Electronic Circuits & Applications (4321103) - Winter 2023 Solution

Question 1(a) [3 marks]

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What is transistor biasing? What is its need?

Answer: Transistor biasing is the process of establishing a stable DC operating point (Q-point) for proper amplification of AC signals.

Table: Need for Transistor Biasing

Mnemonic: “SOLE operation” (Stability, Operating point, Linearity, Efficiency)

Question 1(b) [4 marks]

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Explain load line for CE amplifier

Answer: Load line is a graphical representation of all possible operating points of a transistor circuit.

Diagram:

graph LR
    A[DC Load Line] --- B[CE Amplifier]
    B --- C[AC Load Line]
    C --- D[Q-point]

    style A fill:#f9f,stroke:#333,stroke-width:1px
    style B fill:#bbf,stroke:#333,stroke-width:1px
    style C fill:#f9f,stroke:#333,stroke-width:1px
    style D fill:#bfb,stroke:#333,stroke-width:1px

• DC load line: Drawn between saturation point (Ic=Vcc/Rc, Vce=0) and cutoff point (Ic=0, Vce=Vcc)
• AC load line: Passes through Q-point with slope = -1/rc (rc = AC collector resistance)
• Q-point: Operating point where DC biasing conditions are established

Mnemonic: “SCQ points” (Saturation, Cutoff, Q-point)

Question 1(c) [7 marks]

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List various biasing method of transistor and explain any one of them.

Answer: Various biasing methods for transistors include:

Table: Transistor Biasing Methods

Explanation of Voltage Divider Bias:

Diagram:

• Operation: R1 and R2 form a voltage divider to set base voltage
• Stability: Excellent thermal stability due to stiff voltage divider
• Efficiency: Most widely used due to independence from β variations
• Calculation: Base voltage = Vcc × R2/(R1+R2)

Mnemonic: “VISE grip” (Voltage divider, Independent of β, Stable, Efficient)

Question 1(c) OR [7 marks]

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Explain voltage divider biasing method with help of circuit diagram

Answer: Voltage divider biasing is the most stable method to bias a transistor.

Diagram:

Table: Features of Voltage Divider Biasing

• Working principle: R1 and R2 form a voltage divider that sets the base voltage
• Thermal stability: Re provides negative feedback for excellent thermal stability
• Advantage: Q-point remains stable despite variations in temperature and β

Mnemonic: “BEST bias” (Base voltage, Emitter stability, Stiff divider, Temperature stable)

Question 2(a) [3 marks]

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Write methods of cascading amplifiers

Answer: Cascading amplifiers means connecting multiple amplifier stages in series to increase overall gain.

Table: Methods of Cascading Amplifiers

Mnemonic: “RTDL connection” (RC, Transformer, Direct, LC)

Question 2(b) [4 marks]

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Compare CE and CB amplifiers

Answer:

Table: Comparison of CE and CB Amplifiers

Mnemonic: “PIVOT differences” (Phase shift, Impedance, Voltage gain, Output impedance, Throughput)

Question 2(c) [7 marks]

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Draw the circuit of RC coupled amplifier. Give the frequency response and explain

Answer: RC coupled amplifier uses resistor-capacitor network for interstage coupling.

Diagram:

Frequency Response:

graph LR
    A[Low Frequency] --- B[Mid Frequency]
    B --- C[High Frequency]

    style A fill:#f9f,stroke:#333,stroke-width:1px
    style B fill:#bbf,stroke:#333,stroke-width:1px
    style C fill:#f9f,stroke:#333,stroke-width:1px

• Low frequency region: Gain drops due to coupling and bypass capacitors
• Mid frequency region: Flat response with maximum gain
• High frequency region: Gain falls due to transistor internal capacitances
• Bandwidth: Determined by the lower and upper cutoff frequencies

Mnemonic: “LMH regions” (Low, Mid, High frequency regions)

Question 2(a) OR [3 marks]

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Write definition of gain, Bandwidth and Gain Bandwidth product of an amplifier.

Answer:

Table: Key Amplifier Parameters

Mnemonic: “GBP constants” (Gain, Bandwidth, Product constants)

Question 2(b) OR [4 marks]

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Explain frequency response of single stage amplifier and indicate its cutoff frequencies.

Answer: Frequency response shows variation of gain with frequency in a single stage amplifier.

Diagram:

graph TD
    A[Frequency Response] --> B[Low f Region]
    A --> C[Mid f Region]
    A --> D[High f Region]
    B --> E[f₁: Lower Cutoff]
    D --> F[f₂: Upper Cutoff]
    C --> G[Maximum Gain]

    style A fill:#bbf,stroke:#333,stroke-width:1px
    style B fill:#f9f,stroke:#333,stroke-width:1px
    style C fill:#bfb,stroke:#333,stroke-width:1px
    style D fill:#f9f,stroke:#333,stroke-width:1px

• Cutoff frequencies: Points where gain drops to 0.707 times maximum gain
• Lower cutoff frequency (f₁): Determined by coupling and bypass capacitors
• Upper cutoff frequency (f₂): Limited by transistor junction capacitances
• Bandwidth: Frequency range between f₁ and f₂ (BW = f₂ - f₁)

Mnemonic: “LUG points” (Lower cutoff, Upper cutoff, Gain maximum)

Question 2(c) OR [7 marks]

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Draw and Explain circuit diagram of common collector amplifier

Answer: Common collector (CC) amplifier is also known as emitter follower.

Diagram:

Table: Features of Common Collector Amplifier

• Working principle: Output is taken from emitter, collector is common to input and output
• Key feature: Voltage follower with output voltage following input voltage
• Main advantage: High input impedance and low output impedance

Mnemonic: “BIVOP characters” (Buffer, Impedance matching, Voltage follower, One gain, Phase matched)

Question 3(a) [3 marks]

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Draw transistor two port network and describe h-parameters for it.

Answer: Transistor can be represented as a two-port network with h-parameters.

Diagram:

Table: h-parameters

Mnemonic: “IRFO parameters” (Input impedance, Reverse transfer, Forward transfer, Output admittance)

Question 3(b) [4 marks]

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Explain voltage gain Av, current gain Ai and Power gain Ap for CE amplifier

Answer:

Table: Gain Expressions for CE Amplifier

• Voltage gain: Typically 500-1000 for CE amplifier
• Current gain: Approximately equal to h_fe (β) of transistor
• Power gain: Product of voltage gain and current gain

Mnemonic: “VIP gains” (Voltage, Input-output current, Power)

Question 3(c) [7 marks]

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Explain Darlington pair, its features and applications

Answer: Darlington pair consists of two transistors connected to act as a single high-gain transistor.

Diagram:

Table: Features of Darlington Pair

• Applications: Power amplifiers, motor drivers, touch switches, sensors
• Advantages: Very high current gain, high input impedance
• Limitations: Higher saturation voltage, slower switching

Mnemonic: “CHIPS application” (Current amplification, High impedance, Increased gain, Power handling, Slower switching)

Question 3(a) OR [3 marks]

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Discuss applications of LDR.

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

Table: Applications of LDR

Mnemonic: “CASAL applications” (Camera, Alarm, Street light, Automatic control, Light measurement)

Question 3(b) OR [4 marks]

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Comparison of clipper and clamper

Answer:

Table: Comparison between Clipper and Clamper

Diagram:

graph TD
    A[Input Signal] --> B[Clipper]
    A --> C[Clamper]
    B --> D[Amplitude Limited]
    C --> E[DC Level Shifted]

    style A fill:#bbf,stroke:#333,stroke-width:1px
    style B fill:#f9f,stroke:#333,stroke-width:1px
    style C fill:#bfb,stroke:#333,stroke-width:1px
    style D fill:#f9f,stroke:#333,stroke-width:1px
    style E fill:#bfb,stroke:#333,stroke-width:1px

Mnemonic: “CLIPS vs CLAMPS” (Cut Levels In Peak Signal vs Change Level And Maintain Peak Shape)

Question 3(c) OR [7 marks]

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Describe h-parameters circuit for CE amplifier.

Answer: h-parameters provide a simple way to analyze CE amplifier performance.

Diagram:

Table: h-parameters for CE Configuration

• Circuit analysis: Uses h-parameters to calculate voltage gain, current gain, input/output impedance
• Equivalent circuit: Combines h-parameters in a two-port network representation
• Advantage: Simplifies complex transistor behavior into linear parameters

Mnemonic: “FIRO parameters” (Forward gain, Input impedance, Reverse feedback, Output admittance)

Question 4(a) [3 marks]

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Write short note on Darlington pair.

Answer: Darlington pair combines two transistors to create a super-high gain transistor.

Diagram:

• Configuration: Two transistors where first transistor’s emitter drives second transistor’s base
• Total gain: β₁ × β₂ (product of individual transistor gains)
• Input impedance: Extremely high (β₂ × R_e1)

Mnemonic: “HIS properties” (High gain, Impedance boost, Sandwich configuration)

Question 4(b) [4 marks]

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Explain Zener diode as a voltage regulator.

Answer: Zener diode provides a constant voltage reference when operated in reverse breakdown.

Diagram:

Table: Zener Voltage Regulator

• Working: Zener operates in breakdown region, maintaining fixed voltage
• Limitation: Power dissipation capability limits maximum current

Mnemonic: “ZEBRA” (Zener Effect Breakdown Regulates Accurately)

Question 4(c) [7 marks]

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Explain Optocoupler with advantages and disadvantages.

Answer: Optocoupler (also called optoisolator) uses light to transfer signals between isolated circuits.

Diagram:

Table: Advantages and Disadvantages of Optocoupler

• Working: Input signal drives LED, which emits light detected by photodetector
• Applications: Medical equipment, industrial control, power supplies, signal isolation
• Types: Photoresistor, photodiode, phototransistor, photo-SCR based

Mnemonic: “LIGHT transfer” (Linked Isolated Galvanic-free High-voltage Transfer)

Question 4(a) OR [3 marks]

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Draw Half Wave Voltage Doubler.

Answer: Half-wave voltage doubler uses diodes and capacitors to produce DC output approximately twice the peak input voltage.

Diagram:

• Components: Two diodes and two capacitors
• Output: Approximately twice the peak input voltage

Mnemonic: “DC2” (Doubles input using Capacitors and 2 Diodes)

Question 4(b) OR [4 marks]

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Explain the working and applications of OLED.

Answer: Organic Light Emitting Diode (OLED) uses organic compounds that emit light when current flows through them.

Diagram:

graph TD
    A[OLED Structure] --> B[Cathode]
    A --> C[Organic Layer]
    A --> D[Anode]
    A --> E[Substrate]

    style A fill:#bbf,stroke:#333,stroke-width:1px
    style B fill:#f9f,stroke:#333,stroke-width:1px
    style C fill:#bfb,stroke:#333,stroke-width:1px
    style D fill:#f9f,stroke:#333,stroke-width:1px
    style E fill:#bfb,stroke:#333,stroke-width:1px

Table: Working and Applications of OLED

Mnemonic: “VIEWS technology” (Vibrant colors, Incredible contrast, Excellent angle, Wide application, Self-emitting)

Question 4(c) OR [7 marks]

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Explain working of solar battery charger circuits.

Answer: Solar battery charger converts solar energy to electrical energy to charge batteries.

Diagram:

Table: Components and Their Functions

• Working principle: Photovoltaic effect converts sunlight to electricity
• Regulation: Prevents overcharging using voltage/current regulation
• Protection: Includes reverse current protection to prevent battery discharge at night
• Types: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking)

Mnemonic: “SCORE system” (Solar Conversion, Overcharge protection, Regulation, Energy storage)

Question 5(a) [3 marks]

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Draw a block diagram of regulated power supply.

Answer: Regulated power supply provides stable DC output voltage despite variations in input or load.

Diagram:

graph LR
    A[Transformer] --> B[Rectifier]
    B --> C[Filter]
    C --> D[Voltage Regulator]
    D --> E[Output]

    style A fill:#f9f,stroke:#333,stroke-width:1px
    style B fill:#bbf,stroke:#333,stroke-width:1px
    style C fill:#bfb,stroke:#333,stroke-width:1px
    style D fill:#f9f,stroke:#333,stroke-width:1px
    style E fill:#bbf,stroke:#333,stroke-width:1px

• Components: Transformer, rectifier, filter, voltage regulator
• Function: Converts AC to stable DC regardless of load changes

Mnemonic: “TRFO blocks” (Transformer, Rectifier, Filter, Output regulator)

Question 5(b) [4 marks]

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Describe Transistor shunt Voltage Regulator.

Answer: Transistor shunt regulator maintains constant output voltage by diverting excess current through a transistor in parallel with the load.

Diagram:

Table: Transistor Shunt Regulator

• Working: Transistor conducts more when output tries to increase
• Advantage: Simple circuit with good regulation

Mnemonic: “ZEST circuit” (Zener reference, Excess current, Shunt transistor, Tension-free output)

Question 5(c) [7 marks]

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Draw and explain SMPS block diagram with its advantages and disadvantages.

Answer: Switched Mode Power Supply (SMPS) uses switching regulation for high efficiency.

Diagram:

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

    style A fill:#f9f,stroke:#333,stroke-width:1px
    style D fill:#bbf,stroke:#333,stroke-width:1px
    style E fill:#bfb,stroke:#333,stroke-width:1px
    style H fill:#f9f,stroke:#333,stroke-width:1px
    style I fill:#bbf,stroke:#333,stroke-width:1px

Table: Advantages and Disadvantages of SMPS

• Working principle: Rapidly switches power on/off at high frequency
• Size reduction: Higher switching frequency allows smaller transformers
• Applications: Computers, TVs, mobile chargers, LED drivers

Mnemonic: “SWEEP advantages” (Small size, Widerange input, Efficient, Economical, Precise regulation)

Question 5(a) OR [3 marks]

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Draw voltage regulator using three terminal IC 7812.

Answer: Three terminal IC 7812 provides fixed +12V regulated output voltage.

Diagram:

• Components: 7812 regulator IC and filter capacitors
• Pin configuration: Input, Ground, Output
• Features: Internal current limiting and thermal shutdown

Mnemonic: “IGO pins” (Input, Ground, Output)

Question 5(b) OR [4 marks]

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Describe Transistor series Voltage Regulator

Answer: Transistor series regulator controls output voltage by varying the conductivity of a series transistor.

Diagram:

Table: Features of Series Voltage Regulator

• Working principle: Transistor conductivity changes to maintain constant output
• Advantage: More efficient than shunt regulators for higher currents

Mnemonic: “CERT circuit” (Control transistor, Efficient design, Reference voltage, Transistor in series)

Question 5(c) OR [7 marks]

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Draw and explain UPS block diagram with its advantages and disadvantages.

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

Diagram:

graph TD
    A[AC Input] --> B[Surge Protector]
    B --> C[Rectifier/Charger]
    C --> D[Battery]
    C --> E[Inverter]
    D --> E
    E --> F[Output Filter]
    F --> G[AC Output]
    H[Control Circuit] --> C
    H --> E
    H --> D

    style A fill:#f9f,stroke:#333,stroke-width:1px
    style C fill:#bbf,stroke:#333,stroke-width:1px
    style D fill:#bfb,stroke:#333,stroke-width:1px
    style E fill:#f9f,stroke:#333,stroke-width:1px
    style H fill:#bbf,stroke:#333,stroke-width:1px

Table: Advantages and Disadvantages of UPS

• Types: Offline/Standby, Line-interactive, Online/Double-conversion
• Applications: Computers, medical equipment, data centers, telecommunications
• Working: Normally passes main power while charging battery; switches to battery power during outage

Mnemonic: “POWER backup” (Protection from Outages, Waveform conditioning, Emission-free, Reliability boost)