Unit 2: Fundamentals of Electricity and Devices - Subjective Questions

Ohm's law: For a conductor whose physical conditions remain constant, the current is directly proportional to the voltage across it. Therefore, , where is voltage, is current, and is resistance.

Kirchhoff's current law (KCL): The algebraic sum of currents at a node is zero. Equivalently, the total current entering a junction equals the total current leaving it:

It is based on the conservation of electric charge and is used for analyzing parallel circuits.

Kirchhoff's voltage law (KVL): The algebraic sum of all voltages around a closed loop is zero:

It is based on the conservation of energy and is used to analyze series and complex circuits.

Applications: Ohm's law is used to calculate circuit current, KCL is used in node-voltage analysis, and KVL is used to determine unknown voltages in closed-loop networks.

Consider two resistors and connected in series across a supply voltage .

The same current flows through both resistors. The total resistance is:

The circuit current is:

The voltage across is:

Similarly, the voltage across is:

Thus, the voltage division rule is:

For , , and :

Therefore, the voltage across the resistor is .

When two resistors and are connected in parallel, the applied voltage is the same across both resistors, while the total current divides between the branches.

Let the total current be . Using conductance, the current through is:

Simplifying:

Similarly, the current through is:

Therefore:

The smaller resistance carries the larger current. The rule is useful for analyzing parallel networks and designing current-sharing circuits. The total current satisfies:

A PN junction diode is formed by joining a p-type semiconductor with an n-type semiconductor.

Formation of the depletion region: Electrons from the n-side diffuse toward the p-side and holes from the p-side diffuse toward the n-side. Recombination near the junction creates a region without mobile charge carriers called the depletion region. This region produces a potential barrier.

Forward bias:

• The p-side is connected to the positive terminal and the n-side to the negative terminal.
• The depletion width and barrier potential decrease.
• Majority carriers cross the junction easily.
• The diode conducts significant current after the cut-in voltage is reached.
• The cut-in voltage is approximately for silicon and for germanium.

Reverse bias:

• The p-side is connected to the negative terminal and the n-side to the positive terminal.
• The depletion region becomes wider.
• Only a small reverse saturation current due to minority carriers flows.
• At sufficiently high reverse voltage, breakdown occurs and the current rises sharply.

The diode characteristic curve shows the relationship between diode current and applied voltage.

Forward characteristic:

• For small forward voltages, the current is very small because the barrier potential opposes carrier movement.
• At the cut-in or knee voltage, the barrier is sufficiently reduced and current increases rapidly.
• The cut-in voltage is about for silicon and for germanium.
• Beyond this point, a small increase in voltage produces a large increase in current.

Reverse characteristic:

• A small, nearly constant current flows in the reverse direction. This is called the reverse saturation current, .
• When reverse voltage reaches the breakdown voltage, the reverse current increases suddenly.
• Breakdown may be caused by the Zener effect or avalanche effect.

Definitions:

• Cut-in voltage: Minimum forward voltage required for appreciable conduction.
• Reverse saturation current: Small reverse current caused by minority carriers.
• Breakdown voltage: Reverse voltage at which the diode begins to conduct a large current.

A rectifier converts alternating current into unidirectional pulsating direct current.

Half-wave rectifier:

• It uses one diode.
• During the positive half-cycle, the diode is forward biased and current flows through the load.
• During the negative half-cycle, the diode is reverse biased and blocks current.
• Only one half of the input waveform appears across the load.
• Its ripple frequency is equal to the input frequency, and its maximum rectification efficiency is approximately .

Full-wave bridge rectifier:

• It uses four diodes.
• Two diodes conduct during each half-cycle, allowing current through the load in the same direction.
• Both halves of the AC waveform are used.
• Its ripple frequency is twice the input frequency, and its maximum rectification efficiency is approximately .

The bridge rectifier provides a higher average output and lower ripple than a half-wave rectifier, but it requires more diodes.

A diode can act as a switch because it has two main operating states.

ON state:

• The diode is forward biased.
• Its depletion region becomes narrow.
• It conducts current with a small forward voltage drop.
• For an ideal diode, the ON-state voltage is treated as .
• For a silicon diode, the practical forward drop is approximately .

OFF state:

• The diode is reverse biased.
• The depletion region becomes wide.
• It blocks current except for a very small leakage current.
• It behaves approximately as an open circuit, provided the breakdown voltage is not exceeded.

Applications: Diode switching is used in logic circuits, signal detection, polarity protection, clamping circuits, switching power supplies, and digital pulse-shaping circuits.

A BJT is a three-layer, three-terminal semiconductor device. Its terminals are the emitter, base, and collector. The two types are NPN and PNP.

Emitter:

• It is heavily doped.
• It injects majority carriers into the base.

Base:

• It is very thin and lightly doped.
• It controls the number of carriers reaching the collector.

Collector:

• It is moderately doped and physically larger.
• It collects carriers from the base and dissipates heat.

For an NPN transistor operating in the active region, the emitter-base junction is forward biased and the collector-base junction is reverse biased. Electrons are injected from the emitter into the base. Since the base is thin, most electrons pass through it and are attracted to the collector.

The transistor currents satisfy:

The current gain in common-emitter operation is:

A small base current controls a much larger collector current, which enables amplification.

The operating region of a BJT depends on the biasing of its two junctions.

Cutoff region:

• The emitter-base and collector-base junctions are reverse biased.
• Base current and collector current are approximately zero.
• The transistor behaves like an open switch.
• It represents the logic OFF state.

Active region:

• The emitter-base junction is forward biased.
• The collector-base junction is reverse biased.
• Collector current is approximately proportional to base current:
  
• The transistor is used as a voltage or current amplifier.

Saturation region:

• Both junctions are forward biased.
• The collector current cannot increase significantly even if base current increases.
• The transistor behaves like a closed switch.
• It represents the logic ON state.

Thus, cutoff and saturation are used for switching, while the active region is used for amplification.

CMOS stands for complementary metal-oxide-semiconductor. It uses complementary pairs of p-channel MOSFETs and n-channel MOSFETs.

A CMOS inverter consists of a PMOS transistor connected between the supply voltage and the output, and an NMOS transistor connected between the output and ground. Their gates are connected together as the input.

Operation:

• When the input is LOW, the PMOS turns ON and the NMOS turns OFF. The output is pulled to , so it is HIGH.
• When the input is HIGH, the PMOS turns OFF and the NMOS turns ON. The output is pulled to ground, so it is LOW.

Advantages:

• Very low static power consumption because ideally one transistor is OFF in each stable state.
• High noise immunity.
• High packing density.
• Rail-to-rail output voltage levels.
• Suitable for large-scale integrated circuits and digital systems.

Dynamic power is mainly consumed during switching and is approximately related to capacitance, frequency, and voltage by .

CMOS is dominant in modern digital electronics because it combines low power consumption, high density, and good noise margins. BJT circuits remain useful where high transconductance, drive capability, or specialized high-speed operation is important.

Semiconductor memory stores binary information in electronic cells. A memory system contains address lines, data lines, control lines, and an array of memory cells.

RAM:

• RAM allows random access to any memory location.
• DRAM stores each bit as charge in a capacitor controlled by a transistor and requires periodic refreshing.
• SRAM stores each bit using a bistable transistor circuit and does not require refresh while powered.
• RAM is generally volatile, so data is lost when power is removed.

Flash memory in SSDs:

• Flash memory uses floating-gate or charge-trap transistors to store charge.
• It is non-volatile, so data remains stored without power.
• Data is read in pages and erased in larger blocks.
• An SSD uses flash memory with a controller for address translation, error correction, wear leveling, and garbage collection.

RAM is primarily used as working memory for active programs, whereas SSD flash is used for long-term storage.

A solid-state drive contains NAND flash memory packages, a controller, firmware, cache, and an interface such as SATA or NVMe.

NAND flash organization:

• Cells are arranged into pages and erase blocks.
• Data is normally programmed and read page by page.
• Erasure occurs block by block.
• Multi-level cells may store more than one bit per cell.

Wear leveling: Flash cells support a limited number of program-erase cycles. Wear leveling distributes writes across available blocks so that some blocks do not fail prematurely.

Error correction: Flash memory is affected by noise, charge leakage, and cell aging. Error-correcting codes detect and correct bit errors, improving reliability.

Garbage collection: When valid and invalid pages are mixed in a block, the controller copies valid pages to another block and erases the old block. This creates free space for future writes.

These functions allow SSDs to provide reliable, fast, and durable storage despite the limitations of flash memory.

AI accelerator chips are specialized processors designed to perform machine-learning operations more efficiently than general-purpose processors.

Need: AI workloads involve large numbers of matrix multiplications, vector operations, and tensor calculations. Specialized hardware can provide greater throughput and energy efficiency.

Common architectures:

• GPU: Uses many parallel arithmetic units for tensor and vector operations.
• TPU or tensor accelerator: Optimized for matrix multiplication and neural-network tensors.
• NPU: Integrated neural-processing unit designed for inference in phones and edge devices.
• FPGA: Configurable hardware that can be adapted for specific AI algorithms.
• ASIC: Application-specific hardware designed for maximum efficiency for a fixed workload.

Applications: AI accelerators are used in image recognition, speech processing, autonomous vehicles, recommendation systems, medical imaging, robotics, and generative AI.

They reduce execution time and energy consumption, although they may require specialized software and are less flexible than CPUs.

A sensor detects a physical quantity and converts it into an electrical signal. Examples include temperature, pressure, light, humidity, motion, gas concentration, and acceleration sensors.

A typical IoT signal-processing path is:

1. Sensing: The sensor responds to a physical quantity.
2. Transduction: The physical change is converted into voltage, current, resistance, capacitance, or a digital signal.
3. Signal conditioning: Amplification, filtering, isolation, and linearization improve the signal.
4. Analog-to-digital conversion: An ADC converts an analog signal into digital samples.
5. Local processing: A microcontroller or edge processor calibrates, compresses, or analyzes the data.
6. Communication: The data is transmitted using Wi-Fi, Bluetooth, Zigbee, LoRaWAN, cellular, or another protocol.
7. Cloud or application processing: Data is stored, analyzed, visualized, and used to trigger actions.

Important sensor characteristics include accuracy, sensitivity, resolution, response time, power consumption, and reliability.

IoT sensors must operate reliably in environments where power, bandwidth, and maintenance are limited.

Major challenges and solutions:

• Low power: Use sleep modes, low-power components, duty cycling, and energy harvesting.
• Measurement noise: Use shielding, proper grounding, analog filtering, digital filtering, and signal averaging.
• Calibration drift: Perform periodic calibration and use temperature compensation or self-calibration.
• Limited bandwidth: Transmit only important features or events and use data compression.
• Security: Apply authentication, encryption, secure boot, and protected firmware updates.
• Environmental exposure: Use suitable packaging and protection from moisture, dust, vibration, and temperature extremes.
• Scalability: Use standardized protocols, unique device identities, and efficient network management.
• Latency: Perform time-critical processing at the edge rather than sending all data to the cloud.

A successful IoT sensor system balances accuracy, cost, energy use, communication range, security, and service life.

A computer network is a collection of interconnected computing devices that communicate and share data, resources, and services.

Basic components:

• End devices: Computers, servers, printers, sensors, and mobile devices.
• Network interface: Hardware that connects a device to the network.
• Transmission media: Copper cable, optical fiber, or wireless radio.
• Interconnecting devices: Switches, routers, access points, and gateways.
• Protocols: Rules such as Ethernet, IP, TCP, UDP, HTTP, and DNS.

Types by coverage:

• PAN: Personal area network.
• LAN: Local area network within a home, building, or campus.
• MAN: Metropolitan area network.
• WAN: Wide area network covering large geographic regions.

Performance parameters:

• Bandwidth: Maximum data-carrying capacity.
• Throughput: Actual rate of successful data transfer.
• Latency: Time taken for data to travel from source to destination.
• Reliability: Ability to deliver data correctly and consistently.
• Security: Protection against unauthorized access and attacks.

The OSI model divides network communication into seven layers:

• Physical layer: Transmits raw bits through a medium.
• Data link layer: Provides framing, MAC addressing, and local error detection.
• Network layer: Provides logical addressing and routing, as in IP.
• Transport layer: Provides end-to-end delivery, flow control, and reliability, as in TCP.
• Session layer: Establishes, manages, and terminates communication sessions.
• Presentation layer: Handles translation, compression, and encryption.
• Application layer: Provides network services to user applications, such as web and email services.

During transmission, the application creates data. Each lower layer adds its own control information, a process called encapsulation. The transport layer adds a segment header, the network layer adds a packet header, and the data link layer adds a frame header and trailer. The physical layer transmits the resulting bits.

At the receiving device, each layer removes and interprets its corresponding control information. This reverse process is called decapsulation.

Optical-fiber communication transmits information as light pulses through a thin strand of glass or plastic.

Total internal reflection: The fiber has a high-refractive-index core surrounded by a lower-index cladding. When light enters within the acceptance angle, it undergoes repeated total internal reflection and remains confined to the core.

Communication system:

• Transmitter: Converts an electrical signal into light using an LED or laser diode.
• Optical fiber: Carries the modulated light through the core. It provides high bandwidth and immunity to electromagnetic interference.
• Receiver: Uses a photodiode, such as a PIN or avalanche photodiode, to convert light back into an electrical signal.
• Signal processing: Amplification, filtering, and decoding recover the transmitted information.

Fiber systems have low attenuation, high data capacity, electrical isolation, and strong security. Limitations include installation cost, fragility, and the need for specialized splicing and termination equipment.

Fiber is preferred for high-capacity, stable links, while wireless communication is preferred for mobility, flexibility, and remote access. Modern networks often combine both technologies.