Electronic Circuits & Applications (4321103) - Summer 2023 Solution

Question 1(a) [3 marks]

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Explain thermal runaway in details.

Answer: Thermal runaway is a destructive mechanism in BJT transistors where increased temperature creates a self-reinforcing cycle leading to device failure.

flowchart TD
    A[Increase in Temperature] --> B[Increase in Ic]
    B --> C[Increase in Power Dissipation]
    C --> D[Further Increase in Temperature]
    D --> A

• Heat Generation: Temperature rises from normal operation
• Leakage Current: Collector current Ic increases with temperature
• Power Dissipation: More power = Temperature rises further
• Destructive Cycle: Continuous cycle until transistor destroys itself

Mnemonic: “The Higher Temperature, The Higher Current”

Question 1(b) [4 marks]

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Define amplifier with simple block diagram write down amplifier parameters.

Answer: An amplifier is an electronic device that increases the power, voltage or current of an input signal.

flowchart LR
    A[Input Signal] -->|Vin| B[AMPLIFIER]
    B -->|Vout| C[Output Signal]
    D[Power Supply] --> B

Mnemonic: “VIPS-BIO” (Voltage, Input impedance, Power, Supply, Bandwidth, Impedance Output)

Question 1(c) [7 marks]

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Define Biasing in transistor? Write down types of biasing methods. Explain the voltage divider biasing method in details.

Answer: Biasing is the process of establishing a stable operating point (Q-point) for a transistor by applying DC voltages.

Voltage Divider Biasing:

flowchart TD
    subgraph Circuit
    VCC((+VCC)) --- R1
    R1 --- R2 & B((Base))
    R2 --- GND
    B --- C((Collector))
    B --- E((Emitter))
    C --- RC --- VCC
    E --- RE --- GND
    end

• R1 & R2: Form voltage divider to provide stable base voltage
• RE: Provides stabilization through negative feedback
• RC: Determines collector current and voltage gain
• Stability: Best stability against temperature variations

Mnemonic: “Divide Voltage Before Transistor Conducts”

Question 1(c) OR [7 marks]

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Explain Heat sink.

Answer: A heat sink is a passive heat exchanger that transfers heat from electronic devices to the surrounding air.

flowchart TD
    A[Heat Source/Transistor] --> B[Interface Material]
    B --> C[Heat Sink Base]
    C --> D[Heat Sink Fins]
    D --> E[Ambient Air]

• Thermal Resistance: Lower is better for heat dissipation
• Material: Usually aluminum or copper for good conductivity
• Surface Area: More fins means better cooling
• Air Flow: Critical for efficient heat removal

Mnemonic: “Heat Sinks Keep Transistors Running”

Question 2(a) [3 marks]

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Describe the D.C. and A.C. Load Lines.

Answer: Load lines graphically represent possible operating points of a transistor on its characteristic curves.

• DC Load Line: Shows all possible operating points under DC conditions

  • Equation: Ic = (VCC - VCE)/RC
  • Endpoints: (0, VCC/RC) and (VCC, 0)

• AC Load Line: Shows operating points during AC signal handling

  • Steeper Slope: Due to AC resistance being less than DC
  • Centered at Q-point: The operating point established by biasing

Mnemonic: “DC Draws Completely, AC Alters Course”

Question 2(b) [4 marks]

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Briefly explain bandwidth and gain-bandwidth product of an amplifier.

Answer: Bandwidth and gain-bandwidth product are key specifications for amplifier frequency performance.

flowchart LR
    A[Input] --> B[Amplifier
Gain × Bandwidth]
    B --> C[Output]

• Bandwidth Formula: BW = f₂ - f₁
• Gain-Bandwidth: Remains constant when gain changes
• Trade-off: Higher gain means lower bandwidth

Mnemonic: “Better Bandwidth Gets Perfect Transmission”

Question 2(c) [7 marks]

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Explain frequency response of two stage RC coupled amplifier.

Answer: The frequency response of a two-stage RC coupled amplifier shows how gain varies with frequency.

flowchart LR
    A[Input] --> B[First
Amplifier
Stage]
    B -->|RC Coupling| C[Second
Amplifier
Stage]
    C --> D[Output]

• Low Frequency Response: Limited by coupling capacitors

  • Roll-off Rate: -20 dB/decade for each stage

• Mid Frequency Response: Maximum and flat gain region

  • Total Gain: Product of individual stage gains

• High Frequency Response: Limited by transistor capacitances

  • Roll-off Rate: -20 dB/decade for each stage

Mnemonic: “Low Couples Weakly, High Capacitance Blocks”

Question 2(a) OR [3 marks]

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Explain fixed bias circuit for transistor biasing.

Answer: Fixed bias is the simplest biasing method for transistors, using a single resistor connected to the base.

• Circuit Elements: Base resistor (RB) and collector resistor (RC)
• Base Current: IB = (VCC - VBE)/RB
• Collector Current: IC = β × IB
• Drawbacks: Poor stability, affected by temperature changes

Mnemonic: “Fix Bias, Face Burden” (of instability)

Question 2(b) OR [4 marks]

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Explain frequency response of single stage amplifier.

Answer: The frequency response of a single-stage amplifier shows gain variation across different frequencies.

• Lower cutoff frequency: Determined by coupling capacitors
• Upper cutoff frequency: Limited by internal transistor capacitances
• Bandwidth: Range between lower and upper cutoff frequencies

Mnemonic: “Low Middle High - Capacitors Matter Here”

Question 2(c) OR [7 marks]

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Compare transformer coupled amplifier and RC coupled amplifier

Answer:

flowchart TB
    subgraph RC
    A1[Transistor 1] -->|Coupling Capacitor| B1[Transistor 2]
    end
    subgraph Transformer
    A2[Transistor 1] -->|Transformer| B2[Transistor 2]
    end

Mnemonic: “RC Takes Breadth, Transformer Takes Power”

Question 3(a) [3 marks]

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Explain in brief Direct coupled amplifier.

Answer: A direct-coupled amplifier connects stages without coupling capacitors or transformers, allowing DC signal amplification.

flowchart LR
    In[Input] --> A[First Stage]
    A -- Direct Connection --> B[Second Stage]
    B --> Out[Output]

• DC Signal Handling: Can amplify very low frequencies and DC
• No Coupling Elements: Output of first stage directly connects to input of next
• Frequency Response: Excellent low-frequency response
• Drawbacks: Thermal drift, bias stability issues

Mnemonic: “Directly Connected, Down to Complete zero frequency”

Question 3(b) [4 marks]

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Explain effects of emitter bypass capacitor and coupling capacitor on frequency response of an amplifier.

Answer:

flowchart TB
    subgraph "Effects on Gain"
    A[Without Capacitors] -->|"Low Gain"| B[With Coupling Only]
    B -->|"Medium Gain"| C[With Coupling + Bypass]
    C -->|"High Gain"| D[Ideal Response]
    end

• Emitter Bypass Capacitor:

  • Without: Lower gain due to negative feedback
  • With: Higher gain as RE is bypassed for AC signals

• Coupling Capacitor:

  • Too Small: Poor low-frequency response
  • Larger Value: Better low-frequency response

Mnemonic: “Coupling Controls Lows, Bypass Boosts All”

Question 3(c) [7 marks]

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Draw Transistor Two Port Network and describe h-parameters for it. Write down advantages of hybrid parameters.

Answer: A two-port network is a model to analyze transistor behavior using h-parameters (hybrid parameters).

Advantages of H-Parameters:

• Easily Measured: Direct measurement with simple circuits
• Mixed Units: Uses ratios of voltage and current
• Model Accuracy: Close approximation to transistor behavior
• Mathematical Simplicity: Linear equations for analysis

Mnemonic: “Input, Reverse, Forward, Output - IRFO Parameters”

Question 3(a) OR [3 marks]

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Draw frequency response of an amplifier and indicate upper cut-off frequency, lower cut-off frequency, bandwidth, and mid frequency gain of the amplifier on the response.

Answer: The frequency response graph shows how gain varies with frequency for an amplifier.

• Mid-frequency Gain (Av): Maximum gain in the flat region
• Lower Cutoff Frequency (f₁): Frequency where gain drops to 0.707×Av (-3dB)
• Upper Cutoff Frequency (f₂): Frequency where gain drops to 0.707×Av (-3dB)
• Bandwidth: The difference between upper and lower cutoff frequencies (f₂ - f₁)

Mnemonic: “Lower Bandwidth Upper Makes Amplifier Response”

Question 3(b) OR [4 marks]

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Describe the transistor used as a tuned amplifier.

Answer: A tuned amplifier uses LC resonant circuits to amplify signals selectively at specific frequencies.

flowchart TB
    A[Input Signal] --> B[Transistor Amplifier]
    B --> C[LC Tuned Circuit]
    C --> D[Output Signal]

• High Selectivity: Amplifies signals at resonant frequency
• Applications: RF receivers, transmitters, communications
• Types: Single-tuned, double-tuned, stagger-tuned
• Bandwidth: Inversely proportional to Q factor

Mnemonic: “Tuning LC Selects Signals Precisely”

Question 3(c) OR [7 marks]

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Describe the importance of h parameters in two port network. Draw h-parameters circuit for CE amplifier.

Answer: H-parameters provide a complete mathematical model for analyzing transistor circuits as two-port networks.

Importance of h-parameters:

CE Amplifier h-parameter equivalent circuit:

• hie: Input impedance (base-emitter resistance)
• hre: Reverse voltage feedback ratio
• hfe: Forward current gain (β)
• hoe: Output admittance

Mnemonic: “Input Resistance, Feedback Ratio, Forward gain, Output conductance”

Question 4(a) [3 marks]

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Describe the diode clipper circuit with necessary diagram.

Answer: A clipper circuit limits or clips off a portion of the input signal that exceeds a certain voltage level.

flowchart LR
    A[Input Signal] --> B[Diode Clipper]
    B --> C[Output Signal]

• Operation: Diode conducts when voltage exceeds threshold
• Types:
  • Positive Clipper: Clips positive half-cycles
  • Negative Clipper: Clips negative half-cycles
  • Biased Clipper: Clips at voltage level other than zero

Mnemonic: “Clip Portions Passing Preset Points”

Question 4(b) [4 marks]

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Explain Short note on LDR.

Answer: LDR (Light Dependent Resistor) is a photoresistor whose resistance decreases with increasing light intensity.

flowchart LR
    A[Light] --> B[LDR]
    B --> C[Resistance Changes]

• Light Absorption: Generates free carriers
• Resistance: Inversely proportional to light intensity
• Applications: Light sensors, automatic lighting, camera exposure control
• Symbol: Variable resistor with arrow pointing inward

Mnemonic: “Light Decreases Resistance”

Question 4(c) [7 marks]

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Explain Darlington pair and its applications.

Answer: A Darlington pair consists of two transistors connected so that the current amplified by the first is further amplified by the second.

flowchart TB
    A[Input Signal] --> B[Transistor 1]
    B --> C[Transistor 2]
    C --> D[Output Signal]

Applications:

• Power Amplifiers: High current gain applications
• Audio Amplifiers: High input impedance stages
• Buffer Circuits: Minimizing loading effects
• Motor Control: Driving high-current loads
• Touch Sensitive Switches: High sensitivity due to high gain

Mnemonic: “Double Transistors Amplify Really Greatly”

Question 4(a) OR [3 marks]

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Describe the diode clamper circuit with necessary diagram.

Answer: A clamper circuit shifts the entire waveform up or down by adding a DC component without changing its shape.

flowchart LR
    A[Input Signal] --> B[Diode Clamper]
    B --> C[Output Signal
Shifted Waveform]

• Operation: Capacitor charges during one half-cycle, maintaining DC level
• Types:
  • Positive Clamper: Shifts waveform upward
  • Negative Clamper: Shifts waveform downward
  • Biased Clamper: Shifts to specific DC level

Mnemonic: “Clamps Peaks Down Consistently”

Question 4(b) OR [4 marks]

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Explain the working and applications of OLED.

Answer: OLED (Organic Light Emitting Diode) is a display technology using organic compounds that emit light when electric current passes through.

flowchart TB
    A[Electric Current] --> B[OLED Layer]
    B --> C[Light Emission]

• Working Principle: Electron-hole recombination creates photons
• Self-illuminating: No backlight required unlike LCD
• Types: PMOLED (Passive Matrix) and AMOLED (Active Matrix)
• Advantages: Thinner, lighter, wider viewing angles, better contrast

Applications:

• Smartphones and tablets
• Television screens
• Digital camera displays
• Wearable devices
• Lighting panels

Mnemonic: “Organic Layers Emit Diode-light”

Question 4(c) OR [7 marks]

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Describe the transistor used as a relay driver.

Answer: A relay driver uses a transistor to control a relay, allowing a low-current control signal to switch a high-current load.

flowchart LR
    A[Control Signal] --> B[Transistor]
    B --> C[Relay Coil]
    C --> D[Switched Load]

Applications:

• Motor control circuits
• Industrial automation
• Automotive electronics
• Home appliance control
• Power distribution systems

Mnemonic: “Tiny Regulates Driving Relays”

Question 5(a) [3 marks]

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Draw circuit diagram of a variable power supply using LM317 IC.

Answer: LM317 is an adjustable voltage regulator that can be used to create a variable power supply.

• Components:

  • LM317: Adjustable voltage regulator IC
  • R1: Fixed 240Ω resistor
  • R2: Variable resistor (potentiometer)
  • C1, C2: Filter capacitors

• Output Voltage: VOUT = 1.25 × (1 + R2/R1)

Mnemonic: “LM317 Makes Voltage Adjustable”

Question 5(b) [4 marks]

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Explain working of UPS.

Answer: UPS (Uninterruptible Power Supply) provides emergency power when main power fails.

flowchart LR
    A[AC Mains] --> B[Rectifier]
    B --> C[Battery Charger]
    C --> D[Battery]
    D --> E[Inverter]
    E --> F[Output Load]
    A -.Bypass.-> F

• Main Components: Rectifier, battery, inverter, control circuit
• Functions:
  • Power conditioning
  • Voltage regulation
  • Surge protection
  • Battery backup

Mnemonic: “Uninterrupted Power Supplied During Blackouts”

Question 5(c) [7 marks]

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Draw and explain SMPS block diagram.

Answer: SMPS (Switch Mode Power Supply) uses switching regulation to convert electrical power efficiently.

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

• Advantages: High efficiency (70-90%), smaller size, lower weight
• Operation: Uses PWM (Pulse Width Modulation) at 20-200 kHz
• Types: Forward, Flyback, Push-pull, Half bridge, Full bridge
• Applications: Computers, TVs, mobile chargers, LED drivers

Mnemonic: “Switch Makes Power Stable”

Question 5(a) OR [3 marks]

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Draw circuit diagram for +15 v Power Supply using its IC and explain in brief

Answer: A +15V power supply can be built using the 7815 voltage regulator IC.

• Components:

  • 7815: Fixed +15V voltage regulator IC
  • Bridge Rectifier: Converts AC to pulsating DC
  • C1: Input filter capacitor (1000-2200µF)
  • C2: Output filter capacitor (10-100µF)

• Working: Rectifies AC, filters it, then regulates to stable +15V DC

Mnemonic: “7815 Fixes Voltage To Fifteen”

Question 5(b) OR [4 marks]

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Explain working of solar battery charger circuits.

Answer: Solar battery chargers convert sunlight into electrical energy to charge batteries.

flowchart LR
    A[Solar Panel] --> B[Charge Controller]
    B --> C[Battery]
    C --> D[Load]

• Operating Modes:

  • Bulk Charging: Maximum current until ~80% charged
  • Absorption: Constant voltage, decreasing current
  • Float/Trickle: Maintains full charge

• Protection Features: Overcharge, over-discharge, reverse polarity

Mnemonic: “Sun Charges Batteries Safely”

Question 5(c) OR [7 marks]

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Discuss comparison of linear regulated power supply with switch mode power supply.

Answer:

flowchart TB
    subgraph Linear
    A1[Transformer] --> B1[Rectifier]
    B1 --> C1[Filter]
    C1 --> D1[Series Pass Element]
    D1 --> E1[Output]
    end

    subgraph SMPS
    A2[Rectifier] --> B2[Switch]
    B2 --> C2[Transformer]
    C2 --> D2[Rectifier & Filter]
    D2 --> E2[Output]
    F2[Feedback] --> B2
    end

Applications:

• Linear: Audio equipment, laboratory instruments, sensitive circuits
• SMPS: Computers, TVs, mobile chargers, industrial power supplies

Mnemonic: “Linear Loves Low noise, Switching Saves Size”