ and $C_2$ are the capacitors used in the circuit.

• Condition for Oscillation: Similar to the Colpitts oscillator, the Wien Bridge oscillator also requires a phase shift of 360 degrees (or 0 degrees) around the loop. For the oscillations to be sustained, the gain must be equal to or greater than one. Initially, the gain is set by the amplifier, and the amplitude is stabilized by using automatic gain control (AGC) in some designs.

• Applications: The Wien Bridge oscillator is often used in audio frequency generation, function generators, and testing equipment, due to its ability to produce low-distortion sine waves. It is also used in audio and signal processing applications.

4) What are the features of FET when compared to BJT? Draw the characteristics of n-channel JFET. What is pinch- off voltage? (3+4+1=8)

Answer:

Features of FET Compared to BJT

Field-Effect Transistors (FETs) and Bipolar Junction Transistors (BJTs) are both types of transistors used in electronic circuits, but they have different characteristics and advantages.

Advantages of FETs:

1. High Input Impedance: FETs have very high input impedance, which means they draw very little current from the preceding stage, making them ideal for impedance matching.

2. Low Noise: FETs generate less noise compared to BJTs, making them suitable for low-noise applications like audio amplifiers.

3. Thermal Stability: FETs are more thermally stable than BJTs, meaning their performance is less affected by temperature changes.

4. Voltage-Driven Device: FETs are controlled by voltage, whereas BJTs are current-driven devices. This results in simpler drive circuitry for FETs.

5. Simpler Design: FETs have a simpler structure and require fewer components for biasing and operation compared to BJTs.

6. High Switching Speed: FETs have faster switching times, making them suitable for high-frequency applications.

Advantages of BJTs:

1. High Gain: BJTs typically offer higher gain compared to FETs.

2. Better Performance in Low-Voltage Applications: BJTs can perform better in circuits with low voltage supply.

Characteristics of n-Channel JFET

An n-channel JFET (Junction Field-Effect Transistor) has the following key characteristics:

1. Output Characteristics: The output characteristics show the relationship between the drain current ($I_{D}I\_D$) and the drain-source voltage ($V_{DS}V\_\{DS\}$) for various gate-source voltages ($V_{GS}V\_\{GS\}$).

2. Transfer Characteristics: The transfer characteristics show the relationship between the drain current ($I_{D}I\_D$) and the gate-source voltage ($V_{GS}V\_\{GS\}$) at a constant drain-source voltage ($V_{DS}V\_\{DS\}$).

Output Characteristics of n-Channel JFET

Transfer Characteristics of n-Channel JFET

Pinch-Off Voltage

Pinch-off voltage ($V_{P}V\_P$) is a critical parameter in the operation of a JFET. It is defined as the gate-source voltage ($V_{GS}V\_\{GS\}$) at which the drain current ($I_{D}I\_D$) becomes constant and the channel is fully "pinched off." Beyond this point, increasing $V_{DS}V\_\{DS\}$ does not significantly increase $I_{D}I\_D$, and the transistor enters the saturation region.

• For n-channel JFET: Pinch-off voltage is negative.

• For p-channel JFET: Pinch-off voltage is positive.

5) Write and explain one application of Shift Register with diagram. Draw MOD-5 synchronous counter using J-K Flip-Flop with timing diagram. (4+4=8)

Answer: