Linear Integrated Circuit (4341105) - Winter 2023 Solution

Question 1(a) [3 marks]

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What is negative feedback? List out advantages and disadvantages of negative feedback.

Answer: Negative feedback is feeding a portion of output signal back to the input with 180° phase shift to reduce the input signal.

Mnemonic: “SIRS” - Stability Improved, Reduced distortion, Sensitivity decreased

Question 1(b) [4 marks]

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Describe the effect of negative feedback on frequency response and distortion of an amplifier.

Answer: Negative feedback improves both frequency response and reduces distortion in amplifiers.

Diagram:

graph TD
    A[Amplifier without feedback] --> B[Narrow bandwidth]
    C[Amplifier with negative feedback] --> D[Wider bandwidth]
    E[Input with harmonics] --> F[Amplifier without feedback] --> G[Output with more harmonics]
    E --> H[Amplifier with negative feedback] --> I[Output with fewer harmonics]

Mnemonic: “WIDE” - With negative feedback, Improved response, Distortion reduced, Extended bandwidth

Question 1(c) [7 marks]

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Derive an equation for overall gain of negative feedback voltage amplifier.

Answer: The equation for overall gain of negative feedback voltage amplifier can be derived as follows:

Diagram:

• Input equation: V’ = Vi - βVo
• Output equation: Vo = AV'
• Substituting: Vo = A(Vi - βVo)
• Solving for Vo: Vo = AVi - AβVo
• Rearranging: Vo(1 + Aβ) = AVi
• Final equation: Vo/Vi = A/(1 + Aβ) = Af

Mnemonic: “LOOP” - Look at Original Open-loop gain and Proceed with feedback

Question 1(c) OR [7 marks]

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Compare voltage shunt amplifier and current series amplifier.

Answer:

Diagram:

graph TB
    subgraph "Voltage Shunt"
        A1[Input voltage] --> B1[Shunt connected β]
        B1 --> C1[Amplifier]
        C1 --> D1[Output current]
    end
    subgraph "Current Series"
        A2[Input current] --> B2[Series connected β]
        B2 --> C2[Amplifier]
        C2 --> D2[Output voltage]
    end

Mnemonic: “VICS” - Voltage shunt In, Current out Series has opposite

Question 2(a) [3 marks]

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Discuss Barkhausen’s criteria for oscillation.

Answer: Barkhausen’s criteria states that for sustained oscillations, the following conditions must be met:

Diagram:

Mnemonic: “LOOP” - Loop gain One, Oscillation needs Phase shift zero

Question 2(b) [4 marks]

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Draw circuit diagram of Hartley oscillator and Colpitts oscillator.

Answer:

Hartley Oscillator:

Colpitts Oscillator:

Mnemonic: “HaLs CoCs” - Hartley has inductors in series, Colpitts has Capacitors in series

Question 2(c) [7 marks]

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Explain UJT as a relaxation oscillator.

Answer: UJT (Unijunction Transistor) works as a relaxation oscillator by repeatedly charging and discharging a capacitor.

Diagram:

• Intrinsic standoff ratio: η = RB1/(RB1+RB2)
• Peak voltage: VP = η×VBB + VD
• Frequency: f = 1/[R×C×ln(1/(1-η))]

Mnemonic: “CFDR” - Charge, Fire, Discharge, Repeat

Question 2(a) OR [3 marks]

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Classify Oscillators.

Answer:

Diagram:

graph TD
    A[Oscillators] --> B[RC Oscillators]
    A --> C[LC Oscillators]
    A --> D[Crystal Oscillators]
    A --> E[Relaxation Oscillators]
    B --> F[Wien Bridge]
    B --> G[Phase Shift]
    C --> H[Hartley]
    C --> I[Colpitts]
    C --> J[Clapp]
    E --> K[UJT based]
    E --> L[IC 555 based]

Mnemonic: “SRLC” - Sine waves from RC, LC, and Crystal oscillators

Question 2(b) OR [4 marks]

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Explain construction of UJT with its symbol.

Answer: UJT (Unijunction Transistor) consists of a lightly doped N-type silicon bar with electrical connections at both ends (bases) and a P-type emitter junction.

Diagram:

Mnemonic: “BEB” - Bases at Ends, Emitter in Between

Question 2(c) OR [7 marks]

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Explain working of Wien Bridge oscillator circuit. List out its application.

Answer: Wien Bridge oscillator produces sine waves using RC network for positive feedback and negative feedback for amplitude stability.

Diagram:

graph TD
    subgraph "Positive Feedback"
        R1 --- C1
        R2 --- C2
    end
    subgraph "Negative Feedback"
        R3
        R4
    end
    A[Op-Amp] --> Output
    R1 & C1 & R2 & C2 --> A
    A --> R3 --> R4 --> A

Applications:

• Audio signal generators
• Function generators
• Musical instrument tuning
• Test equipment
• Filter circuits

Mnemonic: “APPS” - Audio Production, Pure Sine waves, Stable frequency

Question 3(a) [3 marks]

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Differentiate between voltage and power amplifier.

Answer:

Mnemonic: “VICE” - Voltage amplifiers Increase voltage, Current not important, Efficiency not critical

Question 3(b) [4 marks]

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Derive an equation for Efficiency of class B push pull amplifier.

Answer: Efficiency (η) of a Class B push-pull amplifier is derived as follows:

Diagram:

• AC power output: P₀ = Vrms × Irms = (Vm/√2) × (Im/√2) = Vm × Im/2
• DC power input: PDC = VCC × IDC = VCC × (2×Im/π)
• Efficiency: η = P₀/PDC = (Vm×Im/2)/(VCC×2×Im/π) = (Vm×π)/(4×VCC)
• For maximum swing: Vm = VCC, so η = π/4 = 78.5%

Mnemonic: “POP” - Push-pull Output Power = π/4 or 78.5%

Question 3(c) [7 marks]

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Explain working of Class-B Push Pull Amplifiers along with waveform.

Answer: Class B push-pull amplifier uses two transistors to amplify opposite halves of the input waveform.

Diagram:

graph LR
    A[Input Signal] --> B[Driver Stage]
    B --> C[Upper Transistor]
    B --> D[Lower Transistor]
    C --> E[Output Transformer]
    D --> E
    E --> F[Output Signal]

    subgraph "Waveforms"
    direction LR
    G[Input] --- H[T1 Conducts] --- I[T2 Conducts]
    end

Key points:

• Efficiency: Approximately 78.5% (π/4)
• Conduction angle: 180° for each transistor
• Crossover distortion: Due to both transistors being off near zero crossing
• Advantages: Higher efficiency, less heat, suitable for high power

Mnemonic: “HOPE” - Half cycle Operation, Push-pull, Efficiency high

Question 3(a) OR [3 marks]

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Explain Classification of Power amplifier.

Answer:

Diagram:

graph TD
    A[Power Amplifiers] --> B[Class A]
    A --> C[Class B]
    A --> D[Class AB]
    A --> E[Class C]
    B --> F[Low distortion, Low efficiency]
    C --> G[Medium distortion, High efficiency]
    D --> H[Low distortion, Medium efficiency]
    E --> I[High distortion, Very high efficiency]

Mnemonic: “ABCE” - As Biasing Changes, Efficiency increases

Question 3(b) OR [4 marks]

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Derive an equation for Efficiency of class A power amplifier.

Answer: Efficiency of Class A power amplifier is derived as follows:

Diagram:

• Maximum AC power output: P₀ = (Vrms)²/RL = (VCC/2√2)²/RL = VCC²/8RL
• DC power input: PDC = VCC × IDC = VCC × (VCC/2RL) = VCC²/2RL
• Efficiency: η = P₀/PDC = (VCC²/8RL)/(VCC²/2RL) = 1/4 = 25%

Mnemonic: “ONE” - Output Never Exceeds 25% efficiency in Class A

Question 3(c) OR [7 marks]

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Explain working of Class-A transformer coupled Amplifiers along with waveform.

Answer: Class A transformer coupled amplifier conducts for the full input cycle (360°) using a transformer for output coupling.

Diagram:

Waveform characteristics:

• Input and output waveforms are in phase
• No crossover distortion
• Full cycle amplification
• Low efficiency (25%)
• Low distortion

Mnemonic: “FACT” - Full cycle Amplification in Class-a with Transformer

Question 4(a) [3 marks]

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Define (i) CMRR (ii) Slew Rate

Answer:

CMRR: CMRR = 20 log₁₀(Ad/Acm) where Ad is differential gain and Acm is common mode gain

Slew Rate: SR = dVout/dt (V/μs)

Mnemonic: “CRiSp” - CMRR Rejects common signals, Slew Rate limits speed

Question 4(b) [4 marks]

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Explain inverting amplifier of operational amplifiers with sketch.

Answer: Inverting amplifier provides gain with 180° phase shift using negative feedback.

Diagram:

Key equations:

• Gain: A = -Rf/Ri
• Input impedance: Z = Ri
• Bandwidth: Depends on op-amp and gain

Mnemonic: “IRON” - Inverting, Resistance ratio gives gain, Output Negative phase

Question 4(c) [7 marks]

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Explain Op-amp as a Summing amplifier.

Answer: Summing amplifier adds multiple input signals with weighted contributions.

Diagram:

graph LR
    V1[V1] -->|R1| A((\+))
    V2[V2] -->|R2| A
    V3[V3] -->|R3| A
    A --- B[Op-Amp]
    B --- C[Vout]
    C -.->|Rf| A

Circuit:

Applications:

• Audio mixers
• Signal processing
• Analog computers
• Weighted averages

Mnemonic: “SARI” - Summing Amplifier Requires Inverting configuration

Question 4(a) OR [3 marks]

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Sketch basic Block diagram of an operational amplifier.

Answer:

Diagram:

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

Mnemonic: “DILO” - Differential Input, Level shifting, Output amplification

Question 4(b) OR [4 marks]

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Explain non inverting amplifier of operational amplifiers with sketch.

Answer: Non-inverting amplifier provides gain without phase inversion using negative feedback.

Diagram:

Mnemonic: “NIPS” - Non-inverting, Input and output In Phase, Same polarity

Question 4(c) OR [7 marks]

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Explain Op-amp as an Integrator.

Answer: Op-amp integrator produces output proportional to the time integral of the input.

Diagram:

Applications:

• Analog computers
• Waveform generators
• PID controllers
• Active filters
• Signal processing

Mnemonic: “TIME” - Takes Input and Makes time-dependent Effect

Question 5(a) [3 marks]

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Draw Pin Diagram of IC 555.

Answer:

Diagram:

Mnemonic: “GTOR-CTD” - Ground, Trigger, Output, Reset, Control, Threshold, Discharge

Question 5(b) [4 marks]

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Explain astable multivibrator of timer IC 555.

Answer: Astable multivibrator using IC 555 generates continuous square wave output without any external trigger.

Diagram:

graph LR
    A[VCC] --> B[R1]
    B --> C[Pin 7]
    B --> D[Pin 6/2]
    C --> E[IC 555]
    D --> E
    F[R2] --> D
    F --> G[Pin 7]
    G --> E
    H[C] --> D
    H --> I[GND]
    E --> J[Output Pin 3]

Mnemonic: “FREE” - FREquency Established by External RC network

Question 5(c) [7 marks]

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Explain working of Complementary symmetry Push Pull Amplifiers.

Answer: Complementary symmetry push-pull amplifier uses complementary transistors (NPN and PNP) to amplify both halves of the waveform.

Diagram:

Key features:

• No center-tapped transformer: Simpler design than transformer-coupled push-pull
• Crossover distortion: Requires biasing to minimize
• Efficiency: About 78.5% (Class B operation)
• Thermal runaway: Risk if not properly designed
• Applications: Audio power amplifiers, output stages of op-amps

Mnemonic: “COPS” - Complementary Opposing Pair of transistors for Symmetrical operation

Question 5(a) OR [3 marks]

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Draw the diagram of Sequential Timer.

Answer:

Diagram:

graph LR
    A[Start] --> B[555 Timer 1]
    B --> C[555 Timer 2]
    C --> D[555 Timer 3]
    D --> E[Optional additional timers]
    B --> B1[Output 1]
    C --> C1[Output 2]
    D --> D1[Output 3]

Mnemonic: “SET” - Sequential Events Triggered one after another

Question 5(b) OR [4 marks]

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Explain bistable multivibrator of timer IC 555.

Answer: Bistable multivibrator using IC 555 has two stable states and changes state only when triggered.

Diagram:

Mnemonic: “FLIP” - Firmly Latched In Position until triggered

Question 5(c) OR [7 marks]

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Compare different types of power Amplifiers.

Answer:

Diagram:

graph TD
    A[Power Amplifier Classes] --> B[Class A: 360° conduction]
    A --> C[Class B: 180° conduction]
    A --> D[Class AB: 180°-360° conduction]
    A --> E[Class C: <180° conduction]
    B --- B1[25-30% efficient]
    C --- C1[78.5% efficient]
    D --- D1[50-78.5% efficient]
    E --- E1[>78.5% efficient]

Mnemonic: “ABCE” - As Biasing Condition changes, Efficiency increases