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Past Questions — 2026
In single phase VSI, the harmonic which is not present is:
A)
2nd
B)
3rd
C)
5th
D)
7th
In a single-phase Voltage Source Inverter (VSI), the output voltage waveform is typically a square wave or a modified sine wave. For a standard square wave inverter, the output voltage can be represented by a Fourier series. The Fourier series for a square wave with amplitude Vdc and frequency f is given by: V(t) = Σ [4Vdc / (nπ)] * sin(nωt), where n = 1, 3, 5, 7... (odd integers).
1. Odd Harmonics: The series explicitly shows that only odd harmonics (1st, 3rd, 5th, 7th, etc.) are present in the output of a symmetric square wave inverter.
2. Even Harmonics: Because the waveform possesses half-wave symmetry (V(t) = -V(t + T/2)), all even harmonics (2nd, 4th, 6th, etc.) cancel out and are not present in the spectrum.
Comparing this to the options:
- Option a (2nd): This is an even harmonic, which is absent.
- Option b (3rd): This is an odd harmonic, which is present.
- Option c (5th): This is an odd harmonic, which is present.
- Option d (7th): This is an odd harmonic, which is present.
Therefore, the 2nd harmonic is the one not present in the output.
JFET is:
A)
Voltage-controlled device
B)
Current-controlled device
C)
Both voltage and current-controlled
D)
None of the above
A Junction Field Effect Transistor (JFET) is a type of field-effect transistor where the output current (drain-to-source current, Id) is controlled by the voltage applied to the gate terminal (Vgs).
1. **Mechanism**: In a JFET, the gate-source junction is reverse-biased. By varying the reverse-bias voltage (Vgs), the width of the depletion region within the semiconductor channel is altered.
2. **Control**: As the depletion region grows, it narrows the conductive channel, thereby restricting the flow of charge carriers (electrons in n-channel, holes in p-channel). Because the control mechanism is the electric field created by the voltage, it is classified as a voltage-controlled device.
3. **Comparison**:
- **Bipolar Junction Transistors (BJTs)** are current-controlled devices because a small base current (Ib) controls a larger collector current (Ic).
- **JFETs** have an extremely high input impedance because the gate is reverse-biased, meaning almost no current flows into the gate; only the voltage level matters.
Therefore, option (a) is correct, and option (b) is incorrect as it describes a BJT.
Describe how thyristors are protected from di/dt.
Microprocessors in SCADA systems in power networks are used for:
A)
Monitoring, control, and automation
B)
Voltage step-up
C)
Reactive compensation
D)
Generator excitation only
SCADA stands for Supervisory Control and Data Acquisition. In power networks, SCADA systems are used to manage large-scale electrical grids. Microprocessors serve as the 'brains' of the Remote Terminal Units (RTUs) and Programmable Logic Controllers (PLCs) within these systems. Their primary functions are: 1) Monitoring: Collecting real-time data (voltage, current, switch status) from the field. 2) Control: Sending commands to actuators or circuit breakers to change the state of the network. 3) Automation: Executing pre-programmed logic to respond to faults or load changes without human intervention.
Why other options are incorrect:
- Option b (Voltage step-up) is the function of a Power Transformer, which is a passive electromagnetic device, not a microprocessor-based system.
- Option c (Reactive compensation) is achieved using capacitors, inductors, or FACTS devices (like SVCs). While microprocessors may control these devices, the microprocessor itself does not perform the compensation.
- Option d (Generator excitation only) is too narrow. While microprocessors are used in Automatic Voltage Regulators (AVR) for excitation, SCADA is a system-wide architecture covering the entire network, not just the generator excitation.
Negative feedback in amplifiers generally:
A)
increases gain
B)
reduces bandwidth
C)
improves stability and bandwidth
D)
causes oscillation
Negative feedback occurs when a portion of the output signal is fed back to the input with a 180-degree phase shift, opposing the input signal.
1. **Stability**: Negative feedback reduces the sensitivity of the amplifier's gain to variations in temperature, component aging, or manufacturing tolerances. This makes the gain more constant and stable.
2. **Bandwidth**: The gain-bandwidth product of an amplifier is generally constant. Since negative feedback reduces the overall voltage gain (Gain_with_feedback = Gain_without_feedback / (1 + Aβ)), the bandwidth increases proportionally.
Analysis of other options:
- **Option a (increases gain)**: Incorrect. Negative feedback always reduces the overall gain of the amplifier.
- **Option b (reduces bandwidth)**: Incorrect. Negative feedback increases the bandwidth (extends the frequency range over which the amplifier operates).
- **Option d (causes oscillation)**: Incorrect. Positive feedback causes oscillation. Negative feedback is used specifically to prevent instability and oscillations.
A circuit with two stable states is called:
A)
Astable multivibrator
B)
Monostable multivibrator
C)
Relaxation oscillator
D)
Bistable multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems. They are classified based on the number of stable states they possess:
1. **Bistable Multivibrator (Option d):** As the prefix 'bi-' suggests, this circuit has two stable states. It will remain in either state indefinitely until an external trigger pulse forces it to switch to the other state. A common example is the Flip-Flop used in digital memory.
2. **Monostable Multivibrator (Option b):** The prefix 'mono-' means one. This circuit has only one stable state. When triggered, it switches to a quasi-stable state for a predetermined time and then automatically returns to its only stable state. It is often used as a 'one-shot' timer.
3. **Astable Multivibrator (Option a):** The prefix 'a-' means without. This circuit has no stable states. It continuously switches back and forth between two quasi-stable states without any external trigger, creating a square wave. This is essentially a free-running oscillator.
4. **Relaxation Oscillator (Option c):** This is a general term for an oscillator that generates a non-sinusoidal waveform (like a sawtooth or square wave) by charging and discharging a capacitor. While an astable multivibrator is a type of relaxation oscillator, the specific term for a circuit with 'two stable states' is the bistable multivibrator.
Frequency division multiplexing (FDM) allows:
A)
Multiple signals on different frequencies
B)
Multiple signals on same frequency
C)
Only one signal
D)
Time-sharing
Frequency Division Multiplexing (FDM) is a technique used in communication systems to transmit multiple independent signals over a single shared medium simultaneously. It achieves this by dividing the total available bandwidth of the communication channel into several non-overlapping frequency bands (sub-channels). Each signal is modulated onto a different carrier frequency, ensuring that they do not interfere with one another.
Analysis of options:
- Option (a) is correct because FDM assigns each signal a unique frequency slot.
- Option (b) is incorrect because transmitting multiple signals on the same frequency without a specific protocol (like CDMA) would result in catastrophic interference (collisions).
- Option (c) is incorrect because the primary purpose of multiplexing is to allow more than one signal to share a medium.
- Option (d) is incorrect because 'time-sharing' refers to Time Division Multiplexing (TDM), where signals take turns using the entire bandwidth for short time slots, rather than splitting the frequency.
Describe switching behavior of BJT.
Explain the formation of a PN junction and describe the concept of depletion region and barrier potential. Discuss how forward and reverse bias affect the PN junction characteristics.
Explain ideal op-amp characteristics.
Draw and explain the V-I characteristics of SCR.
A full-adder has:
A)
Two inputs and one output
B)
Three inputs and two outputs
C)
Two inputs and two outputs
D)
One input and one output
A Full-Adder is a combinational digital circuit that performs the addition of three binary bits.
1. **Inputs**: It takes three inputs: two significant bits to be added (A and B) and a carry-in bit (Cin) coming from a previous less-significant stage of addition. Total = 3 inputs.
2. **Outputs**: It produces two outputs: the Sum (S) and the Carry-out (Cout). Total = 2 outputs.
Comparison with other options:
- Option (a) describes a Half-Adder's inputs but not its outputs (a Half-Adder has 2 inputs and 2 outputs).
- Option (c) describes a Half-Adder (2 inputs: A, B; 2 outputs: Sum, Carry).
- Option (d) is incorrect as no standard adder operates with a single input and output.
Describe sampling, quantization, and encoding.
If the firing angle becomes negative, then the rectifier begins to work as
A)
A rectifier
B)
An inverter
C)
A chopper
D)
A regulator
In power electronics, specifically regarding controlled rectifiers (like those using Thyristors or GTOs), the firing angle (α) determines the point in the AC cycle where the device begins to conduct.
1. **Rectification Mode (0° < α < 180°):** The device converts AC to DC. The average output voltage is positive, and power flows from the AC source to the DC load.
2. **Inversion Mode (180° < α < 360°, or effectively negative firing angles in certain control contexts):** When the firing angle is shifted such that the device conducts during the negative half of the supply voltage cycle (or when the load voltage is higher than the source voltage and the firing occurs at a negative angle relative to the natural commutation point), the average output voltage becomes negative.
3. **Power Flow:** In this state, the power flow reverses; energy is transferred from the DC side (e.g., a battery or a DC motor acting as a generator) back to the AC source. This process of converting DC back to AC is the definition of an inverter.
Why other options are incorrect:
- **a. A rectifier:** This is the standard operation for positive firing angles (0 to 180 degrees).
- **c. A chopper:** A chopper is a DC-to-DC converter; it does not involve firing angles relative to an AC sine wave.
- **d. A regulator:** While a rectifier can be used for regulation, the specific act of shifting the firing angle into the negative region changes the fundamental nature of the power conversion (direction of flow), turning it into an inverter.
An op-amp used for mathematical integration is called:
A)
Differentiator
B)
Oscillator
C)
Comparator
D)
Integrator
An operational amplifier (op-amp) circuit designed to perform the mathematical operation of integration is specifically called an Integrator. In a typical inverting integrator circuit, a resistor is placed at the input and a capacitor is placed in the feedback loop. The output voltage is proportional to the integral of the input voltage over time.
Analysis of other options:
- a. Differentiator: This circuit performs the opposite operation (differentiation), where the output is proportional to the rate of change of the input voltage. It uses a capacitor at the input and a resistor in the feedback loop.
- b. Oscillator: This is a circuit that generates a periodic waveform (like a sine or square wave) without an external input signal; it is not a mathematical integration tool.
- c. Comparator: This circuit compares two input voltages and outputs a digital signal (high or low) based on which input is larger; it does not perform integration.
Memory-mapped I/O means:
A)
I/O devices share the address space with memory
B)
Separate address space for I/O
C)
Only RAM is used
D)
Only ROM is used
In microprocessor architecture, there are two primary methods for interfacing I/O devices: Memory-Mapped I/O and I/O-Mapped I/O (Isolated I/O).
1. **Memory-Mapped I/O**: In this configuration, the microprocessor treats I/O devices as if they were memory locations. The I/O devices are assigned specific addresses within the same address space used by the RAM and ROM. Consequently, the processor uses the same instructions (like MOV or LDA in 8085) to access both memory and I/O devices. The trade-off is that a portion of the available memory address space is 'consumed' by the I/O devices.
2. **I/O-Mapped I/O (Isolated I/O)**: This uses a separate address space for I/O devices, distinct from the memory address space. Special instructions (like IN and OUT in 8085) are used to communicate with these devices, and the processor uses a control signal (like IO/M) to distinguish between a memory access and an I/O access.
Analysis of options:
- Option (a) is correct as it defines the core concept of memory-mapping.
- Option (b) describes 'Isolated I/O' or 'I/O-Mapped I/O'.
- Option (c) and (d) are incorrect as they refer to types of memory, not the method of interfacing I/O devices.
Explain basic block diagram of digital communication system.
Memory interfacing with a microprocessor requires:
A)
Address bus, data bus, control signals
B)
Only data bus
C)
Only power supply
D)
Only clock signal
Memory interfacing is the process of connecting the microprocessor to memory chips so that the CPU can read from or write to specific memory locations. To achieve this, three primary components are required:
1. Address Bus: The microprocessor uses the address bus to specify the unique location (address) in memory that it wants to access.
2. Data Bus: This is the bidirectional path used to transfer the actual data between the microprocessor and the memory chip.
3. Control Signals: These signals (such as RD (Read) and WR (Write) in the 8085) tell the memory whether the CPU wants to read data from the memory or write data into it.
Without any one of these three, the microprocessor cannot communicate with the memory. Options b, c, and d are incorrect because they list only a single component, which is insufficient for the complete operation of memory interfacing.
In a step-up chopper circuit, if Vs is the source voltage and α is duty cycle, then the output voltage is:
A)
Vs/ (1 + α)
B)
Vs (1 + α)
C)
Vs (1 - α)
D)
Vs/ (1 - α)
A step-up chopper (also known as a Boost Converter) is used to produce an output voltage higher than the input source voltage.
1. **Operation Phase 1 (Switch ON):** When the switch is closed for a time period 'T_on', the inductor stores energy from the source (Vs). The diode is reverse-biased, and no current flows to the load.
2. **Operation Phase 2 (Switch OFF):** When the switch is open for a time period 'T_off', the inductor voltage adds to the source voltage, and the combined voltage is delivered to the load through the diode.
**Mathematical Derivation:**
- Let T be the total switching period.
- Duty cycle (α) = T_on / T.
- The average voltage across the inductor over one complete cycle is zero (steady-state assumption).
- Average Voltage = (Vs * T_on) + (Vs - Vo) * T_off = 0
- Vs * T_on + Vs * T_off - Vo * T_off = 0
- Vs * (T_on + T_off) = Vo * T_off
- Vs * T = Vo * (T - T_on)
- Vo = Vs * [T / (T - T_on)]
- Divide numerator and denominator by T: Vo = Vs / [1 - (T_on/T)]
- Since α = T_on/T, the formula becomes: Vo = Vs / (1 - α).
**Analysis of Options:**
- Option (a) Vs/(1 + α) is incorrect.
- Option (b) Vs(1 + α) is incorrect.
- Option (c) Vs(1 - α) is the formula for a step-down (Buck) chopper.
- Option (d) Vs/(1 - α) is the correct formula for a step-up (Boost) chopper.
Explain the working principle and V–I characteristics of a Zener diode. Describe how it is used as a voltage regulator in power supply circuits.
Explain interrupt priority in microprocessor.
The value of anode current required to maintain the conduction of an SCR even though the gate signal is removed is called as the:
A)
Latching Current
B)
Holding Current
C)
Switching Current
D)
All of these
In a Silicon Controlled Rectifier (SCR), there are two critical current levels related to the transition between the OFF state and the ON state:
1. **Latching Current (I_L):** This is the minimum anode current required to keep the SCR in the ON state immediately after the gate trigger signal is removed. If the anode current does not reach the latching level before the gate pulse ends, the SCR will return to the OFF state. Therefore, it is associated with the 'turn-on' process.
2. **Holding Current (I_H):** This is the minimum anode current below which the SCR will stop conducting and return to the OFF state. It is associated with the 'turn-off' process. Once the SCR is already conducting, if the current drops below the holding current, the device turns off.
Since the question specifically asks for the current required to maintain conduction *even though the gate signal is removed*, it refers to the Latching Current.
Comparison: Latching Current > Holding Current.
In an AM system, the maximum and minimum envelope voltages are 10 V and 4 V respectively. The modulation index is approximately:
A)
0.25
B)
0.43
C)
0.67
D)
0.80
Option C is correct: 0.67.
Carrier communication requires:
A)
Modulation
B)
Demodulation
C)
Transmission over channel
D)
All of the above
Carrier communication (wireless or wired high-frequency communication) is a complete system process that involves several critical stages to successfully move information from a source to a destination.
1. **Modulation (Option a):** Baseband signals (like voice or data) have low frequencies and cannot be transmitted efficiently over long distances. Modulation is the process of superimposing the message signal onto a high-frequency 'carrier wave' to allow for efficient radiation via antennas and to avoid signal interference.
2. **Transmission over channel (Option c):** Once modulated, the signal must be physically transported from the transmitter to the receiver through a medium (the channel), such as free space (air/vacuum), coaxial cables, or optical fibers.
3. **Demodulation (Option b):** At the receiving end, the high-frequency carrier is no longer needed. Demodulation is the process of extracting the original baseband message signal from the received modulated carrier wave.
Since a functional carrier communication system fundamentally requires the process of preparing the signal (modulation), sending it (transmission), and recovering it (demodulation), all three options are essential components of the system.
XOR gate output is HIGH when:
A)
Both inputs are LOW
B)
Both inputs are HIGH
C)
Only one input is HIGH
D)
Inputs are equal
An XOR (Exclusive OR) gate is a digital logic gate that implements an exclusive disjunction. The output of an XOR gate is HIGH (1) if and only if the inputs are different.
Let's analyze the Truth Table for a 2-input XOR gate:
1. Input A=0, B=0: Output = 0 (Both are LOW)
2. Input A=0, B=1: Output = 1 (Only one is HIGH)
3. Input A=1, B=0: Output = 1 (Only one is HIGH)
4. Input A=1, B=1: Output = 0 (Both are HIGH)
Evaluating the options:
- Option (a) 'Both inputs are LOW' results in a LOW output.
- Option (b) 'Both inputs are HIGH' results in a LOW output.
- Option (c) 'Only one input is HIGH' results in a HIGH output, which matches the XOR logic.
- Option (d) 'Inputs are equal' (both 0 or both 1) results in a LOW output (this is actually the logic for an XNOR gate).