Electronics Devices & Circuits (1323202) - Winter 2023 Solution

Question 1(a) [3 marks]

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Explain the concept of dc load line with the help of neat diagram.

Answer: DC load line is a straight line on output characteristics that shows all possible operating points of a transistor.

Diagram:

graph LR
    style O fill:#fff,stroke:#000
    style Vcesat fill:#fff,stroke:#000
    style Icsat fill:#fff,stroke:#000
    style Vcc fill:#fff,stroke:#000
    O((O)) --- Icsat((Icsat))
    O --- Vcc((Vcc))
    Icsat --- Vcesat((Vcesat))
    Vcesat --- Vcc

• Collector saturation current: When VCE = 0, IC = VCC/RC
• Cutoff voltage: When IC = 0, VCE = VCC
• Q-point: Operating point along load line

Mnemonic: “LEVEL” - “Load line Establishes Voltage and current for Every Load condition”

Question 1(b) [4 marks]

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Explain thermal runaway in detail.

Answer: Thermal runaway is a condition where heat causes transistor’s collector current to increase, which generates more heat, leading to destruction.

Diagram:

flowchart LR
    A[Temperature Increases] --> B[Leakage Current Increases]
    B --> C[Collector Current Increases]
    C --> D[More Power Dissipation]
    D --> E[Further Temperature Rise]
    E --> A

• Heat generation: Power dissipation = VCE × IC
• Critical effect: Increased junction temperature decreases VBE
• Prevention: Heat sinks, thermal stabilization circuits, proper biasing
• Danger: Can destroy transistor if not controlled

Mnemonic: “HEAT” - “Higher Emission Amplifies Temperature”

Question 1(c) [7 marks]

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Draw the circuit diagram and frequency response of a two stage R-C coupled amplifier. Explain the importance of each component.

Answer: R-C coupled amplifier uses capacitors to connect multiple transistor stages for higher gain.

Diagram:

Frequency Response:

xychart-beta
    title "Frequency Response"
    x-axis [10Hz, 100Hz, 1kHz, 10kHz, 100kHz, 1MHz]
    y-axis "Gain(dB)" 0 --> 40
    line [10, 30, 40, 40, 30, 10]
    annotations
        600Hz "Low frequency cutoff"
        50kHz "High frequency cutoff"

• Coupling capacitors: Block DC, allow AC signal transfer between stages
• Biasing resistors: Establish proper Q-point for transistor operation
• Bypass capacitors: Prevent gain reduction from negative feedback
• Bandwidth: Range between low and high cutoff frequencies

Mnemonic: “CARS” - “Coupling capacitors Allow Resistance Separation”

OR

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Question 1(c) [7 marks]

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Compare negative and positive feedback in amplifier.

Answer: Feedback systems return a portion of output to the input with different effects based on polarity.

Table:

• Negative feedback: Output is out of phase with input (180° shifted)
• Positive feedback: Output is in phase with input (0° shifted)
• Barkhausen criteria: Positive feedback with unity gain creates oscillation

Mnemonic: “SIGN” - “Stability Increases with Gain Negation”

Question 2(a) [3 marks]

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State and explain Barkhausen’s criteria for oscillations.

Answer: Barkhausen’s criteria define conditions for sustained oscillations in a feedback system.

Diagram:

flowchart LR
    A[Amplifier] --> B[Feedback Network]
    B --> A
    A -- "Loop Gain = 1" --> C[Sustained Oscillation]
    A -- "Loop Gain < 1" --> D[Damped Oscillation]
    A -- "Loop Gain > 1" --> E[Growing Oscillation]

• Gain condition: Loop gain (A×β) must equal 1 (unity)
• Phase condition: Total phase shift must be 0° or 360°
• Practical implementation: Initial loop gain > 1, then stabilizes at 1

Mnemonic: “LOOP” - “Loop’s Overall Output Phase”

Question 2(b) [4 marks]

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Compare Fixed bias, Collector to base bias & Voltage divider bias methods.

Answer: Different biasing techniques provide varying degrees of stability and temperature compensation.

Table:

• Fixed bias: Single resistor from base to VCC
• Collector-base bias: Feedback resistor from collector to base
• Voltage divider: Two resistors create stable reference voltage

Mnemonic: “STORM” - “Stability Through Optimized Resistor Methods”

Question 2(c) [7 marks]

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Write short note on Hartley oscillator.

Answer: Hartley oscillator is an LC oscillator with a tapped inductor for feedback.

Diagram:

graph LR
    A[Amplifier] --- B[Feedback Network]
    B --- A
    subgraph "Feedback Network"
    L1[L1] --- L2[L2]
    L1 --- C1[C]
    L2 --- C1
    end

• Circuit components: Amplifier, tapped inductor (L1+L2), capacitor C
• Frequency formula: f = 1/[2π√(LC)] where L = L1+L2
• Advantages: Simple design, good frequency stability
• Drawbacks: Size of inductors, limited frequency range
• Applications: RF signal generators, radio receivers, communication

Mnemonic: “TILC” - “Tapped Inductor with LC Circuit”

OR

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Question 2(a) [3 marks]

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Explain working of transistor as a switch.

Answer: Transistor switches between cutoff (OFF) and saturation (ON) regions for digital applications.

Diagram:

flowchart LR
    A[Input] --> B{Transistor}
    B -- "Saturation (ON)" --> C[Output LOW]
    B -- "Cutoff (OFF)" --> D[Output HIGH]

• Cutoff region: VBE < 0.7V, acts as open switch, VCE ≈ VCC
• Saturation region: VBE > 0.7V, acts as closed switch, VCE ≈ 0.2V
• Switching time: Limited by junction capacitance

Mnemonic: “COPS” - “Cutoff-On-Produces Switching”

Question 2(b) [4 marks]

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Define heat sink. List types of heat sink and give its applications.

Answer: Heat sink is a thermal conductor that transfers heat away from electronic components.

Diagram:

Types of Heat Sinks:

• Purpose: Prevents thermal runaway and component failure
• Materials: Aluminum, copper, or alloys with high thermal conductivity

Mnemonic: “COOL” - “Conducting Out Of Local heat”

Question 2(c) [7 marks]

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Explain advantages and disadvantages of negative feedback in amplifiers in detail.

Answer: Negative feedback returns a portion of output signal to input with opposite phase.

Table:

Diagram:

graph LR
    A[Input] --> B[Amplifier]
    B --> C[Output]
    C -- "Feedback Network" --> D[Subtractor]
    D --> B

• Gain stabilization: Makes gain dependent on passive components
• Bandwidth extension: Increases by factor equal to gain reduction
• Feedback factor: β determines amount of improvement

Mnemonic: “STABLE” - “Stabilized Transmission And Bandwidth with Less Error”

Question 3(a) [3 marks]

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Draw symbol of SCR and explain working of SCR.

Answer: Silicon Controlled Rectifier (SCR) is a four-layer PNPN device with three terminals.

Symbol:

• Structure: P-N-P-N four-layer semiconductor device
• Operation: Remains OFF until gate triggered, then conducts until current falls below holding value
• Terminals: Anode, Cathode, Gate

Mnemonic: “AGK” - “Anode-Gate controls Kathode current”

Question 3(b) [4 marks]

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Explain two transistor analogy of SCR with circuit diagram.

Answer: SCR can be represented as interconnected PNP and NPN transistors sharing junctions.

Diagram:

• PNP section: Upper transistor with collector connected to NPN base
• NPN section: Lower transistor with collector connected to PNP base
• Triggering: Small gate current turns on NPN, which turns on PNP
• Regenerative action: Each transistor supplies base current to other

Mnemonic: “PNPN” - “Positive-Negative-Positive-Negative layers”

Question 3(c) [7 marks]

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Explain the working of TRIAC based fan regulator with circuit diagram.

Answer: TRIAC-based fan regulator controls AC power through phase control.

Circuit Diagram:

• Phase control: Varies firing angle of TRIAC to control power
• Diac: Provides bidirectional triggering for TRIAC
• RC timing circuit: R1 and C1 set phase delay
• Variable resistor: Adjusts phase delay for speed control
• Protection: RC snubber prevents false triggering

Mnemonic: “TRIAC” - “Triggered Response In AC Circuits”

OR

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Question 3(a) [3 marks]

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Draw V-I characteristics of DIAC and TRIAC.

Answer: DIACs and TRIACs are bidirectional devices with symmetrical characteristics.

DIAC Characteristics:

xychart-beta
    title "DIAC V-I Characteristics"
    x-axis [-40, -30, -20, -10, 0, 10, 20, 30, 40]
    y-axis "Current (mA)" -30 --> 30
    line [30, 5, 0, 0, 0, 0, 0, 5, 30]
    annotations
        -VBO "Breakover Voltage (-VBO)"
        VBO "Breakover Voltage (VBO)"

TRIAC Characteristics:

xychart-beta
    title "TRIAC V-I Characteristics"
    x-axis [-40, -30, -20, -10, 0, 10, 20, 30, 40]
    y-axis "Current (mA)" -40 --> 40
    line [40, 40, 40, 5, 0, 5, 40, 40, 40]
    annotations
        -VBO "Breakover (-VBO)"
        VBO "Breakover (VBO)"

• DIAC: Bidirectional diode that conducts after breakover voltage
• TRIAC: Three-terminal device that conducts in both directions when triggered

Mnemonic: “BIBO” - “Bidirectional In, Bidirectional Out”

Question 3(b) [4 marks]

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Explain the Gate triggering method of SCR.

Answer: Gate triggering is the most common method to activate an SCR.

Diagram:

• Gate pulse: Small current applied between gate and cathode
• Triggering methods: DC, AC, or pulse signals
• Current requirements: Typically 5-20mA gate current
• Advantages: Low power control of high-power circuits

Mnemonic: “GATE” - “Gain Activation Through Electron flow”

Question 3(c) [7 marks]

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Explain SCR application for DC power control.

Answer: SCR controls DC power by chopping the supply voltage at variable duty cycles.

Circuit:

• Phase control: Varies firing angle to control average power
• PWM control: Pulse width modulation for efficient control
• Applications: DC motor speed control, dimming, heating
• Advantages: High efficiency, no moving parts, reliable
• Limitations: Unidirectional current flow, needs commutation

Mnemonic: “POWER” - “Pulse Operation With Electronic Regulation”

Question 4(a) [3 marks]

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List characteristics of Ideal OP-AMP.

Answer: Ideal operational amplifiers have perfect characteristics that real devices approximate.

Table:

• Practical values: Actual op-amps have limitations
• Implications: Circuit design must account for real limitations

Mnemonic: “IBOCSS” - “Infinite Bandwidth, Open-loop gain, CMRR, Slew rate, and Sensitivity”

Question 4(b) [4 marks]

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Explain working of differential amplifier using OP-AMP with circuit diagram.

Answer: Differential amplifier amplifies the voltage difference between two inputs.

Circuit:

• Gain formula: Vout = (V1-V2) × (R2/R1)
• Common mode rejection: Suppresses signals common to both inputs
• Applications: Instrumentation, medical equipment, audio

Mnemonic: “DIFF” - “Dual Input For Feedback”

Question 4(c) [7 marks]

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Explain OP-AMP as an inverting amplifier (Closed loop) and derive the formula of voltage gain.

Answer: Inverting amplifier produces output that is inverted and amplified version of input.

Circuit:

Gain Derivation:

• Apply KCL at inverting input: I₁ + I₂ = 0

• I₁ = (Vin - V⁻)/Ri and I₂ = (Vout - V⁻)/Rf

• At virtual ground, V⁻ ≈ 0

• Therefore: Vin/Ri + Vout/Rf = 0

• Solving for Vout/Vin: Av = -Rf/Ri

• Characteristics: Output 180° out of phase with input

• Feedback: Creates virtual ground at inverting input

• Closed loop gain: Controlled by external resistors

Mnemonic: “VAIN” - “Virtual ground Amplification Inverts Negative”

OR

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Question 4(a) [3 marks]

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Define the following parameters of OPAMP: 1) CMRR
2) Slew rate
3) Gain Bandwidth Product

Answer: These parameters define key performance characteristics of operational amplifiers.

Table:

• CMRR: Typically 80-120dB in quality op-amps
• Slew Rate: Limits output for high-frequency, high-amplitude signals
• GBP: Remains constant as frequency increases

Mnemonic: “CSG” - “Common-mode rejection, Speed, and Gain”

Question 4(b) [4 marks]

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Draw and explain summing amplifier using OP-AMP.

Answer: Summing amplifier produces output proportional to weighted sum of input voltages.

Circuit:

• Output formula: Vout = -Rf(V₁/R₁ + V₂/R₂ + V₃/R₃)
• Applications: Audio mixer, analog computers, signal processing
• Advantage: Multiple inputs can be processed simultaneously

Mnemonic: “SUM” - “Several Unified Multipliers”

Question 4(c) [7 marks]

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Draw the pin diagram of IC 555 and explain Monostable multivibrator using IC555 with waveform.

Answer: IC 555 timer in monostable mode produces a single pulse of fixed duration when triggered.

Pin Diagram:

Circuit and Waveform:

graph TB
    subgraph "Monostable Circuit"
    VCC --- R1 --- A
    A --- C1 --- GND
    A --- Pin6 & Pin7
    Pin2 --- Trigger
    Pin3 --- Output
    Pin4 --- Reset
    Pin8 --- VCC
    Pin1 --- GND
    end
    
    subgraph "Waveforms"
    direction TB
    Trig[Trigger] --> O1[Output]
    end

• Operation: Negative trigger starts timing cycle
• Time period: T = 1.1 × R × C
• Applications: Timers, pulse generation, debouncing
• Advantages: Simple, reliable, widely available

Mnemonic: “TIMER” - “Triggered Input Makes Extended Response”

Question 5(a) [3 marks]

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Draw block diagram of SMPS and give its applications.

Answer: Switch Mode Power Supply (SMPS) uses switching elements for efficient power conversion.

Block Diagram:

flowchart LR
    A[AC Input] --> B[EMI Filter]
    B --> C[Rectifier]
    C --> D[Filter]
    D --> E[Switching Circuit]
    E --> F[Transformer]
    F --> G[Output Rectifier]
    G --> H[Output Filter]
    H --> I[Output]
    J[Feedback Control] --> E
    I --> J

Applications:

• Computer power supplies

• Mobile phone chargers

• TV power supplies

• Industrial power systems

• LED lighting drivers

• Advantages: High efficiency, small size, lightweight

• Types: Buck, boost, buck-boost, flyback converters

Mnemonic: “SAFE” - “Switching Achieves Filtered Energy”

Question 5(b) [4 marks]

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Explain working of Regulated Power Supply with diagram.

Answer: Regulated power supply maintains constant output despite input or load variations.

Block Diagram:

flowchart LR
    A[AC Input] --> B[Transformer]
    B --> C[Rectifier]
    C --> D[Filter]
    D --> E[Regulator]
    E --> F[Output]
    G[Feedback] --> E
    F --> G

• Transformer: Steps down AC voltage to required level
• Rectifier: Converts AC to pulsating DC (diode bridge)
• Filter: Smooths DC with capacitors
• Regulator: Maintains constant output voltage
• Feedback: Compensates for input/load variations

Mnemonic: “TRFRO” - “Transform, Rectify, Filter, Regulate, Output”

Question 5(c) [7 marks]

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Explain basic block diagram of OP-AMP with diagram.

Answer: Operational amplifier’s internal structure consists of several stages performing specific functions.

Block Diagram:

flowchart LR
    A[Differential Input Stage] --> B[Intermediate Stage]
    B --> C[Level Shifter]
    C --> D[Output Stage]
    E[Bias Circuit] --> A & B & C & D

• Differential input stage: High impedance, amplifies difference
• Intermediate stage: Provides additional gain
• Level shifter: Adjusts DC level between stages
• Output stage: Low impedance, current amplification
• Bias circuit: Establishes operating points for all stages
• Compensation: Internal capacitor for stability

Mnemonic: “DILO” - “Differential Input, Level shift, Output”

OR

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Question 5(a) [3 marks]

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Explain adjustable voltage regulator using LM317 with diagram.

Answer: LM317 is a versatile adjustable positive voltage regulator with output range of 1.25V to 37V.

Circuit:

• Formula: Vout = 1.25(1 + R2/R1)
• Advantages: Simple adjustment, built-in protection
• Applications: Variable power supplies, battery chargers

Mnemonic: “AVR” - “Adjustable Voltage Regulation”

Question 5(b) [4 marks]

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Give the difference between Fixed voltage regulator IC and Variable voltage regulator IC.

Answer: Voltage regulator ICs differ in their configurability and application requirements.

Table:

• Fixed regulators: Simple to use, limited adjustment
• Variable regulators: More versatile, require calculation

Mnemonic: “FOCUS” - “Fixed Output Compared to User-Set”

Question 5(c) [7 marks]

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List applications of OP-AMP. Explain working operation of D to A converter with circuit diagram using OP-AMP.

Answer: Op-amps have numerous applications; D/A converters transform digital signals to analog.

Applications of OP-AMP:

• Amplifiers (inverting, non-inverting)
• Filters (active filters)
• Oscillators
• Comparators
• Integrators and differentiators
• Voltage followers
• Instrumentation circuits

R-2R Ladder DAC Circuit:

• Working principle: Digital inputs weight currents through resistor network
• Resistance values: Binary-weighted or R-2R ladder network
• Conversion: Output voltage proportional to digital input value
• Resolution: Determined by number of bits (2ⁿ levels)

Mnemonic: “DART” - “Digital to Analog Resistor Translation”