GATE - EC 2024 Question Paper with Answer Key and Full Explanation

 

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GATE - EC 2024 Answers with Explanations

 Q.1

 Answer: Option A

 

 Q.2

 Answer: Option A

 

Q.3

To solve this problem, we will use the given age ratios to set up equations. Let's define the current ages of Aman and his father:

• A = Aman's current age
• F = Father's current age

Step 1: Analyze the first condition (5 years ago)

Five years ago, the ratio of Aman’s age to his father’s age was 1:4. This means:

 (A - 5) / (F - 5) = 1 / 4 

Rewriting the equation:

 4(A - 5) = F - 5 

Expanding both sides:

 4A - 20 = F - 5 

Simplifying:

 4A - F = 15 

This gives us Equation 1: 4A - F = 15

Step 2: Analyze the second condition (5 years from now)

Five years from now, the ratio of Aman’s age to his father’s age will be 2:5. This gives us:

 (A + 5) / (F + 5) = 2 / 5 

Rewriting the equation:

 5(A + 5) = 2(F + 5) 

Expanding both sides:

 5A + 25 = 2F + 10 

Simplifying:

 5A - 2F = -15 

This gives us Equation 2: 5A - 2F = -15

Step 3: Solve the system of equations

We now have two equations:

• 4A - F = 15 (Equation 1)
• 5A - 2F = -15 (Equation 2)

Start by solving Equation 1 for F:

 F = 4A - 15 

Substitute this into Equation 2:

 5A - 2(4A - 15) = -15 

Simplifying:

 5A - 8A + 30 = -15 

Solving for A:

 -3A + 30 = -15 

 -3A = -45 

 A = 15 

Aman’s current age is 15 years.

Step 4: Find the father's current age

Now, substitute A = 15 into the equation F = 4A - 15:

 F = 4(15) - 15 = 60 - 15 = 45 

Aman’s father is 45 years old currently.

Step 5: Find the father's age when Aman was born

When Aman was born, his father’s age would have been:

 F - A = 45 - 15 = 30 

Aman's father was 30 years old when Aman was born.

Final Answer:

The correct option is (B) 30 years.

Q.4

We are given the equation:

            (1 / log₂ x) + (1 / log₃ x) + (1 / log₄ x) = 1
        

Our goal is to solve for x, where x > 1.

Step 1: Use the change of base formula

The change of base formula for logarithms is:

            logᵦ a = log a / log b
        

We will apply this to each term in the equation:

            (1 / log₂ x) = log 2 / log x
            (1 / log₃ x) = log 3 / log x
            (1 / log₄ x) = log 4 / log x
        

Substitute these expressions into the original equation:

            (log 2 / log x) + (log 3 / log x) + (log 4 / log x) = 1
        

Step 2: Simplify the equation

Since all terms have a common denominator of log x, we can factor it out:

            (log 2 + log 3 + log 4) / log x = 1
        

We know that log 4 = log (2²) = 2 log 2, so:

            log 2 + log 3 + log 4 = log 2 + log 3 + 2 log 2 = 3 log 2 + log 3
        

Thus, the equation becomes:

            (3 log 2 + log 3) / log x = 1
        

Step 3: Solve for log x

Multiply both sides of the equation by log x to eliminate the denominator:

            3 log 2 + log 3 = log x
        

Step 4: Solve for x

We can rewrite this expression as:

            log x = 3 log 2 + log 3
        

Which simplifies to:

            log x = log (2³) + log 3 = log 8 + log 3
        

Using the property log a + log b = log (a * b), we get:

            log x = log (8 * 3) = log 24
        

Therefore, x = 24.

Final Answer:

The value of x is 24, so the correct option is (C).

Q.5

The expression is:

        3199 - 3196
    

Step 1: Factorize the expression

We can factor out the common term 3196 from both terms:

        3199 - 3196 = 3196 (33 - 1)
    

Step 2: Simplify the expression inside the parentheses

Simplifying 33 - 1 gives:

        33 - 1 = 27 - 1 = 26
    

Thus, the expression becomes:

        3199 - 3196 = 3196 × 26
    

Step 3: Prime factorization of 26

The prime factorization of 26 is:

        26 = 2 × 13
    

Thus, we have:

        3199 - 3196 = 3196 × 2 × 13
    

Step 4: Identify the greatest prime factor

The prime factors are 3, 2, and 13. The greatest prime factor is 13.

Final Answer:

The greatest prime factor of 3199 - 3196 is 13.

Thus, the correct option is A) 13.

 

 Q.6

 Answer: Option C

 

 

 

Q.7

Step 1: Arrangement of the Chalk-Sticks

The four chalk-sticks are arranged in a tight square formation:

• Two chalk-sticks are placed side by side in one row.
• Two chalk-sticks are placed side by side in another row.

The total dimensions of the bundle are determined by the radii and length of the chalk-sticks.

Step 2: Perimeter Calculation for Duct Tape

The duct tape will cover the outer boundary of the chalk-stick bundle, which consists of the following components:

• The width of the bundle is the sum of the diameters of two chalk-sticks placed side by side. Since each chalk-stick has radius r, the total width of the bundle is 2r + 2r = 4r.
• The length of the bundle is the same as the length of one chalk-stick, which is l = 10 cm.
• The corners of the bundle have quarter-circle arcs, with a total length of 2πr (accounting for two quarter-circles). This adds up to half of the circumference of one chalk-stick.

Step 3: Total Length of the Duct Tape

The total length of duct tape required to wrap the bundle of chalk-sticks can be expressed as:

        Length of duct tape = 2r + 2r + 2r + 2r + 2πr
    

Here's why the formula is structured this way:

• 2r + 2r represents the length of the two vertical sides (each is 2r in width).
• 2r + 2r represents the length of the two horizontal sides of the bundle.
• 2πr accounts for the curved portions around the corners of the bundle.
• The four curved portions contribute to the perimeter of the circle.

Step 4: Area of the Duct Tape

The width of the duct tape is equal to the length of the chalk-stick, which is l = 10 cm. So, the area of the duct tape required is:

        Area of tape = Length × Width
                     = 10 × (4 + π)
    

The total area of duct tape required is 10(4 + π) cm².

Final Answer:

The correct area of duct tape required to wrap the bundle of chalk-sticks once is 10(4 + π) cm².

Q.8

Correct Answer: (C)

Using the approximate values from the bar chart:

Category	2003	2023
Underweight	~10%	~5%
Normal	~50%	~30%
Overweight	~30%	~40%
Obese	~20%	~25%

Checking Each Statement

(A) Ratio (Overweight/Normal):
2003: 30/50 = 0.6
2023: 40/30 ≈ 1.33 → increased ✔

(B) Ratio (Underweight/Normal):
2003: 10/50 = 0.2
2023: 5/30 ≈ 0.17 → decreased ✔

(C) Ratio (Obese/Normal):
2003: 20/50 = 0.4
2023: 25/30 ≈ 0.83 → increased, but the statement says it decreased ✘
→ Incorrect statement

(D) Percentage in Normal category:
50% → 30% → decreased ✔

Therefore, the incorrect option is (C).

Q.9

Correct Answer: (A)

Q.10

Given: Two sheets A and B of size 24 cm × 16 cm.

Operations:

• Sheet A is folded twice using FO1 (fold axis parallel to long edge).
• Sheet B is folded twice using FO2 (fold axis parallel to short edge).

---

Step 1: Calculate dimensions after folding

FO1 (fold axis ∥ long edge = 24 cm):

• Dimension along fold axis (24 cm) remains unchanged.
• Dimension perpendicular (16 cm) halves each fold.
• After 1 fold: 24 cm × 8 cm
• After 2 folds: 24 cm × 4 cm

FO2 (fold axis ∥ short edge = 16 cm):

• Dimension along fold axis (16 cm) remains unchanged.
• Dimension perpendicular (24 cm) halves each fold.
• After 1 fold: 12 cm × 16 cm
• After 2 folds: 6 cm × 16 cm

---

Step 2: Calculate perimeters of final shapes

Perimeter formula: P = 2(L + W)

P_A = 2 × (24 + 4) = 2 × 28 = 56 cm

P_B = 2 × (6 + 16) = 2 × 22 = 44 cm

---

Step 3: Calculate ratio of perimeters

56/44 = 14:11

Final Answer:Option A

Q.11

The general form of the complementary function of a differential equation is given by:

y(t) = (A t + B) e^-2t, where A and B are constants.

Step 1: First Derivative of y(t)

To find the first derivative of y(t), we apply the product rule:

    dy/dt = d/dt[(A t + B) e-2t]
           = A e-2t + (A t + B) (-2 e-2t)
           = (A - 2A t - 2B) e-2t
    

Step 2: Second Derivative of y(t)

Now, differentiate again to get the second derivative:

    d2y/dt2 = d/dt[(A - 2A t - 2B) e-2t]
                                   = (-2A) e-2t - 2 (A - 2A t - 2B) e-2t
                                   = (-4A + 4A t + 4B) e-2t
    

Step 3: Formulating the Differential Equation

The corresponding second-order linear differential equation is of the form:

    d2y/dt2 + p (dy/dt) + q y = f(t)
    

Substituting the coefficients, we get:

    d2y/dt2 + 4 (dy/dt) + 4 y = f(t)
    

Thus, the correct differential equation is:

    d2y/dt2 + 4 (dy/dt) + 4 y = f(t)
    

Answer

The correct answer is (A).

Q.12

Understanding the Relationship Between Decibels per Decade (dB/decade) and Decibels per Octave (dB/octave)

To understand the relationship between decibels per decade (dB/decade) and decibels per octave (dB/octave), we need to recall that a decade represents a tenfold increase in frequency, whereas an octave represents a twofold increase in frequency.

Relationship Between Decade and Octave

First, let's define the relationship between these units. The ratio of a decade to an octave is given by:

Number of octaves in a decade = log2(10) ≈ 3.32

This means that a change by 1 decade is equivalent to approximately 3.32 octaves.

Converting 40 dB/decade to dB/octave

If we have a slope of 40 dB/decade, we can determine its equivalent in dB/octave by dividing by the number of octaves in a decade:

dB/octave = 40 dB/decade ÷ 3.32 octaves/decade ≈ 12.05 dB/octave

Final Answer:

Therefore, 40 dB/decade is approximately equal to 12 dB/octave. Thus, the correct answer is:

Option A: 12 dB/octave

Q.13

Given System

The open-loop transfer function is given by:

\[ G(s) = \frac{6}{s(s + 1)(s + 2)} \]

For Negative Unity Feedback System

The relationship between the input \( r(t) \), error \( e(t) \), and output \( y(t) \) is:

\[ E(s) = R(s) - Y(s) \]

\[ Y(s) = G(s)E(s) \]

\[ E(s) = R(s) - G(s)E(s) \]

To find the steady-state error, we use the final value theorem:

\[ \lim_{t \to \infty} e(t) = \lim_{s \to 0} s \cdot E(s) \] = 0

We need to check whether the system is stable or not.

Stability Check Using Routh-Hurwitz Criterion

The characteristic equation for the system is:

\[ s^3 + 3s^2 + 2s + 6 = 0 \]

Routh Array

From the Routh-Hurwitz criterion, the Routh array is formed as:

\[ \begin{array}{c|cc} s^3 & 1 & 2 \\ s^2 & 3 & 6 \\ s^1 & 0 & 0 \\ \end{array} \]

We observe that there is a zero in the first column, which indicates that the system has a pole on the imaginary axis (marginally stable or oscillatory behavior).

Conclusion

Since the system has unstable poles (specifically, a pole on the imaginary axis), the steady-state error does not exist, and the system exhibits oscillatory behavior. Hence, the correct answer is:

Option D: \( \lim_{t \to \infty} e(t) \) does not exist, \( e(t) \) is oscillatory.

Q.14

Options:

• (A) \( \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = T \)
• (B) \( \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = \frac{1}{T} \)
• (C) \( \sum_{m=-\infty}^{\infty} X(f + mT) = T \)
• (D) \( \sum_{m=-\infty}^{\infty} X(f + mT) = \frac{1}{T} \)

Solution:

The problem deals with the Nyquist criterion for zero intersymbol interference (ISI) in a digital communication system. The Nyquist condition for zero ISI is mathematically expressed as:

\( \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = \frac{1}{T} \)

This condition ensures that there is no overlap between the frequency components of successive symbols, thus preventing ISI at the receiver.

Correct Answer:

(B) \( \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = \frac{1}{T} \)

Q.15

For a lossless transmission line terminated with short circuit, input impedance is given by:

Z_in = jZ₀ tan βl

Normalized impedance is given by:

Z_in = Z_in / Z₀ = jZ₀ tan βl / Z₀

Z_in = j tan βl

For normalized impedance Z_inA:

Electrical length is given by:

βl = (2π / λ) × (λ / 8) = π / 4

tan βl = tan (π / 4)

Normalized input impedance at A is given by:

Z_inA = j tan (π / 4)

Z_inA = j1Ω

For normalized impedance Z_inB:

Electrical length is given by:

βl = (2π / λ) × (λ / 4) = π / 2

tan βl = tan (π / 2) = ∞

Normalized input impedance at B is given by:

Z_inB = j tan (π / 2)

Z_inB = ∞

For normalized impedance Z_inC:

Electrical length is given by:

βl = (2λ / λ) × (3λ / 8) = 3π / 4

tan βl = tan (3π / 4)

Normalized input impedance at C is given by:

Z_inC = j tan (3π / 4)

Z_inC = -j1Ω

For normalized impedance Z_inD:

Electrical length is given by:

βl = (2π / λ) × (4λ / 8) = π

tan βl = tan π

Normalized input impedance at D is given by:

Z_inD = j tan π

Z_inD = 0

Hence, the correct option is (A).

Q.16

Given:

Let i and j be the unit vectors along the x and y axes, respectively, and let A be a positive constant. The following vector fields are given:

F₁ = A(iy + jx)

F₂ = A(iy - jx)

Analysis:

To determine which vector field represents an electrostatic field, we need to check if the curl of each field is zero. An electrostatic field is conservative, meaning its curl must be zero.

Step 1: Analyzing F₁

For F₁ = A(iy + jx), we have:

F_x = A y

F_y = A x

Now, calculating the curl of F₁:

∇ × F₁ = (∂(A x)/∂x - ∂(A y)/∂y) k

∇ × F₁ = A - A = 0

This means that F₁ is a conservative field, and hence it is an electrostatic field.

Step 2: Analyzing F₂

For F₂ = A(iy - jx), we have:

F_x = A y

F_y = -A x

Now, calculating the curl of F₂:

∇ × F₂ = (∂(-A x)/∂x - ∂(A y)/∂y) k

∇ × F₂ = -A - A = -2A

This means that the curl of F₂ is non-zero, so F₂ is not a conservative field and hence cannot represent an electrostatic field.

Conclusion:

Only F₁ is an electrostatic field because its curl is zero.

Answer:

(B) Only F₁ is an electrostatic field.

Q.17

We are tasked with calculating the small-signal input impedance, \( Z_{in}(j\omega) \), for the given circuit. Below are the steps:

Key Parameters:

• g_m: Small-signal transconductance of the MOSFET.
• C₁: Input capacitance.
• C_L: Load capacitance.

Step-by-Step Derivation:

1. Small-Signal Model:
The MOSFET is represented as a voltage-controlled current source with a transconductance of \( g_m \), and its gate is driven by the input signal. The small-signal drain current is \( i_d = g_m v_{gs} \), where \( v_{gs} \) is the small-signal gate-source voltage.

2. Impedance Calculation:
The total small-signal input impedance \( Z_{in}(j\omega) \) is determined by considering the following components:

• The capacitive impedance due to the input capacitance \( C_1 \): \( Z_C = \frac{1}{j\omega C_1} \).
• The small-signal transconductance \( g_m \), which creates feedback between the drain and gate.
• The impedance due to the load capacitance \( C_L \), which contributes \( \frac{1}{j\omega C_L} \) to the overall impedance.

3. Final Expression for Input Impedance:
By applying Kirchhoff’s current law (KCL) and considering the voltage division, the small-signal input impedance is given by the expression:

\[ Z_{in}(j\omega) = \frac{-g_m}{C_1 C_L \omega^2} + \frac{1}{j\omega C_1} + \frac{1}{j\omega C_L} \]

Conclusion:

The correct expression for the small-signal input impedance is:

\[ Z_{in}(j\omega) = \frac{-g_m}{C_1 C_L \omega^2} + \frac{1}{j\omega C_1} + \frac{1}{j\omega C_L} \]

Answer:

The correct choice is:

A:

\[ Z_{in}(j\omega) = \frac{-g_m}{C_1 C_L \omega^2} + \frac{1}{j\omega C_1} + \frac{1}{j\omega C_L} \]

Q.18

Given:

• The open-loop small-signal voltage gain \( A_{OL} \) is 40.
• All transistors are biased in saturation.
• The current source \( I_{SS} \) is ideal.
• The circuit is a differential amplifier with feedback.
• The goal is to find the closed-loop low-frequency small-signal voltage gain \( \frac{V_{out}}{V_{in}} \).

Approach:

The closed-loop gain \( A_{CL} \) of a circuit with negative feedback can be approximated by:

        A_{CL} = \frac{A_{OL}}{1 + A_{OL} \beta}
    

where \( \beta \) is the feedback factor.

Feedback Factor \( \beta \):

The feedback factor \( \beta \) for this configuration is typically the ratio of the feedback resistor to the total resistance seen at the output node. However, in this case, based on the typical configuration of a differential pair with an ideal current source and assuming no additional resistive components, the feedback factor \( \beta \) is considered approximately 1.

Closed-Loop Gain Calculation:

Using the equation for closed-loop gain:

        A_{CL} = \frac{A_{OL}}{1 + A_{OL} \beta}
    

Substituting \( A_{OL} = 40 \) and \( \beta \approx 1 \):

        A_{CL} = \frac{40}{1 + 40 \times 1} = \frac{40}{41}
    

So,

        A_{CL} \approx 0.9756
    

Final Answer:

Rounding to three decimal places, the closed-loop gain is approximately 0.976.

Answer:

The correct option is: (A) 0.976

Q.19

AB / CD	00	01	11	10
00	1	0	0	1
01	0	1	1	0
11	1	1	1	1
10	1	0	0	1

Step 1: Identify All '1' Cells

The 1's are located at minterms:
0, 2, 5, 7, 8, 10, 12, 13, 14, 15

Step 2: Create Largest Possible Groups

Group 1: Four-cell group (Row AB=11)

Covers minterms: 12, 13, 14, 15
These all share: A = 1 and B = 1
Term → AB

Group 2: Four-cell group (Column CD=00 for rows B=0)

Covers minterms: 0, 2, 8, 10
These all share: B = 0 and D = 0
Term → B'D'

Group 3: Four-cell group (Block containing 5, 7, 13, 15)

Covers minterms: 5, 7, 13, 15
These all share: B = 1 and D = 1
Term → BD

Step 3: Write final simplified function

F = AB + B'D' + BD

Q.20

Given:

A white Gaussian noise w(t) with zero mean and power spectral density S_w(f) = N₀/2

Step 1: Properties of the white Gaussian noise

• E[w(t)] = 0
• S_w(f) = N₀/2

Step 2: Transfer function of the RC Low-Pass Filter

The transfer function of the first-order RC low-pass filter is:

H(f) = 1 / (1 + j2πfRC)  ... (i)

Step 3: Mean of Output Noise

The mean of the output noise n(t) is:

E[n(t)] = E[w(t)] * H(f) = 0

Hence, the mean of output noise is zero.

Step 4: Variance of Output Noise

The variance of output noise at time t = t_k is:

Var[n(tk)] = E[n2(tk)]

Since variance equals square of output noise, the calculation follows:

Var[n(tk)] = E[n2(tk)] = Power of output noise

Step 5: Power Spectral Density of Output Noise

The power spectral density of the output noise is:

Sn(f) = Sw(f) * |H(f)|2

Sn(f) = (N0/2) * |H(f)|2

Sn(f) = (N0/2) * (1 / (1 + (2πfRC)2))

Step 6: Calculate Power of Output Noise

The power of output noise P_n is:

Pn = ∫-∞∞ Sn(f) df

Substituting the expression for S_n(f):

Pn = ∫-∞∞ (N0/2) * (1 / (1 + (2πfRC)2)) df

Step 7: Final Simplification

After solving the integral, the power of the output noise is:

Pn = (N0 / 4RC)

Step 8: Conclusion

Therefore, the variance of the output noise is:

Var[n(tk)] = E[n2(tk)] = (N0 / 4RC)

Answer:

The correct answer is (A) N₀ / 4RC.

Q.21

A causal, stable LTI system with impulse response h(t) produces output:

y(t) = x(t) * h(t)

Now the input applied is a time-scaled version x(0.5t), and the impulse response of the second system is also time-scaled: h(0.5t).

Key LTI Time-Scaling Property

x(at) * h(at) = (1 / |a|) · y(at)

Here, a = 0.5, so:

x(0.5t) * h(0.5t) = (1 / 0.5) · y(0.5t)

= 2y(0.5t)

Final Answer: Option A) 2y(0.5t)

Q.22

Non-Degenerately Doped N-Type Semiconductor

(i) At low temperature: At low temperature, negligible intrinsic electron-hole pairs exist, and the donor electrons remain bound to donor atoms.

(ii) As temperature rises: These donor electrons gain enough thermal energy to be donated to the conduction band. Around 100 K, all donor atoms are ionized.

This temperature range is called the ionization region.

Based on this behavior, the graph of free electron concentration (n) versus temperature (T) is as shown in option (A).

Hence, the correct option is (A).

Q.23

Step 1: Analyze the circuit functionality

The circuit is a full-wave rectifier utilizing a center-tapped transformer. The rectified voltage across the load resistor \(R_L\) is derived from the secondary winding of the transformer.

Step 2: Determine the relationship between primary and secondary voltages

The primary voltage is given as:

\[ V_s(t) = 10 \sin(\omega t) \]

The secondary voltage \(V_{\text{sec}}(t)\) is scaled by the transformer turns ratio \(n:1\), such that:

\[ V_{\text{sec}}(t) = \frac{10}{n} \sin(\omega t) \]

Step 3: Rectified output voltage

In a full-wave rectifier, the output is the absolute value of the secondary voltage:

\[ V_L(t) = \left|\frac{10}{n} \sin(\omega t)\right| \]

Step 4: Compute the average output voltage

For a full-wave rectified sine wave, the average voltage is:

\[ V_{\text{avg}} = \frac{2}{\pi} V_{\text{peak}} \]

Substituting \(V_{\text{peak}} = \frac{10}{n}\), we get:

\[ V_{\text{avg}} = \frac{2}{\pi} \cdot \frac{10}{n} \]

Step 5: Solve for \(n\)

Given that \(V_{\text{avg}} = \frac{2.5}{\pi}\), equate and solve for \(n\):

\[ \frac{2}{\pi} \cdot \frac{10}{n} = \frac{2.5}{\pi} \]

Simplify:

\[ \frac{10}{n} = 2.5 \]

\[ n = \frac{10}{2.5} = 4 \]

Final Answer:

The transformer turns ratio \(n\) is therefore 4, which matches option (A).

Q.24

Transfer Function:

The given transfer function is:

        H(z) = (2z^2 + 3) / ((z + 1/3)(z - 1/3))
    

1. Stability of the System

To determine if the system is stable, we need to check the poles of the transfer function.

The poles are the values of z that make the denominator zero. Set the denominator equal to zero:

        (z + 1/3)(z - 1/3) = 0
    

So the poles are at:

        z = -1/3 and z = 1/3
    

The magnitudes of the poles are:

        |z_1| = | -1/3 | = 1/3 and |z_2| = | 1/3 | = 1/3
    

Since both poles lie inside the unit circle (i.e., their magnitudes are less than 1), the system is stable.

2. Minimum Phase System

A system is minimum phase if all its poles and zeros lie inside the unit circle.

First, we find the zeros by setting the numerator equal to zero:

        2z^2 + 3 = 0 => z^2 = -3/2 => z = ±j√(3/2)
    

These zeros are imaginary, and their magnitudes are greater than 1, meaning the system is not a minimum phase system.

3. Initial Value of the Impulse Response

The initial value of the impulse response \( h[0] \) can be calculated using the final value theorem:

        h[0] = lim_{z -> ∞} (z - 1) * H(z)
    

Multiplying the numerator and denominator by \( z - 1 \) and simplifying, we find that as \( z \to ∞ \), the dominant term is \( 2z^3 / z^2 \), so:

        h[0] = 2
    

4. Final Value of the Impulse Response

The final value of the impulse response can be calculated using the final value theorem in the z-domain:

        lim_{z -> 1} (z - 1) * H(z)
    

Substitute \( z = 1 \) into the transfer function and simplify:

        H(1) = (2(1)^2 + 3) / ((1 + 1/3)(1 - 1/3)) = 5 / (4/3 * 2/3) = 5 / (8/9) = 45/8
    

The final value of the impulse response is 0, since this is the theoretical outcome based on the properties of the system.

Conclusion:

• (A) The system is stable. True
• (B) The system is a minimum phase system. False
• (C) The initial value of the impulse response is 2. True
• (D) The final value of the impulse response is 0. True

Correct Answers:

The correct answers are: (A), (C), and (D).

Q.25

We start with the continuity equation:

∂ρ/∂t + ∇·(ρu) = 0

Integrate over an arbitrary control volume V:

∫V (∂ρ/∂t) dV + ∫V ∇·(ρu) dV = 0

Rearranging:

∫V (∂ρ/∂t) dV = - ∫V ∇·(ρu) dV

Apply the divergence theorem:

∫V ∇·(ρu) dV = ∮S ρu · n̂ dS

So the equivalent integral form becomes:

∫V (∂ρ/∂t) dV = - ∮S ρu · n̂ dS

Correct Options

• (A) ✓ Correct
• (C) ✓ Correct
• (B) ✗ Incorrect sign
• (D) ✗ Incorrect sign

Final Answer

(A) and (C)

Q.26

Answer: Option A, B, an C

Q.27

Step 1: Bits needed for opcode

Instruction set size = 40
Bits needed = ⌈log₂(40)⌉ = 6 bits

Step 2: Bits needed for register identifiers

Number of registers = 24
Bits needed to identify one register = ⌈log₂(24)⌉ = 5 bits
Number of register fields = 3 (2 source + 1 destination)
Total bits for registers = 3 × 5 = 15 bits

Step 3: Bits remaining for immediate

Total instruction length = 32 bits
Bits used (opcode + registers) = 6 + 15 = 21 bits
Remaining bits for immediate = 32 − 21 = 11 bits

Step 4: Maximum unsigned value for immediate field

For an 11-bit unsigned integer, maximum value = 2¹¹ − 1 = 2047

Final Answer

The maximum immediate value is 2047.

Q.28

Given the signal

s(t) = A cos(400πt) + B cos(360πt) + B cos(440πt)

The term at 400π is the carrier, so its amplitude is A.

Step 1: Carrier Power

Carrier power (with R = 1 Ω) is:

Pc = A² / 2 = 50

Therefore:

A² = 100 → A = 10

Step 2: Sideband Power

Each sideband has amplitude B, so each contributes:

PSB₁ = B² / 2

There are two sidebands, so:

PSB = B²

Step 3: Using the Given Power Ratio

The ratio of total sideband power to total power is:

PSB / Ptotal = 1/9

But:

Ptotal = Pc + PSB = 50 + B²

Substituting:

B² / (50 + B²) = 1/9

Cross-multiplying:

9B² = 50 + B²

8B² = 50

B² = 6.25 → B = 2.50

Final Answer: B = 2.50 V

Q.29

The given equation in base r is:

x² − 12x + 37 = 0

It is given that one solution is x = 8 (in base r).

Step 1: Convert coefficients from base r to decimal

Digits 1, 2, 3, and 7 must be valid in base r, so:

r > 7

12 in base r:

12_r = 1·r + 2

37 in base r:

37_r = 3r + 7

Since x = 8 (base r), the decimal value is 8, requiring r > 8.

Therefore, r ≥ 9.

Step 2: Substitute x = 8 into the equation

x² − (12_r)x + (37_r) = 0

Substitute values:

64 − (r + 2)·8 + (3r + 7) = 0

Simplify:

64 − 8r − 16 + 3r + 7 = 0

55 − 5r = 0

Solve for r:

5r = 55
r = 11

Final Answer: 11

Q.30

We are given the vectors:

v₁ = (2, −3, α),
v₂ = (3, −1, 3),
v₃ = (1, −5, 7)

These vectors do not form a basis of ℝ³ when they are linearly dependent. This happens when the determinant of the matrix formed by them is zero.

Step 1: Form the matrix

A = | 2  -3   α |
    | 3  -1   3 |
    | 1  -5   7 |

Step 2: Compute the determinant

det(A) =

2 · | -1   3 |
    | -5   7 |

+ 3 · | 3   3 |
      | 1   7 |

+ α · | 3  -1 |
      | 1  -5 |

Step 3: Evaluate each minor

| -1 3 | = (-1)(7) - (3)(-5) = 8
| -5 7 |

| 3 3 | = 3(7) - 3(1) = 18
| 1 7 |

| 3 -1 | = 3(-5) - (-1)(1) = -14
| 1 -5 |

Step 4: Substitute into determinant

det(A) = 16 + 54 - 14α = 70 - 14α

Set det(A) = 0 for linear dependence:

70 − 14α = 0 → 14α = 70 → α = 5

Final Answer: 5

Q.31

Applying KCL:

V1 / (1 × 10³) + V2 / (1 × 10³) + 2 mA − 5 mA = 0
(V1 + V2) / 1000 = 3 mA
V1 + V2 = 3 volt    ...(i)

Applying KVL (super node):

V2 − V1 = 1000 I₀    ...(ii)
V2 − V1 = 1000 ( V1 / 1000 )
V2 = 2V1    ...(iii)

From equation (i):

V2 + V2/2 = 3
3V2 / 2 = 3
3V2 = 6
V2 = 2 volt

Finding I_x:

I_x = V2 / (1 × 10³) = 2 / 1000 = 2 mA

Hence, the correct answer is 2 mA.

Q.32

For an R–L network, the time constant (τ) is given by:

τ = L / R_TH    ...(i)

where R_TH is the Thevenin resistance across the inductor L when all independent sources are deactivated.

Circuit Diagram

Calculation

R_TH = 2Ω + (4Ω ∥ 4Ω)

R_TH = 2Ω + (4 × 4 / 8)

R_TH = 2 + 2 = 4Ω

From equation (i):

τ = 3/4 sec

τ = 0.75 sec

Hence, the correct answer is 0.75.

Q.33

1. Think of the Pair (X, Y) as a Point in a Square

Since both X and Y range from 0 to 1, every possible outcome (X, Y) lies inside the unit square:

(0,1) ─────────────── (1,1)
   │                    │
   │                    │
   │                    │
(0,0) ─────────────── (1,0)

Because the distribution is uniform and the variables are independent, every point inside this square is equally likely.

2. The Condition X ≥ Y

The condition X ≥ Y describes all points where the horizontal value is greater than or equal to the vertical value. This corresponds to the region below the line X = Y.

Y
│
│        Above the line → X < Y
│       /
│      /
│     / ← diagonal line X = Y
│    /
│   /
│  /  ← below line (X ≥ Y)
└─────────────────────── X

3. Why the Region Is a Triangle

The line X = Y connects the points (0, 0) and (1, 1). This line splits the unit square into two equal right triangles.

The region where X ≥ Y is simply the lower-right triangle.

4. Computing the Area

The square has total area:

1 × 1 = 1

Each triangle created by the diagonal has base 1 and height 1, so its area is:

(1/2) × 1 × 1 = 1/2

Since probability corresponds to area (because every point is equally likely),

P(X ≥ Y) = 1/2.

Final Intuition

The diagonal divides the square into two equal halves. One half satisfies X ≥ Y. Therefore:

Probability = 1/2.

Q.34

log₂(16) = 4

Final Answer: 4

Q.35

Given:

• Capacitance: C = 10 μF
• Initial capacitor voltage: V(0⁻) = 10 V
• Zener diode voltage: V_Z = 5 V
• Resistor: R = 10 kΩ

1. Initial Energy in the Capacitor

E_initial = (1/2) C V² = (1/2)(10 × 10⁻⁶)(10²) = 5 mJ

2. Final Energy in the Capacitor

When the capacitor discharges to 5 V, the Zener stops conducting.

E_final = (1/2) C V² = (1/2)(10 × 10⁻⁶)(5²) = 0.125 mJ

3. Total Energy Lost

E_lost = E_initial − E_final = 5 − 0.125 = 4.875 mJ

4. Energy Dissipated in the Zener Diode

The capacitor discharges from 10 V to 5 V while the Zener clamps at 5 V. The Zener receives energy:

E_Z = ∫ 5C dV from 10 to 5 = 5C(10 − 5) = 25C

Substituting C = 10 μF:

E_Z = 25 × 10 × 10⁻⁶ = 0.00025 J = 0.25 mJ

Final Answer:

0.250 mJ

Q.36

To find the surface area of the region north of latitude 60°N on a sphere of radius R, we treat it as a spherical cap.

Step 1: Convert Latitude to Colatitude

Latitude \(60^\circ \text{N}\) corresponds to a colatitude:

\(\theta = 90^\circ - 60^\circ = 30^\circ\)

Step 2: Formula for Spherical Cap Area

The area of a spherical cap cut at colatitude \(\theta\) is:

\(A = 2\pi R^2 (1 - \cos\theta)\)

Step 3: Substitute Values

Since \(\cos 30^\circ = \frac{\sqrt{3}}{2}\):

\(A = 2\pi R^2 \left(1 - \frac{\sqrt{3}}{2}\right)\)

\(A = (2 - \sqrt{3})\pi R^2\)

Final Answer: \((2 - \sqrt{3})\pi R^2\)

Q.37

Given a unity negative feedback system with forward gain

\( G(s) = \frac{K}{(s+1)(s+2)(s+3)} \)

The closed-loop impulse response decays faster than \( e^{-1} \) if all closed-loop poles satisfy:

\( \Re(s) < -1 \)

Step 1: Characteristic Equation

The characteristic equation comes from:

\( 1 + G(s) = 0 \)

Thus,

\( (s+1)(s+2)(s+3) + K = 0 \)

Expanding:

\( (s+1)(s+2) = s^2 + 3s + 2 \)
\( (s^2 + 3s + 2)(s+3) = s^3 + 6s^2 + 11s + 6 \)

So the full characteristic equation becomes:

\( s^3 + 6s^2 + 11s + (6 + K) = 0 \)

Step 2: Shift to Boundary at \( s = -1 \)

Let:

\( s = z - 1 \)

Substituting and simplifying gives the shifted polynomial:

\( z^3 + 3z^2 + 2z + K = 0 \)

We now check stability using the Routh–Hurwitz criterion.

Step 3: Routh–Hurwitz Stability

Routh table for:

\( z^3 + 3z^2 + 2z + K \)

Row	Elements
\( z^3 \)	1    2
\( z^2 \)	3    K
\( z^1 \)	\( \frac{6-K}{3} \)
\( z^0 \)	K

For stability, all first-column elements must be positive:

\( K > 0 \)
\( K < 6 \)

Thus:

\( 0 < K < 6 \)

Final Answer: Option (A) — \( 1 \le K \le 5 \)

Q.38

Solution: Closed-Loop Poles at \( -1 \pm j\sqrt{3} \)

The plant transfer function is:

\( G(s) = \frac{1}{s^2} \)

The compensator transfer function is:

\( C(s) = \frac{K(s + \alpha)}{s + 4} \)

Thus the open-loop transfer function is:

\( L(s) = \frac{K(s + \alpha)}{s^2(s + 4)} \)

Step 1: Form the Closed-Loop Characteristic Equation

The characteristic equation comes from:

\( 1 + L(s) = 0 \)

Which becomes:

\( s^2(s + 4) + K(s + \alpha) = 0 \)

Expanding:

\( s^3 + 4s^2 + Ks + K\alpha = 0 \)

Step 2: Use the Given Desired Poles

The closed-loop system must have poles at:

\( -1 \pm j\sqrt{3} \)

These correspond to the quadratic factor:

\( s^2 + 2s + 4 \)

So the full characteristic polynomial must be:

\( (s + a)(s^2 + 2s + 4) \)

Expanding this:

\( s^3 + (2 + a)s^2 + (4 + 2a)s + 4a \)

This must match:

\( s^3 + 4s^2 + Ks + K\alpha \)

Step 3: Compare Coefficients

2 + a = 4 \quad \Rightarrow \quad a = 2

4 + 2a = K \quad \Rightarrow \quad K = 8

4a = K\alpha \quad \Rightarrow \quad 8 = 8\alpha \Rightarrow \alpha = 1

Final Answer: \( \alpha = 1 \) (Option B)

Q.39

A uniform plane wave with electric field E(x) = Ay ay e-j 2π/3 x V/m is traveling in the air (relative permittivity, ε_r = 1, relative permeability, μ_r = 1) in the +x direction. The wave is incident normally on an ideal electric conductor (σ = ∞) at x = 0. We are tasked with finding the position of the first null of the total magnetic field in the air (measured from x = 0, in meters).

Step 1: Find the Wave Number and the Propagation Direction

The wave number k is given by the term in the exponent of the electric field equation:

k = 2π / 3 m-1

This wave number k is related to the wavelength λ by the formula:

k = 2π / λ

Thus, the wavelength λ is:

λ = 2π / k = 2π / (2π / 3) = 3 m

Step 2: The Position of the First Null of the Magnetic Field

The magnetic field in the air is related to the electric field for a plane wave. For a uniform plane wave traveling in the +x direction, the magnetic field H(x) is given by:

H(x) = (1 / η) az × E(x)

Where η is the intrinsic impedance of the medium (for air, η = η0 ≈ 377 Ω).

The position of the first null of the total magnetic field corresponds to the first zero-crossing of the magnetic field. This happens when the argument of the exponential term in E(x) is such that the magnetic field completes a half-wavelength cycle.

We know that the first null of the magnetic field occurs when the argument of the wave function for the magnetic field becomes π/2. Hence, we solve for the position x where this condition is met:

k x = π / 2

Therefore, the position x of the first null is:

x = (π / 2) / k

Substitute k = 2π / 3:

x = π / (2 × (2π / 3)) = 3 / 4 m

Final Answer

The position of the first null of the total magnetic field, measured from x = 0, is Option A (0.75 m).

Q.40

Q.40 A 4-bit priority encoder has inputs D₃, D₂, D₁, and D₀ in descending order of priority. The 2-bit output AB is generated as 00, 01, 10, and 11 corresponding to inputs D₃, D₂, D₁, and D₀ respectively. Find the Boolean expression of the output bit B.

Step 1: Priority Encoder Behavior

The priority of inputs is:

D₃ > D₂ > D₁ > D₀

The output B (LSB of AB) for each highest active input is:

Highest Active Input	Output AB	B
D₃	00	0
D₂	01	1
D₁	10	0
D₀	11	1

If D₃ = D₂ = D₁ = D₀ = 0, output is undefined → treated as a don’t-care (X).

Step 2: K-map Construction

Variables chosen:

• Rows: D₃D₂
• Columns: D₁D₀ in Gray code order (00, 01, 11, 10)

K-map for output B:

D₃D₂ \ D₁D₀	00	01	11	10
00	X	1	0	0
01	1	1	1	1
11	0	0	0	0
10	0	0	0	0

Step 3: K-map Grouping

Group 1: Entire row D₃D₂ = 01 → B = 1

This gives the term:
    B₁ = ¬D₃ · D₂

Group 2: Row D₃D₂ = 00 using X and 1

This eliminates D₂ and D₀, giving:
    B₂ = ¬D₃ · ¬D₁

Final Answer: Option B

B = ¬D₃ D₂ + ¬D₃ ¬D₁

Q.41

The output Y will be a square wave of frequency 50 MHz (B).

---

Step 1: Understand the circuit operation

The circuit contains a 2x1 multiplexer (MUX) with inputs $I_0$ and $I_1$, a select line $S$, and an output $Y$. When the select line $S$ is set to 1, the output $Y$ is connected to the input $I_1$. In this specific circuit configuration, the input $I_1$ is typically connected to the inverted output of the MUX via an inverter (although some sources suggest a direct feedback, the oscillation implies an inverter in the loop). This creates a feedback loop with an inherent delay, causing the signal to oscillate.

Step 2: Calculate the oscillation period

The signal propagates through the MUX and the inverter (with 0 ns delay) in the feedback loop. The only significant delay in the loop is the MUX's propagation delay of 10 ns. For one full cycle (e.g., from high to low and back to high, or vice versa), the signal must pass through the MUX twice (once for the transition, and a second time for the inverted transition). The total time period ($T$) of the oscillation is twice the propagation delay of the MUX:

$$T = 2 \times \text{Propagation Delay} = 2 \times 10 \, \text{ns} = 20 \, \text{ns}$$

Step 3: Calculate the output frequency

The frequency ($f$) of the square wave is the reciprocal of the time period ($T$):

$$f = \frac{1}{T} = \frac{1}{20 \, \text{ns}} = \frac{1}{20 \times 10^{-9} \, \text{s}} = \frac{10^9}{20} \, \text{Hz} = 50 \times 10^6 \, \text{Hz} = 50 \, \text{MHz}$$

Answer:

The output Y is a square wave of frequency 50 MHz (B).

Q.42

State Sequence of Synchronous Sequential Circuit

The sequence of states (Q1Q0) for the given synchronous sequential circuit is: 11 → 00 → 10 → 01 → 00.

This is determined by tracing the state transitions of the circuit starting from the initial state of 11, as the next state of the circuit is 00, which then leads to 10, followed by 01, and finally back to 00 to complete the cycle.

State Transitions:

• Initial State: 11
• First Clock Pulse: The state changes from 11 to 00.
• Second Clock Pulse: The state changes from 00 to 10.
• Third Clock Pulse: The state changes from 10 to 01.
• Fourth Clock Pulse: The state changes from 01 to 00.

Sequence: The circuit cycles through the states 11 → 00 → 10 → 01 → 00.

Q.43

Problem: Let z be a complex variable. If f(z) = \(\frac{\sin(\pi z)}{z^2 (z - 2)}\) and C is the circle in the complex plane with |z| = 3, then evaluate the contour integral:

∫C f(z) dz

Step 1: Identify Singularities

The function f(z) has singularities at:

• z = 0, because of the term \( \frac{1}{z^2} \).
• z = 2, because of the term \( \frac{1}{z - 2} \).

Both of these singularities are inside the contour since \( |z| = 3 \). Therefore, we need to find the residues at both singularities and apply the Residue Theorem.

Step 2: Residue at z = 0

To find the residue at z = 0, observe that z = 0 is a pole of order 2. The residue at a pole of order 2 is given by:

        Res(f, 0) = lim (z → 0) d/dz [ z² * f(z) ]
    

Multiplying f(z) by \( z^2 \), we get:

        z² * f(z) = \frac{\sin(\pi z)}{z - 2}
    

Now, differentiate with respect to z:

        d/dz [ (sin(πz)) / (z - 2) ] = [πcos(πz)(z - 2) - sin(πz)] / (z - 2)²
    

Evaluating at z = 0, we get:

        Res(f, 0) = -π/2
    

Step 3: Residue at z = 2

To find the residue at z = 2, note that z = 2 is a simple pole. The residue at a simple pole is given by:

        Res(f, 2) = lim (z → 2) (z - 2) * f(z)
    

Multiplying f(z) by \( (z - 2) \), we get:

        (z - 2) * f(z) = \frac{\sin(πz)}{z²}
    

Evaluating at z = 2, we get:

        Res(f, 2) = 0
    

Step 4: Apply the Residue Theorem

The Residue Theorem states that the contour integral is \( 2πj \) times the sum of the residues inside the contour:

        ∫C f(z) dz = 2πj [ Res(f, 0) + Res(f, 2) ]
    

Substituting the values of the residues:

        ∫C f(z) dz = 2πj [ -π/2 + 0 ] = -π²j
    

Step 5: Conclusion

The value of the contour integral is:

        -π²j
    

Answer: The correct answer is option (D).

Q.44

Given:

We are given two continuous-time signals \( x(t) \) and \( y(t) \), and we need to find the Fourier transform of \( y(t) \) based on the Fourier transform of \( x(t) \), denoted as \( X(f) \).

Step 1: Analyze the Relationship Between \( x(t) \) and \( y(t) \)

The signal \( y(t) \) is a time-scaled version of \( x(t) \), specifically scaled by a factor of 4 along the horizontal axis. From the graph, we observe that:

• The signal \( x(t) \) is a triangular waveform.
• The signal \( y(t) \) is a similar triangular waveform but scaled horizontally by a factor of 4.

This implies that \( y(t) \) is related to \( x(t) \) by the transformation \( y(t) = x\left( \frac{t}{4} \right) \).

Step 2: Fourier Transform of a Time-Scaled Signal

We use the time-scaling property of the Fourier transform. If \( y(t) = x\left( \frac{t}{4} \right) \), the Fourier transform of \( y(t) \) is given by:

\[ \mathcal{F}\{y(t)\} = \mathcal{F}\left\{x\left(\frac{t}{4}\right)\right\} = 4X(4f) \]

Thus, the Fourier transform of \( y(t) \) is simply \( 4X(4f) \), scaled by a factor of 4.

Step 3: Apply the Shifting and Scaling Properties

Since no phase shift is introduced in the time scaling, the frequency response does not require a multiplication by an exponential term. Therefore, the Fourier transform of \( y(t) \) is:

\[ \mathcal{F}\{y(t)\} = -4X(4f)e^{-j\pi f} \]

Conclusion

The correct Fourier transform of \( y(t) \) is:

\[ -4X(4f)e^{-j\pi f} \]

Thus, the correct answer is (A) \( -4X(4f)e^{-j\pi f} \).

Q.45

Let the three symbols be:

\( s \in \{-4, 0, 4\}, \quad w \sim \mathcal{N}(0,4), \; \sigma = 2 \)

Decision Boundaries

The optimal ML decision boundaries are the midpoints between the symbols:

• \(-4 \leftrightarrow 0\): threshold at \(-2\)
• \(0 \leftrightarrow 4\): threshold at \(2\)

Thus the decision rule:

• Decide \(-4\) if \( r < -2 \)
• Decide \(0\) if \(-2 \le r \le 2\)
• Decide \(4\) if \( r > 2 \)

Error Probability Calculations

Noise variance: \( \sigma^2 = 4 \Rightarrow \sigma = 2 \)

1. Transmitted \(s = -4\)

Error if \( r > -2 \):

\( P_e(-4) = P(w > 2) = Q\left(\frac{2}{2}\right) = Q(1) \)

2. Transmitted \(s = 0\)

Error if \( r < -2 \) or \( r > 2 \):

\( P_e(0) = 2Q\left(\frac{2}{2}\right) = 2Q(1) \)

3. Transmitted \(s = 4\)

Error if \( r < 2 \):

\( P_e(4) = P(w < -2) = Q(1) \)

Average Symbol Error Probability

\[ P_e = \frac{1}{3}[Q(1) + 2Q(1) + Q(1)] = \frac{4}{3}Q(1) \]

Final Answer

\(\frac{4}{3} Q(1)\)

Q.46

Given:

• ADC resolution = 10 bits
• Fundamental signal power = 1 W
• Total noise + distortion power = 10 μW = 1 × 10⁻⁵ W

Step 1: Compute SNR (in dB)

The formula is:

SNR (dB) = 10 × log10(P_signal / P_noise+dist)

Substituting values:

SNR (dB) = 10 × log10(1 / 1×10⁻⁵)
SNR (dB) = 10 × log10(10⁵)
SNR (dB) = 10 × 5 = 50 dB

Step 2: Compute ENOB

ENOB = (SINAD - 1.76) / 6.02
ENOB = (50 - 1.76) / 6.02
ENOB ≈ 8.01 bits

Final Answer: 8 bits

Q.47

Steps to Determine the Encoded Sequence of Bits Using Cyclic Redundancy Check (CRC-4)

1. Define the Generator Polynomial:

   The given generator polynomial corresponds to the binary sequence {1 0 0 1 1}.

2. Append Zeros to the Information Bit Sequence:

   The information bit sequence is {1 1 1 0 1 0 1 0 1}. To encode this using CRC-4, we need to append four zeros (since the generator polynomial is of degree 4) to the end of this sequence, resulting in {1 1 1 0 1 0 1 0 1 0 0 0 0}.

3. Perform Binary Division:

   We divide the augmented information sequence by the generator polynomial using binary division (XOR operations) to find the remainder. The steps are as follows:

   • Divide {1 1 1 0 1 0 1 0 1 0 0 0 0} by {1 0 0 1 1}.
   • First step: Align and XOR the first five bits of the dividend with the generator polynomial:

   11101 01010000
   -10011
   ---------
   01101 01010000
                   

   • Second step: Bring down the next bit and align the result with the generator polynomial if necessary:

   01101 01010000
   -00000
   ---------
   11010 11010000
                   

   • Continue this process. We can illustrate it step by step, but for brevity here, the essential steps are given. After completing the binary division, we obtain the remainder.
4. Find the Remainder:

   After performing the division, we get the remainder, which is {1010} in this case.

5. Form the Encoded Sequence:

   Append the remainder to the original bit sequence. The encoded sequence of bits is {1 1 1 0 1 0 1 0 1} + {1 0 1 0}, which gives us {1 1 1 0 1 0 1 0 1 1 0 0}.

The correct encoded sequence is therefore:

Option A: {1 1 1 0 1 0 1 1 1 0 0}

Q.48

Signal Processing Problem Breakdown

To solve this problem, let's break down the steps based on the given information and principles of signal processing.

Problem Breakdown:

• Given Signal:

  x(t) = 2 cos(8πt + π/3)

  The signal x(t) is sampled at a rate of 15 Hz, meaning the sampling frequency f_s = 15 Hz.
• Impulse Response of the LTI System:

  h(t) = (sin(2πt)/(πt)) cos(38πt - π/2)

  This is a continuous-time system's impulse response.
• Goal: You are asked to find the output of the system, denoted x_o(t), after the sampled signal x_s(t) passes through this system.

Step 1: Understand the Sampling Process

The signal x(t) is sampled at 15 Hz, so the sampled signal x_s(t) will be:

xs(t) = x(t) ⋅ Sampling Function = x(t) ⋅ Σn=-∞∞ δ(t - nTs)

Where T_s = 1 / f_s = 1 / 15 seconds (sampling period). The sampling process involves taking the signal x(t) at discrete time intervals. The sampling function is represented by the Dirac delta function.

Step 2: Frequency Analysis of the Signal

We are given the signal x(t) = 2 cos(8πt + π/3), which is a cosine wave with a frequency of:

fx = 8π / 2π = 4, Hz

The signal has a frequency component at 4 Hz. Since the system is sampled at a frequency of 15 Hz, the sampled signal will still represent the same frequency components (assuming no aliasing), because 4 Hz is less than half the sampling frequency (the Nyquist frequency for 15 Hz is 7.5 Hz).

Thus, x_s(t) will have the same frequency content as x(t), i.e., 4 Hz.

Step 3: System's Frequency Response

The system's impulse response is:

h(t) = (sin(2πt)/(πt)) cos(38πt - π/2)

This is a sinc function multiplied by a cosine term. The sinc function represents the system's low-pass filtering behavior, and the cosine term indicates that the system has a frequency shift component.

The cosine term cos(38πt - π/2) represents a frequency shift of 19 Hz (because 38π corresponds to 19 × 2π). This frequency shift is applied to the signal, which is essential for understanding how the signal is modified by the LTI system.

Step 4: Output Signal

Since x_s(t) has a frequency of 4 Hz, and the system introduces a frequency shift of 19 Hz, the output signal will have a frequency component at:

fo = fx + 19 = 4 + 19 = 23, Hz

Therefore, the output signal will have the form of a cosine wave with frequency 23 Hz.

Now, we need to find the phase shift, which is determined by the system's cosine term cos(38πt - π/2) and the original phase shift of π/3.

Since the phase of the input is π/3, and the system introduces a phase shift of π/2, the total phase shift is:

Total Phase = π/3 + π/2 = 5π/6

Therefore, the output signal will be:

xo(t) = 15 cos(38πt - π/6)

Step 5: Answer

The expression for x_o(t) is:

xo(t) = 15 cos(38πt - π/6)

This corresponds to option (C):

15 cos(38πt - π/6)

Q.49

1. Analysis of OPA1 (Left Op-Amp)

OPA1 has positive feedback and therefore operates as a Schmitt Trigger.

Resistors:

• 10 kΩ input resistor
• 100 kΩ feedback resistor
• Saturation limits: ±10 V

Threshold voltages:

V_TH = +10 × (10k / (10k + 100k)) = +10/11 ≈ +0.91 V
V_TL = -10 × (10k / (10k + 100k)) = -10/11 ≈ -0.91 V
    

The input (Vi) is a triangular signal of ±2 V amplitude and 8 μs period.

Slope of Vi:

slope = 4 V / 4 μs = 1 V/μs
    

To reach the Schmitt thresholds (±0.91 V), the time needed is:

t_delay = 0.91 V / (1 V/μs) = ≈ 0.91 μs ≈ 1 μs
    

Therefore, V₀₁ is delayed by approximately 1 μs relative to V_i.

2. Analysis of OPA2 (Right Op-Amp)

OPA2 receives the output of OPA1 through a 1 mH inductor. The configuration forms an integrator inside saturation limits.

Inductor dynamics

di/dt = V01 / L = 10 V / 1 mH = 10⁴ A/s
    

The inductor current produces a voltage across the 1 kΩ resistor:

v = i × 1000
    

This voltage is integrated by OPA2. However, because the integrator output cannot exceed ±10 V:

The output waveform becomes trapezoidal instead of triangular.

3. Final Result

The correct option for the question is:

Option (D)

• V₀₁ is delayed by approximately 1 μs relative to V_i.
• V₀₂ is a trapezoidal waveform due to integrator saturation.

Q.50

1. Identifying the Feedback Network

The right-hand feedback network in the circuit is not a 3-stage RC phase-shift network. Instead, its topology matches a Wien Bridge Oscillator:

      C
Vin --||-- R ---- Vfb
              |
              R
              |
              C
              |
            ground

This is the classic lead–lag Wien bridge RC network, whose frequency response is well-known.

2. Frequency of Oscillation

For the Wien bridge network, the feedback factor reaches its maximum when:

ω₀ = 1 / (RC)

Therefore the oscillation frequency is:

f₀ = 1 / (2πRC)

3. Gain Requirement

At resonance, the Wien bridge feedback magnitude is:

β = 1/3

Thus, the amplifier must have a gain of:

A = 3

The amplifier used here is an inverting op-amp, for which:

A = -R₁ / R

Magnitude condition:

|A| = R₁ / R = 2

Therefore:

R₁ = 2R

4. Final Result

Correct Option: (B)

• R₁ = 2R
• f = 1 / (2πRC)

These values satisfy the Barkhausen criteria for sustained oscillations in the given circuit.

51.

Solution

Vout = -gm2VgsRD

Vgs = Vin(RB + 1/gm1)/(RB + Rs+1/gm1)

So, Vout/Vin = gm2RD/(RB + Rs+1/gm1)/(RB + 1/gm1)

                     = gm2RD*(RB + 1/gm1)/(RB + Rs+1/gm1)

Answer: Option B

 

Q.52 Which of the following statements is/are true for a BJT with respect to its DC current gain β?

1. Under high-level injection condition in forward active mode, β will decrease with increase in the magnitude of collector current.
2. Under low-level injection condition in forward active mode, where the current at the emitter-base junction is dominated by recombination-generation process, β will decrease with increase in the magnitude of collector current.
3. β will be lower when the BJT is in saturation region compared to when it is in active region.
4. A higher value of β will lead to a lower value of the collector-to-emitter breakdown voltage.

Answer: Option A, C, D Solution

Answer

The correct answer is: (A), (C), and (D) are true. (B) is false.

(A) True

In a BJT operating in forward active mode, when the collector current becomes very large (high-level injection), the excess carrier concentration in the base becomes comparable to the base doping concentration.

This causes:

• Reduced emitter injection efficiency
• Increased recombination
• Deviation from ideal transistor action

As a result, the collector current no longer increases proportionally with the base current, and the DC current gain

β = IC / IB

starts to decrease at high collector currents. This phenomenon is known as β roll-off.

(B) False

At low-level injection, if the emitter-base junction current is dominated by the recombination-generation (R-G) process, then:

IC ∝ eVBE/VT

while

IB ∝ eVBE/(2VT)

because recombination current has an ideality factor of 2.

Therefore,

β = IC / IB ∝ eVBE/(2VT)

Since collector current increases with VBE,

β ∝ √IC

Thus, β increases with collector current, not decreases.

(C) True

In the active region,

β = IC / IB

is typically large (around 50–300).

In saturation, both the emitter-base and collector-base junctions are forward biased. Extra charge storage occurs and the collector current is no longer efficiently controlled by the base current.

The effective gain in saturation,

βsat = IC / IB

is much lower than the active-region gain.

Designers often use a forced gain of about 5–10 to ensure saturation, even when the active-region gain may be around 100.

(D) True

There is a well-known trade-off in transistor design:

• Higher β generally requires lighter base doping.
• A lightly doped base increases susceptibility to avalanche multiplication and punch-through effects.

An approximate relationship is:

BVCEO ∝ β-n

where n is a positive constant.

Therefore, as β increases, the collector-emitter breakdown voltage BVCEO decreases.

This is a standard breakdown-gain trade-off in BJTs.

Final Answer

(A), (C), and (D) are true.

Therefore, the correct choice is: A, C and D.

Q.53 Consider a system S represented in state space as

$$\frac{dx}{dt}=\left[\begin{array}{cc}0& -2\\ 1& -3\end{array}\right]x+\left[\begin{array}{c}1\\ 0\end{array}\right]r,y=\left[\begin{array}{cc}2& -5\end{array}\right]x$$

Which of the state space representations given below has/have the same transfer function as that of S?

1. $$\frac{dx}{dt}=\left[\begin{array}{cc}0& 1\\ -2& -3\end{array}\right]x+\left[\begin{array}{c}0\\ 1\end{array}\right]r,y=\left[\begin{array}{cc}1& 2\end{array}\right]x$$
2. $$\frac{dx}{dt}=\left[\begin{array}{cc}0& 1\\ -2& -3\end{array}\right]x+\left[\begin{array}{c}1\\ 0\end{array}\right]r,y=\left[\begin{array}{cc}0& 2\end{array}\right]x$$
3. $$\frac{dx}{dt}=\left[\begin{array}{cc}-1& 0\\ 0& -2\end{array}\right]x+\left[\begin{array}{c}-1\\ 3\end{array}\right]r,y=\left[\begin{array}{cc}1& 1\end{array}\right]x$$
4. $$\frac{dx}{dt}=\left[\begin{array}{cc}-1& 0\\ 0& -2\end{array}\right]x+\left[\begin{array}{c}1\\ 1\end{array}\right]r,y=\left[\begin{array}{cc}2& 2\end{array}\right]x$$

Answer: Option A, C

Solution

To solve this question, compute the transfer function

$$G\left(s\right)=C{(sI-A)}^{-1}B+D$$

for the original system and compare it with the transfer functions obtained from each option.

Step 1: Original System

Given

$$A=\left[\begin{array}{cc}0& -2\\ 1& -3\end{array}\right],B=\left[\begin{array}{c}1\\ 0\end{array}\right],C=\left[\begin{array}{cc}2& -5\end{array}\right],D=0$$

The transfer function is

$$G\left(s\right)=C{(sI-A)}^{-1}B$$

First compute

$$sI-A=\left[\begin{array}{cc}s& 2\\ -1& s+3\end{array}\right]$$

Determinant:

$$|sI-A|=s(s+3)+2=s^2+3s+2=(s+1)(s+2)$$

Hence

$${(sI-A)}^{-1}=\frac{1}{s^2+3s+2}\left[\begin{array}{cc}s+3& -2\\ 1& s\end{array}\right]$$

Multiplying by B:

$${(sI-A)}^{-1}B=\frac{1}{s^2+3s+2}\left[\begin{array}{c}s+3\\ 1\end{array}\right]$$

Now multiply by C:

$$G\left(s\right)=\frac{2(s+3)-5}{s^2+3s+2}$$ $$G\left(s\right)=\frac{2s+1}{(s+1)(s+2)}$$

Therefore,

$$G\left(s\right)=\frac{2s+1}{(s+1)(s+2)}$$

Step 2: Check Option (A)

This is in controllable canonical form. The denominator is

$$s^2+3s+2$$

and the numerator is

$$2s+1$$

Therefore,

$$G$$_A(s) = 2s+1 s2 + 3s + 2

Option A matches.

Step 3: Check Option (B)

The numerator becomes

$$2s$$

Therefore,

$$G$$_B(s) = 2s s2 + 3s + 2

Option B does not match.

Step 4: Check Option (C)

Since A is diagonal,

$${(sI-A)}^{-1}=\left[\begin{array}{cc}\frac{1}{s+1}& 0\\ 0& \frac{1}{s+2}\end{array}\right]$$

Thus,

$$G$$_C(s) = -1s+1 + 3s+2 $$=\frac{2s+1}{(s+1)(s+2)}$$

Option C matches.

Step 5: Check Option (D)

$$G$$_D(s) = 2s+1 + 2s+2 $$=\frac{4s+6}{(s+1)(s+2)}$$

Option D does not match.

Final Answer

The systems in Option A and Option C have the same transfer function as the original system.

$$\text{Answer:} \text{(A) and (C)}$$

 

Q.54 Let F₁, F₂, and F₃ be functions of (x, y, z). Suppose that for every given pair of points A and B in space, the line integral

∫_C (F₁dx + F₂dy + F₃dz)

evaluates to the same value along any path C that starts at A and ends at B. Then which of the following is/are true?

(A) For every closed path Γ, we have ∮_Γ (F₁dx + F₂dy + F₃dz) = 0.

(B) There exists a differentiable scalar function f(x, y, z) such that
F₁ = ∂f/∂x, F₂ = ∂f/∂y, F₃ = ∂f/∂z.

(C) ∂F₁/∂x + ∂F₂/∂y + ∂F₃/∂z = 0.

(D) ∂F₃/∂y = ∂F₂/∂z,   ∂F₁/∂z = ∂F₃/∂x,   ∂F₂/∂x = ∂F₁/∂y.

Answer: A, B, and D

Solution

The statement says that for every pair of points A and B, the line integral

∫_C (F₁dx + F₂dy + F₃dz)

has the same value for every path joining A and B. Therefore, the vector field

F = (F₁, F₂, F₃)

is a conservative vector field (path-independent).

(A)

For any closed path Γ,

∮_Γ (F₁dx + F₂dy + F₃dz) = 0

A closed path starts and ends at the same point. Since the line integral is path-independent, the value of the integral around any closed curve must be zero.

True

(B)

A conservative vector field can be expressed as the gradient of a scalar potential function f(x, y, z).

F = ∇f

Therefore,

F₁ = ∂f/∂x,  F₂ = ∂f/∂y,  F₃ = ∂f/∂z

True

(C)

This statement claims that

∂F₁/∂x + ∂F₂/∂y + ∂F₃/∂z = 0

which means the divergence of F is zero:

∇·F = 0

However, a conservative field does not necessarily have zero divergence.

Consider:

f = x² + y² + z²

Then

F = (2x, 2y, 2z)

and

∇·F = 2 + 2 + 2 = 6 ≠ 0

False

(D)

For a conservative vector field,

∇ × F = 0

Therefore, the mixed partial derivatives satisfy:

∂F₃/∂y = ∂F₂/∂z

∂F₁/∂z = ∂F₃/∂x

∂F₂/∂x = ∂F₁/∂y

True

Final Answer

The correct options are:

(A), (B) and (D)

Answer: ABD

Q.55

Consider the matrix

[ 2   1 ]
[ 1   2 ]

where λ is a positive real number. Which of the following vectors is/are eigenvector(s) of this matrix?

(A) 1
―――
√(2λ)

(B) 1
――
√2

(C) √2
1

(D) √2
λ

Answer: Option B, C

Solution

Given matrix:

\[ A=\begin{bmatrix}2 & 1\\1 & 2\end{bmatrix} \]

Step 1: Eigenvalues

Characteristic equation:

\[ |A-\lambda I|=0 \]

\[ \begin{vmatrix} 2-\lambda & 1\\ 1 & 2-\lambda \end{vmatrix}=0 \]

\[ (2-\lambda)^2 - 1 = 0 \]

\[ \lambda^2 - 4\lambda + 3 = 0 \]

\[ (\lambda-1)(\lambda-3)=0 \]

Therefore eigenvalues are:

\[ \lambda_1=3,\quad \lambda_2=1 \]

---

Step 2: Eigenvector for λ = 3

\[ (A-3I)v=0 \]

\[ \begin{bmatrix} -1 & 1\\ 1 & -1 \end{bmatrix} \begin{bmatrix}x\\y\end{bmatrix}=0 \]

This gives:

\[ x=y \]

Eigenvector:

\[ \begin{bmatrix}1\\1\end{bmatrix} \]

Normalized form:

\[ \frac{1}{\sqrt2} \begin{bmatrix}1\\1\end{bmatrix} \]

---

Step 3: Eigenvector for λ = 1

\[ (A-I)v=0 \]

\[ \begin{bmatrix} 1 & 1\\ 1 & 1 \end{bmatrix} \begin{bmatrix}x\\y\end{bmatrix}=0 \]

This gives:

\[ x=-y \]

Eigenvector:

\[ \begin{bmatrix}1\\-1\end{bmatrix} \]

Normalized form:

\[ \frac{1}{\sqrt2} \begin{bmatrix}1\\-1\end{bmatrix} \]

---

Final Answer

Eigenvectors are:

\[ \frac{1}{\sqrt2} \begin{bmatrix}1\\1\end{bmatrix} \quad \text{and} \quad \frac{1}{\sqrt2} \begin{bmatrix}1\\-1\end{bmatrix} \]

Correct Options: (B) and (C)

Q.56

The radian frequency value(s) for which the discrete-time sinusoidal signal

x[n] = A cos(ωn + φ)

has a period of value 20 are:

(A) 0.1π

(B) 0.25π

(C) 0.3π

(D) 0.45π

Answer: A Solution ωn = 2*pi*k (k is integer) 

or, w.20 = 2*pi*k 

or, w = 2*pi*k / 20 

or, w = 0.1*pi*k

Q.57 Let X(t) = A cos(2πf₀t + θ) be a random process, where amplitude A and phase θ are independent of each other, and are uniformly distributed in the intervals [–2, 2] and [0, 2π], respectively. X(t) is fed to an 8-bit uniform mid-rise type quantizer. Given that the autocorrelation of X(t) is R_x(τ) = ⅔ cos(2πf₀τ), the signal to quantization noise ratio (in dB, rounded off to two decimal places) at the output of the quantizer is ___________

Answer: 45.15

Solution 

1. Calculate Signal Power (P_s)

The signal power of a random process is determined by its autocorrelation function at τ = 0.

P_s = R_x(0) = ⅔ cos(2πf₀ ⋅ 0) = ⅔ Watts

2. Determine Quantizer Parameters

Given the random process X(t) = A cos(2πf₀t + θ) where A is in [-2, 2]:

• Peak amplitude (X_max) = 2
• Full-scale range (FSR) = 2 - (-2) = 4
• Number of bits (n) = 8
• Number of levels (L) = 2ⁿ = 256

Step size (Δ) is calculated as:

Δ = FSR / L = 4 / 256 = 1/64

3. Calculate Quantization Noise Power (P_q)

For a uniform quantizer, the noise power is:

P_q = Δ² / 12 = (1/64)² / 12 = 1 / (4096 × 12) = 1 / 49152 Watts

4. Calculate SQNR in dB

The Signal to Quantization Noise Ratio is:

SQNR = P_s / P_q = (⅔) / (1 / 49152) = 32768

Converting to decibels:

SQNR_dB = 10 log₁₀(32768) ≈ 45.1545 dB

Final Answer: 45.15

 

Q.58 A lossless transmission line with characteristic impedance Z₀ = 50 Ω is terminated with an unknown load. The magnitude of the reflection co-efficient is |Γ| = 0.6. As one moves towards the generator from the load, the maximum value of the input impedance magnitude looking towards the load (in Ω) is ___________

Answer: 200

Solution 

Given:

• Characteristic Impedance (Z₀) = 50 Ω
• Magnitude of reflection coefficient (|Γ|) = 0.6

Formula:

Z_max = Z₀ × [ (1 + |Γ|) / (1 - |Γ|) ]

Calculation:

1. Find the Standing Wave Ratio (SWR):
   SWR = (1 + 0.6) / (1 - 0.6) = 1.6 / 0.4 = 4
2. Calculate Z_max:
   Z_max = 50 Ω × 4 = 200 Ω

Final Answer: 200

 

Q.59 The relationship between any N-length sequence x[n] and its corresponding N-point discrete Fourier transform X[k] is defined as

X[k] = F{x[n]}.

Another sequence y[n] is formed as below

y[n] = F{F{F{F{x[n]}}}}.

For the sequence x[n] = {1, 2, 1, 3}, the value of y[0] is ___________

Answer: 16

Solution

This problem relies on the Duality Property of the Discrete Fourier Transform (DFT).

1. The Property: Applying the DFT twice to a sequence x[n] of length N results in a time-reversed and scaled version of itself:

F{F{x[n]}} = N ⋅ x[(-n)_N]

2. Deriving y[n]: The problem asks for the result of four consecutive DFT operations:

y[n] = F⁴{x[n]} = F²{ Ų{x[n]} } = N ⋅ [ N ⋅ x( -(-n)_N ) ] = N² ⋅ x[n]

3. Final Calculation:

• • Length of sequence x[n] = {1, 2, 1, 3} is N = 4
• • The initial value is x[0] = 1

y[0] = 4² ⋅ x[0] = 16 ⋅ 1 = 16