We can solve this efficiently by using the properties of determinants. The key property is that for any two square matrices B and C, the determinant of their product is the product of their determinants, i.e., ∣BC∣=∣B∣∣C∣.
Applying this to our problem, we get: ∣ATA−1∣=∣AT∣∣A−1∣
Next, we use two more fundamental determinant rules:
The determinant of a matrix transpose is equal to the determinant of the original matrix: ∣AT∣=∣A∣.
The determinant of an inverse matrix is the reciprocal of the original matrix's determinant: ∣A−1∣=∣A∣1.
Substituting these into our equation yields: ∣ATA−1∣=∣A∣⋅∣A∣1=1
This result holds for any invertible matrix A, so we don't even need to calculate the specific values for this particular matrix.
Q2GATE 2015MCQ1MEngineering Mathematics
The contour on the x-y plane, where the partial derivative of x2+y2 with respect to y is equal to the partial derivative of 6y + 4x with respect to x, is
The problem asks us to find the curve on the x-y plane where two partial derivatives are equal.
First, let's find the partial derivative of x2+y2 with respect to y. We treat x as a constant, so the derivative of x2 is zero. This gives us: ∂y∂(x2+y2)=2y
Next, we find the partial derivative of 6y+4x with respect to x. We treat y as a constant, so the derivative of 6y is zero. This gives us: ∂x∂(6y+4x)=4
The question states that these two results are equal. Setting them equal gives the equation 2y=4. Solving for y, we find the contour is the horizontal line y=2.
Q3GATE 2015MCQ1MEngineering Mathematics
If C is a circle of radius r with centre z0 , in the complex z-plane and if n is a non-zero integer, then ∮C(z−z0)n+1dz equals
This integral can be solved using Cauchy's Integral Formula for derivatives. The general form of this powerful theorem is: ∮C(z−z0)k+1f(z)dz=k!2πif(k)(z0)
Here, f(z) is a function that is analytic inside the contour C, and f(k)(z0) is its k-th derivative evaluated at z0.
In our specific problem, the integral is ∮C(z−z0)n+1dz. We can match this to the formula by choosing our analytic function to be f(z)=1. The problem requires the (n+1)-th power in the denominator, so we have k=n.
Since n is a non-zero integer, we must find the n-th derivative of f(z)=1. The first derivative of a constant is 0, and so are all higher-order derivatives. Therefore, f(n)(z0)=0. Substituting this into the formula gives a final result of 0.
Q4GATE 2015NAT1MSignals and Systems
Consider the function g(t)=e−tsin(2πt)u(t) where u(t) is the unit step function. The area under g(t) is _______.
To find the area under the curve, we must calculate the integral of g(t) over all time. The unit step function u(t) makes the function zero for t<0, so the integral becomes ∫0∞e−tsin(2πt)dt.
A clever way to solve this is to recognize that this integral is the exact definition of the Laplace transform of e−tsin(2πt), which we'll call G(s), evaluated at s=0.
Using a standard Laplace transform pair for a damped sine wave, we have: G(s)=L{e−tsin(2πt)}=(s+1)2+(2π)22π
The area is therefore G(s) at s=0:
Area =G(0)=(0+1)2+4π22π=1+4π22π≈0.155
This sum can be elegantly solved using the Z-transform. The given expression is equivalent to the Z-transform of the sequence n(21)nu(n), evaluated at the point z=1.
We will use the standard Z-transform pair for a ramp-multiplied exponential sequence, which states that the transform of nanu(n) is X(z)=(1−az−1)2az−1.
For our specific problem, the constant is a=21.
Substituting a=21 and z=1 into the general formula gives the value of the sum: (1−21(1)−1)221(1)−1=(1−1/2)21/2=(1/2)21/2=1/41/2=2
Q6GATE 2015NAT1MNetwork Theory
For the circuit shown in the figure, the Thevenin equivalent voltage (in Volts) across terminals a-b is ________.
The Thevenin voltage, VTh, is the open-circuit voltage across terminals a-b. We can find this value by calculating the voltage at the main node, let's call it Va, using nodal analysis.
By applying Kirchhoff's Current Law (KCL) at node 'a', we sum the currents leaving the node and set the total to zero: 3Va−12+6Va−1=0
The first term is the current through the 3Ω resistor, the second is through the 6Ω resistor, and the -1 accounts for the 1A source entering the node.
To solve for Va, we can rearrange the equation: Va(31+61)=1+312=5
Simplifying the fractions gives Va(21)=5.
Therefore, Va=10 V, which is our Thevenin voltage.
Q7GATE 2015NAT1MNetwork Theory
In the circuit shown, the voltage VX (in Volts) is _______.
To find the voltage VX, we need to solve the system of two equations that describe the circuit's behavior. The first equation, derived from applying Kirchhoff's Current Law, relates the two node voltages: 13VX=100+2VY. The second equation gives the dependency between them: VY=0.25VX.
We can rearrange the dependency equation to express VX in terms of VY: VX=4VY. Now, we substitute this into the first equation to eliminate the VX variable: 13(4VY)=100+2VY
This simplifies the equation to 52VY=100+2VY. By isolating VY, we find that 50VY=100, which gives VY=2 V. Finally, we use this value to solve for VX: VX=4VY=4(2)=8 V.
Q8GATE 2015NAT1MNetwork Theory
At very high frequencies, the peak output voltage V0 (in Volts) is ________.
At very high frequencies, a capacitor's impedance, given by ZC=1/(jωC), approaches zero. This allows us to treat all the capacitors in the circuit as ideal short circuits.
Under these conditions, the two series capacitors become wires, applying the full source voltage, v∈(t)=1.0sin(ωt) V, across the resistor network. The network itself simplifies to two parallel voltage dividers. The output V0 is measured across the top resistor of the right-hand divider.
Using the voltage divider formula for this branch: V0=v∈×1 kΩ+1 kΩ1 kΩ=21v∈
The peak output voltage is therefore half of the peak input voltage, which is 21×1.0 V=0.5 V.
Q9GATE 2015MCQ1MElectronic Devices
Which one of the following processes is preferred to form the gate dielectric ( SiO2 ) of MOSFETs ?
The gate dielectric in a MOSFET must be an exceptionally pure and uniform insulating layer to ensure reliable device performance. Dry oxidation, which uses pure oxygen (O2), is the preferred method for this critical step. While this process is significantly slower than wet oxidation, its slow growth rate allows for precise control over the thickness of the very thin oxide layer. This results in a denser, higher-quality SiO2 film with a lower defect density at the crucial silicon-silicon dioxide interface, which is essential for a high-performance transistor.
Q10GATE 2015MCQ1MElectronic Devices
If the base width in a bipolar junction transistor is doubled, which one of the following statements will be TRUE?
The Early effect, or base-width modulation, describes how the effective base width changes with the collector-emitter voltage (VCE). When the physical base width (WB) is doubled, the depletion region's encroachment into the base becomes a much smaller fraction of the total width. This significantly reduces the impact of the Early effect. Consequently, the collector current (IC) becomes less sensitive to variations in VCE, resulting in a flatter slope on the IC vs. VCE output characteristic curves. The Early Voltage (VA) is the magnitude of the voltage axis intercept of these extrapolated curves, so a flatter slope directly corresponds to a larger Early Voltage.
Q11GATE 2015NAT1MAnalog Circuits
In the circuit shown in the figure, the BJT has a current gain ( β ) of 50. For an emitter-base voltage VEB = 600 mV, the emitter-collector voltage VEC (in Volts) is _______.
This circuit contains a PNP transistor. We begin by finding the base current (IB). The emitter is at VE=3V. Given the specified emitter-to-base voltage drop VEB=0.6V, the voltage at the base is VB=VE−VEB=2.4V. This voltage drives a current through the base resistor to ground, so IB=RBVB=60kΩ2.4V=0.04mA.
Next, we calculate the collector current using the current gain β=50: IC=βIB=50×0.04mA=2mA. This current flows from the collector through the 500 Ω resistor, so the collector voltage is VC=ICRC=2mA×500Ω=1V.
Finally, the emitter-collector voltage (VEC) is the potential difference between the emitter and collector: VEC=VE−VC=3V−1V=2V.
Q12GATE 2015NAT1MAnalog Circuits
In the circuit shown using an ideal opamp, the 3-dB cut-off frequency (in Hz) is _______.
This circuit is a first-order active low-pass filter. Its behavior is defined by two distinct parts: an initial RC filter network and a subsequent non-inverting amplifier stage. The 3-dB cutoff frequency is determined entirely by the passive components in the filter section.
The formula for the cutoff frequency (f3dB) of a simple RC filter is: f3dB=2πRC1
Using the given values, R=10kΩ and C=0.1μF: f3dB=2π(10×103Ω)(0.1×10−6F)1≈159.15Hz
Q13GATE 2015NAT1MAnalog Circuits
In the circuit shown, assume that diodes D1andD2 are ideal. In the steady state condition, the average voltage Vab (in Volts) across the 0.5 μ F capacitor is _____.
This circuit is an asynchronous ripple counter where each J-K flip-flop is in toggle mode (J=K=1). Since the normal output Q of each flip-flop provides the negative-edge clock for the next stage, the counter counts upwards.
The reset logic consists of a NAND gate connected to the active-low reset inputs (Rdˉ). The reset signal is given by Q2⋅Q0. This signal becomes low (active) only when both inputs Q2 and Q0 are high.
The counter state is represented by (Q2Q1Q0). It counts up from 000. The first time the reset condition (Q2=1 and Q0=1) is met is at the state 101 (decimal 5). As soon as the counter reaches this state, it is immediately and asynchronously reset to 000. Thus, the counter has five stable states (0, 1, 2, 3, 4), defining it as a modulo-5 binary up counter.
Q15GATE 2015MCQ1MDigital Circuits
In the circuit shown, diodes D1,D2andD3 are ideal, and the inputs E1,E2andE3 are "0 V" for logic '0' and "10 V" for logic '1'. What logic gate does the circuit represent?
Let's analyze the circuit's logic by considering the two main scenarios for the inputs.
First, consider the case where at least one input (E1,E2, or E3) is at logic '0', which is 0 V. The ideal diode connected to this low input will become forward-biased, acting like a closed switch. This creates a direct path from the output node V0 to the 0 V input, forcing the output voltage V0 to be 0 V (logic '0').
Next, consider the case where all three inputs are at logic '1', or 10 V. In this situation, there is no potential difference across any of the diodes to turn them on. They are effectively reverse-biased and act as open circuits. With the diodes off, no current flows through them or the resistor, so the output V0 is pulled up to the supply voltage of 10 V (logic '1').
Since the output V0 is high only when all inputs are high, this circuit performs the function of a 3-input AND gate.
Q16GATE 2015MCQ1MMicroprocessors
Which one of the following 8085 microprocessor programs correctly calculates the product of two 8-bit numbers stored in registers B and C?
The fundamental concept of multiplication, such as finding the product B×C, is repeated addition. This program executes this by adding the value in register C to a running total, a total of B times.
The accumulator (register A) is first initialized to zero (MVI A, 00H). The program then enters a loop where it adds the value of register C to the accumulator (ADD C). Register B acts as a loop counter, which is decremented after each addition (DCR B). The loop continues (JNZ LOOP) until the counter B reaches zero, leaving the final product in the accumulator.
Q17GATE 2015MCQ1MSignals and Systems
The impulse response of an LTI system can be obtained by
Let's start by defining our terms. The impulse response, h(t), is the output of a system when the input is a Dirac delta function. The unit step response, which we'll call s(t), is the output when the input is a unit step function, u(t).
The output of any LTI system is the convolution of the input with the impulse response. So, the step response is given by s(t)=h(t)∗u(t).
A key property of convolution is that convolving a function with the unit step is equivalent to integrating that function. This gives us the relationship: s(t)=∫−∞th(τ)dτ
This equation shows that the unit step response is the time integral of the impulse response. To reverse this and solve for the impulse response h(t), we apply the fundamental theorem of calculus by differentiating both sides with respect to t.
This yields the final relationship: h(t)=dtds(t)
Q18GATE 2015MCQ1MSignals and Systems
Consider a four-point moving average filter defined by the equation y[n]=∑i=03αix[n−i] , The condition on the filter coefficients that results in a null at zero frequency is
The filter's transfer function, H(z), is the Z-transform of its impulse response, which for this FIR filter is H(z)=α0+α1z−1+α2z−2+α3z−3.
A "null at zero frequency" means the filter's frequency response, H(ejω), must be zero when the frequency ω=0. This corresponds to evaluating the transfer function at the point z=ej0=1 on the unit circle.
Setting the transfer function to zero at this point gives the condition H(1)=0.
Substituting z=1 into the expression for H(z) yields the requirement that the sum of the filter coefficients must be zero: α0+α1+α2+α3=0.
Of the choices provided, only the condition where α1=α2=0 and α0=−α3 ensures this sum is zero, since α0+0+0−α0=0.
Q19GATE 2015NAT1MControl Systems
Consider the Bode plot shown in the figure. Assume that all the poles and zeros are real-valued. The value of fH−fL (in Hz) is ___________.
The slope of a Bode plot indicates the rate of change of gain with frequency. A slope of +40 dB/decade means the gain increases by 40 dB for every factor of 10 increase in frequency.
To find fL, we see the gain rises by 40 dB (from 0 to 40 dB) as the frequency goes from fL to 300 Hz. This corresponds to one decade, so the frequency ratio is 10. fL300=10⟹fL=30 Hz
Similarly, to find fH, the gain drops by 40 dB (from 40 to 0 dB) as frequency goes from 900 Hz to fH. This also represents one decade. 900fH=10⟹fH=9000 Hz
Finally, we can calculate the difference: fH−fL=9000−30=8970 Hz
Q20GATE 2015NAT1MControl Systems
The phase margin (in degrees) of the system G(s)=s(s+10)10 is _______.
To find the phase margin (PM), we first determine the gain crossover frequency, ωgc, where the system's magnitude equals one. We set ∣G(jω)∣=1: jω(jω+10)10=ωω2+10010=1
Solving the resulting expression, ω4+100ω2−100=0, gives a gain crossover frequency of ωgc≈0.99 rad/s.
Next, we calculate the phase angle of the system at this frequency: ∠G(jωgc)=−90∘−tan−1(10ωgc)=−90∘−tan−1(100.99)
Finally, the phase margin is PM=180∘+∠G(jωgc), which gives: PM=180∘−90∘−tan−1(0.099)≈84.36∘
Q21GATE 2015MCQ1MControl Systems
The transfer function of a first-order controller is given as Gc(s)=s+bK(s+a) where K, a and b are positive real numbers. The condition for this controller to act as a phase lead compensator is
To determine if the controller acts as a phase lead compensator, we can compare its transfer function to the standard time-constant form, G(s)=C1+αsτ1+sτ, where the key condition for phase lead is α<1.
First, let's rearrange the given transfer function, Gc(s)=s+bK(s+a), to match this standard form: Gc(s)=b(1+s/b)Ka(1+s/a)=(bKa)1+s/b1+s/a
By comparing the terms, we can identify τ=1/a and ατ=1/b. Now, we solve for α by substituting the expression for τ: α(a1)=b1⟹α=ba
The condition for a phase lead compensator is α<1. Therefore, we must have a/b<1, which simplifies to a<b since both a and b are positive.
Q22GATE 2015MCQ1MCommunication Systems
The modulation scheme commonly used for transmission from GSM mobile terminals is
Global System for Mobile Communications (GSM) employs Gaussian Minimum Shift Keying (GMSK) for transmissions, particularly from the mobile terminal to the base station. The primary reason for this choice is GMSK's constant envelope property, where the signal's amplitude does not fluctuate. This characteristic is crucial for battery-powered devices, as it enables the use of highly efficient, non-linear power amplifiers without introducing significant distortion. This conserves battery life while the Gaussian filtering also ensures the signal occupies a narrow bandwidth, reducing interference with adjacent channels.
Q23GATE 2015MCQ1MCommunication Systems
A message signal m(t)=Amsin(2πfmf) is used to modulate the phase of a carrier Accos(2πfct) to get the modulated signal y(t)=Accos(2πfct+m(t)) . The bandwidth of y(t).
The bandwidth of an angle-modulated signal is estimated by Carson's rule, B≈2(Δf+fm), where fm is the frequency of the message signal and Δf is the peak frequency deviation.
For phase modulation, the instantaneous frequency deviation is proportional to the time derivative of the message signal, m(t). With m(t)=Amsin(2πfmt), the instantaneous frequency deviation is 2π1dtd[m(t)]=Amfmcos(2πfmt).
The peak value of this frequency deviation is therefore Δf=Amfm.
Substituting this into Carson's rule gives the bandwidth: B≈2(Amfm+fm)=2fm(Am+1). As you can see from this final expression, the bandwidth B is a function of both the message amplitude Am and the message frequency fm.
Q24GATE 2015MCQ1MElectromagnetics
The directivity of an antenna array can be increased by adding more antenna elements, as a larger number of elements
Adding more elements to an antenna array increases its overall physical dimensions. This enlargement directly increases the antenna's effective area (Ae), which represents how well the antenna captures or transmits power in a specific direction. The directivity (D) of an antenna is fundamentally and proportionally related to its effective area. This relationship is described by the equation D=λ24πAe. As the formula shows, if the effective area (Ae) increases, the directivity (D) must also increase.
Q25GATE 2015MCQ1MElectromagnetics
A coaxial cable is made of two brass conductors. The spacing between the conductors is filled with Teflon ( εr′ = 2.1, tan δ =0). Which one of the following circuits can represent the lumped element model of a small piece of this cable having length Δ z?
The lumped element model uses circuit components to represent the physical properties of a transmission line segment. The shunt conductance, G, specifically models the energy loss that occurs within the dielectric material separating the two conductors. This dielectric loss is quantified by a parameter called the loss tangent, tanδ.
The problem states that the Teflon dielectric is ideal, with a loss tangent of zero (tanδ=0). The shunt conductance is directly related to the loss tangent by the formula G=ωCtanδ. Consequently, a zero loss tangent means the shunt conductance G must also be zero. This simplifies the general model by removing the shunt resistor (GΔz), leaving the components for conductor resistance (R), inductance (L), and capacitance (C).
Q26GATE 2015NAT2MEngineering Mathematics
The Newton-Raphson method is used to solve the equation f(x)=x3−5x2+6x−8=0 . Taking the initial guess as x = 5, the solution obtained at the end of the first iteration is __________.
The Newton-Raphson method finds the root of an equation by iteratively improving an initial guess. The formula for the first iteration is x1=x0−f′(x0)f(x0).
First, we need the derivative of the given function, f(x)=x3−5x2+6x−8. The derivative is f′(x)=3x2−10x+6.
Next, we evaluate both f(x) and f′(x) at the initial guess, x0=5: f(5)=53−5(52)+6(5)−8=22 f′(5)=3(52)−10(5)+6=75−50+6=31
Finally, we substitute these values into the Newton-Raphson formula to find the solution after one iteration: x1=5−3122≈5−0.7097=4.2903
Q27GATE 2015NAT2MEngineering Mathematics
A fair die with faces {1, 2, 3, 4, 5, 6} is thrown repeatedly till '3' is observed for the first time. Let X denote the number of times the die is thrown. The expected value of X is ____.
This problem describes a sequence of independent Bernoulli trials, where we are waiting for the first "success". This is a classic example of a Geometric distribution.
Let X be the number of throws until a '3' is observed. The probability of success (rolling a '3') in a single trial is p=61.
The random variable X follows a Geometric distribution with parameter p.
The expected value, E[X], for a Geometric distribution is given by the formula: E[X]=p1
Substituting the value of p, we get: E[X]=1/61=6
Therefore, the expected number of throws is 6.
Q28GATE 2015NAT2MEngineering Mathematics
Consider the differential equation dt2d2x(t)+3dtdx(t)+2x(t)=0 . Given x(0) = 20 and x(1) = 10/e, where e = 2.718, the value of x(2) is _________.
First, we find the general solution to the homogeneous differential equation. The characteristic equation is m2+3m+2=0, which factors into (m+1)(m+2)=0. The roots are m1=−1 and m2=−2, so the general solution is x(t)=C1e−t+C2e−2t.
Next, we use the given conditions to find the constants C1 and C2. The condition x(0)=20 gives us C1+C2=20. The condition x(1)=10/e gives C1e−1+C2e−2=10e−1, which simplifies to C1+C2e−1=10.
Solving this system of two linear equations yields C1=e−110e−20 and C2=e−110e.
Finally, we substitute these constants back into the solution and evaluate at t=2: x(2)=C1e−2+C2e−4=(e−110e−20)e−2+(e−110e)e−4≈0.857
Q29GATE 2015NAT2MElectromagnetics
A vector field D=2ρ2aρ+zaz exists inside a cylindrical region enclosed by the surfaces ρ = 1, z = 0 and z = 5. Let S be the surface bounding this cylindrical region. The surface integral of this field on S(∮∮SD.ds) is _______.
To find the total flux, we calculate the surface integral over the closed cylinder by summing the contributions from its three distinct faces: the top, the bottom, and the curved side wall.
Top Cap (z=5): Here, the differential surface is ds=ρdρdϕa^z. The only component of D=2ρ2a^ρ+za^z that contributes is the z-component. The flux is ∫02π∫01(z)∣z=5ρdρdϕ=5π.
Bottom Cap (z=0): Since the z-component of D is z, it becomes zero on this surface. Therefore, the flux through the bottom is 0.
Curved Side (ρ=1): The differential surface is ds=ρdϕdza^ρ. The flux comes from the ρ-component of D. The flux is ∫05∫02π(2ρ2)∣ρ=1dϕdz=2(2π)(5)=20π.
The total flux is the sum of these parts: 5π+0+20π=25π≈78.54.
Q30GATE 2015NAT2MNetwork Theory
In the circuit shown, the current I flowing through the 50 Ω resistor will be zero if the value of capacitor C (in μ F) is ______.
For the current I to be zero, the total impedance of the circuit must be infinite. This requires the impedance of the network to the right of the resistor, which has a parallel structure, to be infinite. A parallel circuit's impedance becomes infinite at resonance, which occurs when the denominator of its equivalent impedance expression equals zero.
The angular frequency is ω=5000 rad/s. The impedance of each 1 mH inductor is ZL=jωL=j(5000)(10−3)=j5Ω. The two parallel branches have impedances Z1=j5Ω and Z2=j5+jωC1.
The condition for infinite impedance is that the sum of the admittances is zero, or equivalently for this structure, the sum of the branch impedances in the denominator of the overall impedance expression is zero. j5Ω+(j5Ω+jωC1)=0 j10=−jωC1=ωCj
Solving for C gives: C=10ω1=10×50001=20×10−6 F=20μF.
Q31GATE 2015MCQ2MNetwork Theory
The ABCD parameters of the following 2-port network are
To determine the ABCD parameters, we start by considering an open circuit at the output port, which sets the output current I2=0. With the output open, the input current I1 flows through the (5+j4)Ω impedance and the 2Ω shunt impedance. The input voltage V1 is the total voltage drop across this path: V1=(5+j4+2)I1=(7+j4)I1. The output voltage V2 is the voltage across the shunt element, so V2=2I1.
Now, we can find parameters A and C from their definitions under the open-circuit condition: A=V2V1I2=0=2I1(7+j4)I1=3.5+j2 C=V2I1I2=0=2I1I1=0.5
These calculated values for A and C match the first column of the matrix in the correct option, allowing us to identify the answer without needing to find B and D.
Q32GATE 2015MCQ2MControl Systems
A network is described by the state model as x1˙=2x1−x2+3ux2˙=−4x2−uy=3x1−2x2 The transfer function H(s)(=U(s)Y(s)) is
To find the transfer function from the state-space model, we first identify the system matrices A, B, and C from the given equations.
A=[20−1−4]
,
B=[3−1]
, and
C=[3−2]
The transfer function is calculated using the standard formula H(s)=C(sI−A)−1B. We start by finding the inverse of (sI−A):
(sI−A)−1=[s−201s+4]−1=(s−2)(s+4)1[s+40−1s−2]
Now, we substitute all the matrices into the transfer function formula and multiply them together:
H(s)=(s−2)(s+4)1[3−2][s+40−1s−2][3−1]
Carrying out the multiplication gives a numerator of 3(3(s+4)+1)−2(−(s−2))=9s+39+2s−4=11s+35. Therefore, the complete transfer function is (s−2)(s+4)11s+35.
Q33GATE 2015MCQ2MElectronic Devices
The electric field profile in the depletion region of a p-n junction in equilibrium is shown in the figure. Which one of the following statements is NOT TRUE?
The potential difference across the depletion region, also known as the built-in potential (Vbi), is the area under the electric field versus position graph. The profile shown is a triangle.
First, let's identify the triangle's dimensions from the graph, ensuring consistent units. The height is the peak electric field, Emax=104 V/cm, which is 106 V/m. The base is the total depletion width, W, spanning from −0.1μm to 1.0μm, so W=1.1μm=1.1×10−6 m.
Now, we calculate the area: Vbi=Area=21×base×height Vbi=21×(1.1×10−6 m)×(106 V/m)=0.55 V
The calculated potential difference is 0.55 V, or 550 mV. This contradicts statement C, which claims the potential difference is 700 mV.
Q34GATE 2015NAT2MElectronic Devices
The current in an enhancement mode NMOS transistor biased in saturation mode was measured to be 1 mA at a drain-source voltage of 5 V. When the drain-source voltage was increased to 6 V while keeping gate-source voltage same, the drain current increased to 1.02 mA. Assume that drain to source saturation voltage is much smaller than the applied drain-source voltage. The channel length modulation parameter λ (in V−1 ) is _______.
In the saturation region, an ideal MOS transistor's drain current (ID) is independent of the drain-source voltage (VDS). However, a real transistor exhibits channel length modulation, causing the current to increase slightly with VDS. This effect is modeled by the equation ID=ID,sat(1+λVDS).
We can determine the channel length modulation parameter, λ, by observing how the current changes with voltage. Using the given data points, we can approximate this relationship with finite differences.
Let's calculate the change in current (ΔID) and the change in voltage (ΔVDS): ΔID=1.02 mA−1.0 mA=0.02 mA ΔVDS=6 V−5 V=1 V
Now, we can rearrange the approximate relationship ΔID≈ID1⋅λ⋅ΔVDS to solve for λ. Using the initial current as our reference, we find: λ=ID1⋅ΔVDSΔID=1 mA×1 V0.02 mA=0.02 V−1
Q35GATE 2015NAT2MElectronic Devices
An npn BJT having reverse saturation current IS=10−15 A is biased in the forward active region with VBE = 700 mV. The thermal voltage ( VT ) is 25 mV and the current gain ( β ) may vary from 50 to 150 due to manufacturing variations. The maximum emitter current (in μ A) is ________.
First, we determine the collector current (IC) using the standard BJT equation for the forward-active region. This current depends on the base-emitter voltage, not the current gain β. IC=ISeVBE/VT
The emitter current (IE) is related to the collector current by IE=ββ+1IC. To find the maximum possible IE, we must maximize the factor ββ+1, which can be written as 1+β1. This expression is largest when β is at its minimum value.
Given the manufacturing variation, the minimum β is 50. Substituting this into the combined expression for IE gives the maximum value: IE,max=5050+1×ISeVBE/VT IE,max=1.02×(10−15 A)×e(700/25)≈1475μA
Q36GATE 2015MCQ2MDigital Circuits
A three bit pseudo random number generator is shown. Initially the value of output Y≡Y2Y1Y0 is set to 111. The value of output Y after three clock cycles is
This circuit functions as a 3-bit synchronous shift register with feedback. The initial output value is given as Y=Y2Y1Y0=111.
At each rising clock edge, the values shift to the right (Y2 moves to Y1, and Y1 moves to Y0), while a new value for Y2 is loaded from the output of the XOR gate, which computes Y1⊕Y0.
Let's trace the state for three cycles:
Initial State:Y=111. The next input for Y2 is 1⊕1=0. After the 1st clock cycle, the state becomes 011.
After 1 cycle:Y=011. The next input for Y2 is 1⊕1=0. After the 2nd clock cycle, the state becomes 001.
After 2 cycles:Y=001. The next input for Y2 is 0⊕1=1. After the 3rd clock cycle, the final state is 100.
Q37GATE 2015MCQ2MDigital Circuits
A universal logic gate can implement any Boolean function by connecting sufficient number of them appropriately. Three gates are shown. Which one of the following statements is TRUE?
A logic gate is considered universal if, by itself, it can be used to build any other logic function. This requires the ability to create a "functionally complete" set of operations, such as {AND, NOT} or {OR, NOT}.
Gate 1 (OR) and Gate 2 (AND) are not universal because neither gate can be used to create an inverter (a NOT gate). You can't produce Xˉ using only OR gates or only AND gates.
Gate 3, however, with the function F3=Xˉ+Y, can act as an inverter. If we set the input Y to a logic 0, the function becomes F3(X,0)=Xˉ+0=Xˉ. Since this single gate can produce both inversion and an OR-like operation, it is functionally complete and therefore universal.
Q38GATE 2015MCQ2MDigital Circuits
An SR latch is implemented using TTL gates as shown in the figure. The set and reset pulse inputs are provided using the push-button switches. It is observed that the circuit fails to work as desired. The SR latch can be made functional by changing
The primary issue is how the input logic levels are generated by the switches. In the provided configuration, the inputs to the inverters are tied to a 5 V source. Pressing a switch connects the inverter input to 5 V, which is a logic 1. Crucially, for TTL logic, leaving the switch open creates a floating input, which is also interpreted as a logic 1.
This means the inverter inputs are always at logic 1, regardless of whether the buttons are pressed. Consequently, the inputs to the NOR-gate latch are always logic 0, preventing it from ever being set or reset. By connecting the switches to ground instead of 5 V, pressing a switch provides a logic 0 input, while leaving it open provides a logic 1 (due to the pull-up nature of TTL inputs). This creates the necessary distinct logic levels to operate the latch.
Q39GATE 2015NAT2MAnalog Circuits
In the circuit shown, assume that the opamp is ideal. If the gain ( vo/v∈ ) is -12, the value of R (in k Ω ) is _____.
Due to the ideal op-amp, the inverting terminal is a virtual ground (0V). Let's label the voltage at the junction of the T-network resistors as Vx.
Applying KCL at the inverting input node gives 10kv∈−0+10kVx−0=0, which simplifies to Vx=−v∈.
Next, applying KCL at node Vx, we sum the currents leaving the node: 10kVx−0+RVx−0+10kVx−vo=0.
Substitute Vx=−v∈ into this second equation and rearrange to find the overall gain: v∈vo=−2−R10k.
Given the gain is -12, we can set up the equation −12=−2−R10k. Solving this gives 10=R10k, which means R=1 kΩ.
Q40GATE 2015NAT2MAnalog Circuits
In the circuit shown, both the enhancement mode NMOS transistors have the following characteristic: kn=μnCox(W/L)=1mA/V2;VTN=1V . Assume that the channel length modulation parameter λ is zero and body is shorted to source. The minimum supply voltage VDD (in volts) needed to ensure that transistor M1 operates in saturation mode of operation is _______.
For transistor M1 to operate in saturation, its drain-source voltage must satisfy VDS1≥VGS1−VTN. To find the minimum supply voltage, we place M1 at the edge of saturation, where VDS1=VGS1−VTN=2V−1V=1V. This sets the drain voltage of M1 to VD1=1V.
The saturation current flowing through M1 is ID1=kn(VGS1−VTN)2=(1)(2−1)2=1 mA. Since the transistors are in series, this same current flows through the diode-connected transistor M2, which is also in saturation.
For M2, we find the required gate-source voltage to sustain this current: ID2=kn(VGS2−VTN)2. Plugging in the values gives 1=1×(VGS2−1)2, which yields VGS2=2V.
Finally, we relate this to the supply voltage. We know that VGS2=VG2−VS2. From the circuit, VG2=VDD and VS2=VD1=1V. Therefore, 2V=VDD−1V, which means the minimum supply voltage is VDD=3V.
Q41GATE 2015NAT2MAnalog Circuits
In the circuit shown, assume that the diodes D1andD2 are ideal. The average value of voltage Vab (in Volts), across terminals 'a' and 'b' is _________.
The circuit's behavior is analyzed over one full period of the input signal, v∈(t)=6πsin(ωt). The diodes cause the output voltage, Vab, to be different for the positive and negative half-cycles of the input.
During the positive half-cycle (v∈>0), the circuit produces an output Vab that is a half-sine wave with a peak amplitude of Vm/2. During the negative half-cycle (v∈<0), the circuit rectifies the input to produce a positive half-sine wave for Vab with a peak amplitude of ∣Vm,neg∣/3=Vm/3.
The total average value of Vab is the sum of the average values from each half-cycle. The average value of a half-wave rectified sine signal with peak amplitude A is A/π. Applying this, we find the contribution from each part.
The total average voltage is Vavg=π(Vm/2)+π(Vm/3). Given the input peak voltage Vm=6π V, we can calculate the result: Vavg=π6π/2+π6π/3=π3π+π2π=3+2=5 V.
Q42GATE 2015MCQ2MSignals and Systems
Suppose x[n] is an absolutely summable discrete-time signal. Its z-transform is a rational function with two poles and two zeroes. The poles are at z=±2j . Which one of the following statements is TRUE for the signal x[n]?
The fact that the signal x[n] is absolutely summable is a critical piece of information. It guarantees that the Discrete-Time Fourier Transform (DTFT) exists. For the DTFT to exist, the Region of Convergence (ROC) of the z-transform, X(z), must include the unit circle (∣z∣=1).
The poles of X(z) are given at z=±2j. Both of these poles have a magnitude of ∣±2j∣=2.
The ROC is a ring in the z-plane that cannot contain any poles. If the signal were causal, its ROC would be the region outside the outermost pole, which is ∣z∣>2. This region does not include the unit circle.
Therefore, for the ROC to include the unit circle, it must be the region ∣z∣<2. An ROC defined as the interior of a circle corresponds to a non-causal signal.
Q43GATE 2015MCQ2MSignals and Systems
A realization of a stable discrete time system is shown in the figure. If the system is excited by a unit step sequence input x[n], the response y[n] is
First, we determine the system's transfer function H(z) from the block diagram's structure. By relating the input X(z) to the output Y(z) through the internal nodes, we find H(z)=X(z)Y(z)=z2−z+9235(1−z).
The input is a unit step sequence, so its Z-transform is X(z)=z−1z. The output transform is then Y(z)=H(z)X(z).
Substituting and factoring the denominator gives Y(z)=(z−31)(z−32)35(1−z)⋅z−1z. The (1−z) and (z−1) terms cancel, leaving a negative sign, which results in Y(z)=(z−31)(z−32)−35z.
To prepare for the inverse transform, we perform partial fraction expansion on zY(z), which yields zY(z)=z−315−z−325. Multiplying by z gives Y(z)=z−315z−z−325z.
Taking the inverse Z-transform of each term gives the final time-domain response: y[n]=5(31)nu[n]−5(32)nu[n].
Q44GATE 2015NAT2MSignals and Systems
Let x~[n]=1+cos(8πn) be a periodic signal with period 16. Its DFS coefficients are defined by ak=161∑n=015x~[n]exp(−j8πkn) for all k . The value of the coefficient a31 is _______.
To find the Discrete Fourier Series (DFS) coefficients, we can first express the signal using Euler's formula. The fundamental frequency for a period of N=16 is ω0=162π=8π.
The signal can be rewritten as: x~[n]=1+21ej8πn+21e−j8πn
This form directly corresponds to the DFS synthesis equation, x~[n]=∑k=015akejk8πn. By comparing the terms, we can identify the non-zero coefficients by inspection: a0=1, a1=0.5, and a−1=0.5.
The DFS coefficients are periodic with period N=16, meaning ak=ak+16. To find the coefficient a31, we can use this property: a31=a31−2×16=a31−32=a−1
Therefore, the value of a31 is equal to a−1, which is 0.5.
Q45GATE 2015NAT2MSignals and Systems
Consider a continuous-time signal defined as x(t)=((πt/2)sin(πt/2))∗∑n=−∞∞δ(t−10n) where '*' denotes the convolution operation and t is in seconds. The Nyquist sampling rate (in samples/sec) for x(t) is _____.
To find the Nyquist rate, we must first determine the maximum frequency, fmax, of the signal x(t). The signal is a convolution of two functions, which corresponds to the multiplication of their spectra in the frequency domain.
The first function, πt/2sin(πt/2), is a sinc function whose Fourier transform is a rectangular pulse limited to ∣f∣<0.25 Hz. The second function is an impulse train with period T=10s, whose Fourier transform is an impulse train with spacing 1/T=0.1 Hz.
Multiplying these two spectra means we only keep the frequency impulses that fall within the sinc function's bandwidth. These impulses are at f=0,±0.1, and ±0.2 Hz. The impulse at ±0.3 Hz and higher are eliminated.
Therefore, the maximum frequency component in the final signal is fmax=0.2 Hz. The Nyquist sampling rate is twice this value, calculated as fs=2×fmax=2×0.2=0.4 samples/sec.
Q46GATE 2015NAT2MControl Systems
The position control of a DC servo-motor is given in the figure. The values of the parameters are KT=1N−m/A , Ra=1Ω , La=0.1H,J=5kg−m2 , B=1N−m(rad/sec) and Kb=1V/(rad/sec) . The steady-state position response (in radians) due to unit impulse disturbance torque Td is_______.
To find the steady-state position error caused by the disturbance torque, we first need the transfer function from the disturbance input Td(s) to the position output θ(s). We can find this by setting the other input, Va(s), to zero and applying block diagram reduction rules.
The transfer function is given by: Td(s)θ(s)=1+Loop GainForward Path=1+(Ra+Las)(Js+B)KTKb−s(Js+B)1
Simplifying this expression by multiplying the numerator and denominator by (Ra+Las)(Js+B) gives: Td(s)θ(s)=s[(Ra+Las)(Js+B)+KTKb]−(Ra+Las)
The problem specifies a unit impulse disturbance, so Td(s)=1. To find the steady-state position, we apply the Final Value Theorem: θss=lims→0sθ(s). θss=lims→0s(s[(Ra+Las)(Js+B)+KTKb]−(Ra+Las)⋅1)=RaB+KTKb−Ra
Substituting the given values Ra=1Ω, B=1, KT=1, and Kb=1: θss=(1)(1)+(1)(1)−1=2−1=−0.5 radians
Q47GATE 2015NAT2MControl Systems
For the system shown in the figure, s = -2.75 lies on the root locus if K is _______.
For the point s=−2.75 to be on the root locus, it must satisfy the magnitude condition, ∣G(s)H(s)∣=1. The system's open-loop transfer function is given as G(s)H(s)=s+210K(s+3).
To find the value of gain K, we substitute s=−2.75 into the magnitude condition: ∣G(s)H(s)∣s=−2.75=−2.75+210K(−2.75+3)=1
Evaluating the absolute values of the terms, we get: ∣−0.75∣10K∣0.25∣=0.7510K(0.25)=1
Solving this equation for K gives: K=10×0.250.75=2.50.75=0.3
Q48GATE 2015NAT2MControl Systems
The characteristic equation of an LTI system is given by F(s)=s5+2s4+3s3+6s2−4s−8=0 The number of roots that lie strictly in the left half s-plane is _________.
To determine the number of roots in the left half-plane, we use the Routh-Hurwitz stability criterion. We begin by constructing the Routh array from the characteristic equation.
In the process, we encounter a row of all zeros at the s3 level. This special case indicates the presence of roots that are symmetric about the origin. We form an auxiliary equation, A(s), using the coefficients from the row above the zeros (s4 row): A(s)=2s4+6s2−8=0.
To complete the array, we differentiate A(s) to get dsdA=8s3+12s and use its coefficients (8 and 12) to replace the zero row. After completing the array, we analyze its first column. There is exactly one sign change (from 3100 to −8), which tells us there is one root in the right half-plane (RHP).
The total number of roots is 5. The auxiliary equation A(s)=0 can be solved to find the symmetric roots: s=±1 and s=±j2. This shows that two roots lie on the imaginary axis. Since these are not strictly in the LHP, we subtract them. Therefore, the number of roots strictly in the LHP is the total number of roots minus those in the RHP and on the imaginary axis: 5−1−2=2.
Q49GATE 2015NAT2MSignals and Systems
Two sequences x1[n]andx2[n] have the same energy. Suppose x1[n]=α0.5nu[n] , where α is a positive real number and u[n] is the unit step sequence. Assume
To find the value of α, we'll calculate the energy of each sequence and set them equal, as stated in the problem. The energy of a signal x[n] is the sum of its squared magnitudes, E=∑n=−∞∞∣x[n]∣2.
First, let's find the energy of x1[n]. The calculation involves summing an infinite geometric series: E1=∑n=0∞∣α(0.5)n∣2=α2∑n=0∞(0.25)n=α2(1−0.251)=34α2.
Next, we find the energy of x2[n] by summing its squared values at the two non-zero points: E2=∣1.5∣2+∣1.5∣2=1.5+1.5=3.
By equating the two energies, we get the equation 34α2=3. Solving for α gives α2=49. Since α is a positive real number, we take the positive square root, which yields α=1.5.
Q50GATE 2015NAT2MCommunication Systems
The variance of the random variable X with probability density function f(x)=21∣x∣e−∣x∣ is_______.
To find the variance, we first need the mean, E[X]. The probability density function f(x)=21∣x∣e−∣x∣ is an even function, as it is symmetric about the y-axis. This symmetry implies that the mean is zero, so E[X]=0.
The formula for variance is \text{Var}(X) = E\[X^2]$ - (E[X])^2.SinceE[X]=0,thevariancesimplifiestojustE[X^2]$.
We calculate E[X2] by solving the integral ∫−∞∞x2f(x)dx: E$[X2]$=∫−∞∞x2(21∣x∣e−∣x∣)dx=21∫−∞∞x2∣x∣e−∣x∣dx
The integrand x2∣x∣e−∣x∣ is also an even function. We can simplify the calculation by changing the integration bounds to [0,∞) and multiplying by 2. For x≥0, ∣x∣=x. Var(X)=2⋅21∫0∞x2(x)e−xdx=∫0∞x3e−xdx
This integral is a standard form of the Gamma function, Γ(z)=∫0∞tz−1e−tdt, which equals (z−1)! for integer z. With z−1=3, we have z=4, so the integral evaluates to Γ(4)=3!=6.
Q51GATE 2015MCQ2MCommunication Systems
The complex envelope of the bandpass signal x(t)=−2(πt/5sin(πt/5))sin(π−4π) , centered about f=21 Hz, is
To find the complex envelope, we first express the given bandpass signal, x(t), in the standard form x(t)=a(t)cos(2πfct+ϕ(t)). The complex envelope is then defined as x~(t)=a(t)ejϕ(t). The carrier frequency is given as fc=1/2 Hz, so the angular frequency is ωc=2πfc=π.
Let's simplify the trigonometric part of the signal, −sin(πt−4π). Using the identity −sin(θ)=cos(θ+2π), we can rewrite this term: −sin(πt−4π)=cos((πt−4π)+2π)=cos(πt+4π).
Substituting this back into the original expression for x(t) yields: x(t)=2(πt/5sin(πt/5))cos(πt+4π).
By comparing this to the standard form, we can identify the real amplitude envelope a(t)=2(πt/5sin(πt/5)) and the phase ϕ(t)=4π. The complex envelope is therefore x~(t)=a(t)ejϕ(t).
Q52GATE 2015NAT2MCommunication Systems
A random binary wave y(t) is given by y(t)=∑−∞∞Xnp(t−nT−ϕ) where u(t) = u(t) - u(t -T), u(t) is the unit step function and ϕ is an independent random variable with uniform distribution in [0,T]. The sequence { Xn } consists of independent and identically distributed binary valued random variables with P{Xn=+1}=P{Xn=−1}=0.5 for each n. The value of the autocorrelation Ryy(43T)≐E[y(t)y(t−43T)] equals ________.
The autocorrelation function for this type of random binary signal, which is made wide-sense stationary by the random phase ϕ, has a well-known form. It is determined by the autocorrelation of the fundamental rectangular pulse, p(t).
For a rectangular pulse of duration T, the resulting process autocorrelation is a triangular function. The formula is given by: Ryy(τ)=1−T∣τ∣ for ∣τ∣≤T.
We need to find the value at τ=43T. Plugging this into the equation: Ryy(43T)=1−T∣3T/4∣=1−43
This simplifies to 41, or 0.25.
Q53GATE 2015MCQ2MElectromagnetics
Consider the 3 m long lossless air-filled transmission line shown in the figure. It has a characteristic impedance of 120 πΩ , is terminated by a short circuit, and is excited with a frequency of 37.5 MHz. What is the nature of the input impedance ( Z∈ )?
To determine the nature of the input impedance, we first need to relate the line's physical length to the signal's wavelength. For a frequency f=37.5 MHz in an air-filled line, the wavelength is λ=c/f=(3×108 m/s)/(37.5×106 Hz)=8 m.
The input impedance of a short-circuited lossless line is given by Z∈=jZ0tan(βl), where the phase constant is β=2π/λ. The argument of the tangent, βl, represents the electrical length of the line: βl=λ2πl=8 m2π⋅3 m=43π radians.
Substituting this into the impedance equation, we get: Z∈=j(120πΩ)tan(43π)
Since tan(43π)=−1, the input impedance is Z∈=−j120πΩ. An impedance with a negative imaginary part is purely capacitive.
Q54GATE 2015NAT2MElectromagnetics
A 200 m long transmission line having parameters shown in the figure is terminated into a load RL . The line is connected to a 400 V source having source resistance RS through a switch, which is closed at t = 0. The transient response of the circuit at the input of the line (z = 0) is also drawn in the figure. The value of RL (in Ω ) is ________.
When the switch closes at t=0, the source voltage divides between the source resistance and the line's characteristic impedance. This launches an initial voltage wave onto the line: V+=VSRS+R0R0=400 V×150Ω+50Ω50Ω=100 V.
The graph shows this 100 V level at the input until t=2.0μs, which is the round-trip time for the wave to travel to the load and back. At this point, the voltage changes to 62.5 V due to the superposition of the initial wave and the first reflection. The voltage at the input after one round trip is V(0,2T0)=V++V+ΓL(1+Γ∈).
The reflection coefficient at the source input is Γ∈=RS+R0RS−R0=150+50150−50=0.5.
Plugging the values into our voltage equation: 62.5=100+100×ΓL(1+0.5).
Solving for the load reflection coefficient, we get 150ΓL=−37.5, so ΓL=−0.25.
Finally, we use the formula for the load reflection coefficient to find RL: ΓL=RL+R0RL−R0⟹−0.25=RL+50RL−50. This gives RL=30Ω.
Q55GATE 2015NAT2MElectromagnetics
A coaxial capacitor of inner radius 1 mm and outer radius 5 mm has a capacitance per unit length of 172 pF/m. If the ratio of outer radius to inner radius is doubled, the capacitance per unit length (in pF/m) is ________.
The capacitance per unit length (CL) for a coaxial capacitor is inversely proportional to the natural logarithm of the ratio of the radii (b/a). The formula is CL=ln(b/a)2πϵ, where the term 2πϵ is a constant for the given dielectric material.
Let's denote the initial state with subscript 1 and the final state with subscript 2. We can set up a ratio to find the new capacitance: CL1CL2=2πϵ/ln(b/a)12πϵ/ln(b/a)2=ln(b/a)2ln(b/a)1
Initially, CL1=172 pF/m and the ratio (b/a)1=5/1=5. The ratio is then doubled, so (b/a)2=2×5=10.
Plugging these values into our ratio equation: CL2=CL1×ln(10)ln(5)=172 pF/m×ln(10)ln(5)
This calculation gives a new capacitance per unit length of approximately 120.22 pF/m.