MPPGCL JE Electronics Mock Test - 2 - Electronics and Communication Engineering (ECE) MCQ

Avalanche breakdown and temperature effect:

• In avalanche breakdown, a bunch of electrons knocks out another electron to conduction band creating electron-hole pair.
• Due to an increase in temperature, vibrations, of atoms increases and thus reduces the mean free path for electrons.
• Hence in avalanche breakdown voltage increases with temperature.

Hence it is a positive temperature coefficient.

Zener breakdown and temperature effect:

• It occurs when the electric field is much stronger to move the electrons from the valence band to the conduction band.
• As temperature increases, bandgap decreases so less electric field is required to
• Hence Zener breakdown voltage decreases as the temperature increases. Thus it has a negative temp coefficient.

Digital Modulation Techniques:

1) Amplitude shift key

2) Frequency shift key

3) Phase Shift key

4) QAM

ASK System:

1. In the ASK modulation scheme, a finite number of amplitudes are used for binary 1 and 0 transmission.

2. For ASK Transmitter on-off keying is used.

3. In Amplitude Shift Keying (ASK) binary 1 is represented with the presence of carrier and binary 0 is represented with the absence of the carrier.

1 : s₁(t) = A_c cos 2πf_ct

0 : s₂ (t) = 0

Hence,

ASK = [± 10 V, 0, ± 10 V, ± 10 V, ± 10 V, ± 10 V, 0, 0]

Important Point

FSK (Frequency Shift Keying):

In FSK (Frequency Shift Keying) binary 1 is represented with a high-frequency carrier signal and binary 0 is represented with a low-frequency carrier, i.e. In FSK, the carrier frequency is switched between 2 extremes.

For binary ‘1’ → S₁ (A) = Acos 2π f_Ht

For binary ‘0’ → S₂ (t) = A cos 2π f_Lt . The constellation diagram is as shown:

PSK(Phase Shift Keying):

In PSK (phase shift keying) binary 1 is represented with a carrier signal and binary O is represented with 180° phase shift of a carrier

For binary ‘1’ → S₁ (A) = Acos 2π f_ct

For binary ‘0’ → S₂ (t) = A cos (2πf_ct + 180°) = - A cos 2π f_ct

The Constellation Diagram Representation is as shown:

Concept:

The sampling frequency of the bandpass signal is given by:

Where

  (Where [ .] denotes the floor value)

Analysis:

Given:

f_L = 390 kHz, f_H = 410 kHz

f_s ≈ 41 kHz 

CONCEPT:

Fermi level: 

• The highest energy level that an electron can occupy at the absolute zero temperature is known as the Fermi Level.
• The Fermi level lies between the valence band and conduction band because at absolute zero temperature the electrons are all in the lowest energy state. Due to a lack of sufficient energy at 0 Kelvin, the Fermi level can be considered as the sea of fermions (or electrons) above which no electrons exist.

​

Hence the valence band and conduction band for a semiconductor are nearer to the fermi level.

The transition band characteristics we are usually interested in deal with the shape or steepness of the roll-off between the passband and the stopband. Usually, the shape factor is defined as shown in Figure (which shows an otherwise ideal filter with just the addition of transition bands). It is defined for two levels of attenuation and is usually taken to be the ratio between 3dB of attenuation (passband) and given stopband attenuation.

For example, we could take 3dB to define the passband and 60dB to use the stopband.

Our ideal filter would have a shape factor of unity, but where this is not physically realizable we seek the smallest shape factor we can. For reference, simple RLC filters might have shape factors in the range of ~3, SAW filters in the range of ~1.5.

     ---(1)

Calculation:

Given:

BW at 60 dB = 10 kHz

BW at 3 dB = 4 kHz

From equation (1)

Concept:

Time-domain specification (or) transient response parameters:

Peak Time (tp):

It is the time taken by the response to reach the maximum value.

Settling time (Ts):

It is the time taken by the response to reach ± 2% tolerance band as shown in the fig above.

  for a 5% tolerance band.

 for 2% tolerance band

If ξ increases, rise time, peak time increases and peak overshoot decreases.

Delay time

The time taken by a response to rising from 0 to 50% of the final value is known as ‘delay time’.

Peak overshoot is the maximum error at the output.

Peak overshoot, 

% peak overshoot,

Important Points

• A practical second-order system should be required to meet all the specifications mentioned above.
• If it is an underdamped system, the system needs to satisfy the overshot to a given step input.
• Settling time is another specification that needs to be as less as possible so that the system will reach a steady-state in less time.
• Steady-state accuracy gives, how close the output of the system is to the required output.
• But, variations in step output are not a required specification for a second-order system.

Concept:

Maximum power transfer theorem:

Maximum power transfer theorem states that " In a linear bilateral network if the entire network is represented by its Thevenin's equivalent circuit then the maximum power transferred from source to the load when the load impedance is equal to the complex conjugate of  Thevenin's impedance".

Let's consider variable resistive load and Thevenin's equivalent network as shown below,

Apply KVL, for the above circuit

V_th - I_LR_th - I_LR_L = 0

Where

V_th is the source or Thevenin's voltage, I_L is the load current, R_L is the load resistance, R_th is the source or Thevenin's resistance

V_th = I_L (R_th + R_L)

I_L = V_th / (R_th + RL)

P = I_L² R_L

P = 

For maximum power transfer, R_L = R_S

Then the maximum power transferred is given by

Calculation:

To find V_th:

By applying the voltage division rule, we get

V_th = (18 × 6) / (12 + 6) 

⇒ V_th = 6 V

To find R_th:

From the above circuit,

R_th = 12 // 6 = (12 × 6) / (12 + 6) 

⇒ R_th = 4 Ω 

Then the maximum power transferred is given by

⇒ P_max = (6)² / 4 × 4 = 2.25 W

Concept:

In the RADAR system, the range of the target is:

And,

PRT = (PW + RT) μ sec

Where

C = 3 × 10⁸ m/s = speed of light at which EMW travel

PRT = Pulse repetition time

PW = Pulse width

RT = rest time i.e. time interval between sending a pulse and return from target to the receiver.

Calculation:

Given that,

PW = 2 μ sec

RT = 10 μ sec

∴ PRT = 2 + 10 = 12 μ sec

If we take an approximate value of PRT, we can write:

PRT = 2 + 10 ≈ 10 μ sec

Concept:

Min Terms: 

• A minterm is a boolean expression written for all those terms whose value is 1 in the K-map.
• It is denoted by: F(x, y, z) = ∑(minterms)

​MaxTerms: 

• A maxterm is a boolean expression written for all those terms whose value is 0 in the K-map.
• It is denoted by: F(x, y, z) = π(max terms)
• Max terms are the compliments of minterms.

Explanation:

Given,  F(x, y, z) = ∑(1, 3, 6, 7)

Binary representation is:

 F(x, y, z) = ∑(001, 011, 010, 011)

It is minterm representation.

F(x, y, z) = ∑(000, 010, 100, 101)

F(x, y, z) = ∏ (0, 2, 4, 5)

It is max max-term representation.

1) When the unwanted thermal noise exceeds the present threshold level, then false alarms are generated.

2) If the threshold is set too low, the large number of false alarms will mask the detection of valid targets. 

3) If the threshold is set too higher there will be very few false alarms, but it can inhibit the detection of valid targets.

EXPLANATION:

Faraday’s Law states that a change in magnetic flux induces an emf in a coil.

Also, Lenz’s Law states that this induced emf produces a flux which opposes the flux that generates this emf, i.e.

     ----(1)

EMF is also defined as:

Also, 

Putting the above in Equation (1), we get:

Additional Information

Maxwell's Equations for time-varying fields is as shown:

The correct answer is option 1):(Reversed biasing)

Concept:

Diode

A diode is a semiconductor device that essentially acts as a one-way switch for current.

It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction

The reverse bias of the diode

• When an external voltage (V ) is applied across the diode such that the n-side is positive and the p-side is negative, it is said to be reverse-biased.
• The applied voltage mostly drops across the depletion region. The direction of the applied voltage is the same as the direction of barrier potential.
• As a result, the barrier height increases, and the depletion region widens due to the change in the electric field.
• When a diode is given an external voltage that increases its potential barrier and restricts the flow of current.

Additional Information
Forward biased diode:

• In the forward bias, the p side of the diode is connected to the positive side of the battery and the n side is connected to the negative side of the battery.
• The direction of the applied voltage is opposite to the junction barrier potential. Therefore, the size of the depletion region decreases.

The given circuit is redrawn as:

DIAGRAM

The output Q will be:

Applying De-Morgan's Law, the above expression becomes:

Q = A ⊕ B, i.e. A EXOR B

The correct answer is Magnetic Tape.

• Magnetic tape is a secondary storage device used for storing files of data; Example: a company’s payroll record.
• Access is sequential and consists of records that can be accessed one after another as the tape moves along a stationary read-write mechanism
• A hard disk is an electromechanical data storage device from which data is accessed in a random-access manner, individual blocks of data can be stored or retrieved in any order
• Both sequential and random access of data is possible in hard disk
• In random-access memory (RAM) the memory cells can be accessed for information transfer from any desired random location
• That is, the process of locating a word in memory is the same and requires an equal amount of time no matter where the cells are located physically in memory; Hence RAM is random access

Concept:

Information associated with the event is “inversely” proportional to the probability of occurrence.

Entropy: The average amount of information is called the “Entropy”.

Rate of information = r.H

Calculation:

Given: r = 16 outcomes/sec

, and  

∴ Rate of information = r.H

R_s = 16 x 19/32

R_s = 19/2 or 38/4 bits/sec

Concept:

A system is linear, if the operations on the input signal are all linear and no signal-independent terms are contained. Basic linear operations are given below-

• Multiplication of input signal with a constant value: y(t) = ax(t)
• Time-shifting the input signal y(t) = x(t−t₁)
• Scaling of the input signal y(t) = x(bt)
• Combinations of scalling,shifting and multiplication , e.g. y(t) = ax(b(t−t₁))
• Summations of terms that are linear, e.g. y(t) = ax(t)+x(t−t₁)
• Convolution of two input signal, eg y(t) = x(t)*h(t)

Some non-linear operations are given below-

• Multiplication of the signal with itself, i.e. y(t) = x(t)⋅x(t)
• Applying any non-linear function to the signal, e.g. y(t) = sin(x(t))
• Adding constant terms, which are independent of the signal, i.e. y(t) = x(t)+a

Analysis:

(I)  d²y(t)/dt² + 9a₁dy(t)/dt + a₂y(t) = u(t) is linear

(ii) y(t) dy(t)/dt + a₁y(t) = a₂u(t) is non-linear due to multiplication of y(t)

(iii).  2d²y(t)/dt² + dy(t)/dt + t²y(t) = 5 is Non Linear because of the contant term 5.

PAM (Pulse Amplitude Modulation): 

The width and location of pulses in a modulated signal (PAM) are constant, while the amplitude of pulses varies proportionally with the amplitude of an analogical useful signal.

The pulses in a PAM signal may of Flat-top type or natural type or ideal type.

Generation of PAM:

There are two operations involved in the generation of PAM signal:

• Instantaneous sampling of the message signal m(t) every T_s seconds, where the sampling rate fs = 1/T_s is chosen in accordance with the sampling theorem. ('a' is True)
• Lengthening the duration of each sample so obtained to some constant value T. ('b' is True)

Concept:

In an extrinsic semiconductor, the conductivity (hence resistivity) depends on the number of carriers present and is given by:

n0 = majority carrier electron concentration

p₀ = majority carrier hole concentrationμn and μp are the electron and hole mobilities respectively.

The resistivity is the inverse of conductivity, i.e.

Calculation:

Since the intrinsic carrier concentration of Germanium is of the order of 10¹³, and the given donor density is of the order of 10²⁰, we can write:

n₀ = Nd = 1020 atoms/m3

Since there is no acceptor impurity, p₀ << n₀ and can be easily neglected and the resistivity becomes:

ρ = 0.164 Ωm

Chebyshev Filter:

• It is also called an equal rip­ple filter.
• It gives a sharper cut-off than Butterworth filter in the passband.
• It has a ripple response in the passband and flat response in the stopband. it is also known as an equal-ripple response.
• Both Butterworth and Chebyshev filters exhibit large phase shifts near the cut-off frequency.
• A drawback of the Chebyshev fil­ter is the appearance of gain maxima and minima below the cut-off frequency.
• This gain ripple, ex­pressed in dB, is an adjustable parameter in filter design.
• A Chebyshev filter is used where a very sharp roll-off is required. However, this is achieved at the expense of a gain ripple in the lower frequency passband.

Butterworth Filter:

• This filter is also called a maximally flat or flat filter. This class of filters approximates the ideal fit in the passband. 
• it's filter response characterized by flat passband. it is also known as a maximally flat response.
• The Butterworth filter has an essentially flat amplitude-fre­quency response up to the cut­off frequency. 
• Butterworth filters have the sharpest attenuation, their phase-shift as a function of frequency is non-linear.
• It has a monotonic drop in gain with frequency in the cut-off region and a maximally flat response below the cut-off frequency.
• The Butterworth filter has characteristics somewhere be­tween those of Chebyshev and Bessel filters.
• It has a moderate roll-off of the skirt and a slightly non­linear phase responses.

The truth table for ALL options is shown

NAND

From the truth table, it is clear that in NAND gate output is low when all inputs are high

Important Points

OR

AND

NOR

TDMA:

• Satellite communication among stations in different areas can be achieved if the satellite has the ability to switch time slots from one beam to another. This is known as time division multiple access (TDMA).
• In TDMA users transmit in rapid succession, one after the other, each using its time slots.

TSMA:

• The tone sense multiaccess protocol (TSMA) is designed for packet satellite communications.
• This protocol is particularly suitable for a satellite system serving an area with a dense population.

FAMA:

• In fixed- assignment multiple access (FAMA) channels are permanently assigned to terminals.
• Hence it is also known as a pre-assigned multiple access technique.

SCPC:

• Single-channel per carrier (SCPC) refers to using a single signal at a given frequency and bandwidth.
• Most often it is used as on broadcast satellite.

Concept:

Current in single tone modulation:

I_T = I_C√ ( 1 + μ²/2)

where;

I_T → Total current

I_C → Carrier's current

μ → Modulation index in single tone modulation

Current in multitone modulation:

 I_T = I_C√ ( 1 + μ_t²/2)

where;

 μ_t = √( μ₁² + μ₂² + μ₃² + ..... ) 

μ_t → Modulation index due to multitone modulation

Calculation:

Given;

Modulation index at single tone: μ = 40% = 0.4

Total current : I_T1 = 11 A

When simultaneous modulation by another audio sine wave.

Total current increased to I_T2 = 12 A

Since;

 IT ∝  √ ( 1 + μ²/2)

Hence,

Concept: 

ICEO is the reverse leakage current in the common-emitter configuration of BJT when the base is open.

ICBO is the reverse leakage current in the common-base configuration of BJT when the emitter is open.

Also, ICEO > ICBO

And they are related by the relation:

ICEO = (1 + β) ICBO

Total collector current is given by:

IC = βIB + ICBO(β + 1)

Application:

Given that

Common base current gain α = 0.97

So, the common-emitter current gain:

ICO = reverse saturation current = 0.4 μA

IB = Base current = 15 μA

Then collector current is given by  

I_C = βI_B + (1 + β) I_CO

I_C = 32.33 × 15 × 10^-6 + (1 + 32.33) 0.4 × 10^-6 

IC = 498.34 μA

The correct answer is All of the above.

Key Points

• The Internet is a huge collection of networks, a networking infrastructure. It encapsulates millions of computers together globally, forming a network in which any computer can communicate with any other computer as long as they are both connected to the Internet. In other words, it is a worldwide system of cross-connected computer networks, connecting millions of devices through which exchange of information such as data, news, and opinions, etc. is possible.
• It utilizes the TCP/IP (Transmission Control Protocol/Internet Protocol) to aid millions of users across the globe. So, TCP/IP can be called the backbone of the Internet. It is considered as a network of networks that consists of thousands of private and public, academic, business, and government interconnections. The Internet is often considered as “The Information Highway”, which implies that there is a straight and clear way of obtaining information. It connects thousands of computer networks. Each device connected to the Internet is known as the host and is independent. Through telephone wires, Fiber optical cable, and satellite links, Internet users can share a variety of information.
• The Internet is defined as an Information superHighway, to access information over the web. However, It can be defined in many ways as follows:
  • The Internet is a worldwide global system of interconnected computer networks.
  • The Internet uses the standard Internet Protocol (TCP/IP).
  • Each device connected to the internet is identified by its unique IP address.

The correct answer is option 4):(the same as that of Y)

Concept:

• The given circuit resembles the wheat stone bridge
• 
• The Wheatstone bridge works on the principle of null deflection, i.e. the ratio of their resistances is equal, and no current flows through the circuit. Under normal conditions, the bridge is in an unbalanced condition where current flows through the galvanometer. The bridge is said to be balanced when no current flows through the galvanometer.
• If the resistance of each resistor is 100 ohm, The bridge is at balance condition no current flows in the centre arm. the potential at X is equal to y

The given

Narrow bandpass filter

T = 0.25 msec

Now applying Fourier series formula on x(t), we get:

ω₀T = 2π

So, odd components are a₁, a₃, a₅, a₇, a₉…., and so on.

Since the filter passes frequencies only between 20 kHz and 40 kHz, the components lying between this band of frequency will be passed by the filter.

The fundamental frequency given is 4 kHz.

a₁ = 4 kHz

a₃ = 12 kHz

These 3 components of 20 kHz, 28 kHz, and 36 kHz will be passed and the rest of the components will be rejected by the filter.

a₁₁ → 44 kHz

a₁₃ → 52 kHz

       ⋮

And so on

∴ Option 2 is correct.

The correct answer is option 2.

Key Points

• Serial Line Internet Protocol (SLIP) is a Data Link layer simple protocol that works with TCP/IP for communication over serial ports and routers. They provide communications between machines that were previously configured for direct communication with each other. 
• Border Gateway Protocol (BGP) is a Network layer used to exchange routing information for the internet and is the protocol used between ISP which are different ASes.
• User Datagram Protocol (UDP) is a Transport Layer protocol. UDP is a part of the Internet Protocol suite, referred to as UDP/IP suite. Unlike TCP, it is an unreliable and connectionless protocol.
• SNMP is an application layer protocol that uses UDP port number 161/162.SNMP is used to monitor the network, detect network faults and sometimes even used to configure remote devices

∴ Hence the correct answer is A-III, B -IV, C -II, D -I.

Additional Information

The correct answer is Satluj.

• Bhakra canal has been drawn from the Satluj River near Nangal.

Key Points

• It’s a concrete gravity dam located in Bilaspur, Himachal Pradesh.
• Govind Sagar reservoir is formed by the dam.
• The length of the dam is 518.25 m and the width of the dam is 9.1 m.
• The reservoir of the dam is known as Govind Sagar.
• Bhakra dam has created a 90 km long reservoir.
• Pandit Jawaharlal Nehru described the dam as the “New temple of resurgent India”.

Gauss is the great mathematician who is credited with discovering the total value of the first 100 natural numbers.