Electronics Devices & Circuits (1323202) - Winter 2024 Solution

Question 1(a) [3 marks]

Explain thermal runaway in detail.

Answer: Thermal runaway is a destructive process where a transistor gets increasingly hotter until it fails.

Diagram:

• Cause: Increased temperature decreases base-emitter voltage
• Effect: Collector current increases with temperature
• Result: Self-reinforcing cycle of heating leads to destruction

Mnemonic: "Heat Rises, Current Climbs, Transistor Dies"

Question 1(b) [4 marks]

Draw and explain fixed bias method.

Answer: Fixed bias uses a single resistor from base to voltage supply for biasing.

Circuit Diagram:

• Working: Base current (IB) = (VCC - VBE)/RB
• Characteristics: Simple circuit but poor stability
• Disadvantage: Highly sensitive to temperature variations
• Application: Used in small signal circuits where stability isn't critical

Mnemonic: "Fixed Bias: One Resistor, Poor Stability"

Question 1(c) [7 marks]

List the biasing methods. Draw the circuit of voltage divider type bias method and explain it.

Answer: The biasing methods for transistors include several techniques for establishing proper operating points.

Table: Transistor Biasing Methods

Circuit Diagram:

• Working: R1-R2 divider creates stable base voltage
• Advantage: Less affected by β variations and temperature
• Key feature: RE provides negative feedback stabilization
• Application: Most widely used in amplifier circuits

Mnemonic: "Divide and Rule for Stable Bias"

Question 1(c OR) [7 marks]

Draw and explain DC load line for common emitter amplifier.

Answer: DC load line represents all possible operating points of a transistor.

Graph:

Equation Table:

• Purpose: Graphically shows relationship between IC and VCE
• Significance: Helps determine operating point (Q-point)
• Application: Essential for amplifier design and analysis

Mnemonic: "Maximum Current or Maximum Voltage, Never Both"

Question 2(a) [3 marks]

Explain term (i) Gain (ii) Bandwidth.

Answer: These are key parameters that describe amplifier performance.

Table: Amplifier Parameters

• Gain Types: Voltage gain (Av), Current gain (Ai), Power gain (Ap)
• Bandwidth Formula: BW = fH - fL (Higher cutoff - Lower cutoff)
• Related Parameter: Gain-Bandwidth Product (constant for a specific amplifier)

Mnemonic: "Gain Makes Bigger, Bandwidth Makes Broader"

Question 2(b) [4 marks]

List advantages and disadvantages of negative feedback in amplifier.

Answer: Negative feedback significantly improves amplifier performance but with tradeoffs.

Table: Negative Feedback Characteristics

Mnemonic: "Stabilize Wide And Clean, Just Give Up Gain"

Question 2(c) [7 marks]

Draw and explain Hartley oscillator.

Answer: Hartley oscillator generates sine waves using inductive feedback.

Circuit Diagram:

• Frequency Determination: By L1, L2 and C1 values (f = 1/2π√(L × C))
• Feedback Mechanism: Inductive voltage divider (L1 and L2)
• Identifying Feature: Tapped inductor or two inductors in series
• Applications: RF signal generation, radio transmitters, communication systems

Mnemonic: "Hartley Has Helpful Inductors"

Question 2(a OR) [3 marks]

State and explain Barkhausen criterion of oscillation.

Answer: Barkhausen criteria define conditions for sustained oscillations.

The Two Main Criteria:

• Loop Gain Condition: |Aβ| = 1 (exactly 1 for sustained oscillation)
• Phase Shift Condition: ∠Aβ = 0° or 360° (signal reinforcement)
• Practical Design: Initial |Aβ| > 1, eventually stabilizes at |Aβ| = 1

Mnemonic: "For Oscillation: Unit Gain, Zero Phase"

Question 2(b OR) [4 marks]

Compare negative and positive feedback amplifier.

Answer: Feedback type dramatically changes amplifier behavior.

Comparison Table:

Mnemonic: "Negative Stabilizes, Positive Oscillates"

Question 2(c OR) [7 marks]

Draw and explain colpitt's oscillator.

Answer: Colpitt's oscillator uses capacitive voltage divider for feedback.

Circuit Diagram:

• Frequency Determination: By L, C1 and C2 values (f = 1/2π√(L × Ceq))
• Feedback Mechanism: Capacitive voltage divider (C1 and C2)
• Identifying Feature: Two capacitors in series across inductor
• Advantage: More stable frequency than Hartley

Mnemonic: "Colpitts Catches Capacitive Current"

Question 3(a) [3 marks]

Explain about DIAC.

Answer: DIAC (Diode for Alternating Current) is a bidirectional trigger diode.

Symbol and Structure:

goat

• Operation: Conducts in both directions after breakdown voltage
• Characteristic: Symmetrical V-I curve in both directions
• Key Parameter: Breakover voltage (typically 30-40V)
• Main Application: Triggering TRIACs in AC power control

Mnemonic: "DIAC: Double Direction Breakdown Device"

Question 3(b) [4 marks]

Explain triggering methods of SCR.

Answer: SCR can be triggered to conduct by several methods.

Table: SCR Triggering Methods

Mnemonic: "Gate Voltage Temperature Rate Light"

Question 3(c) [7 marks]

Draw symbol and construction of SCR. Also draw and explain V-I characteristic of SCR.

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

Symbol:

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Construction:

V-I Characteristic:

• Forward Blocking: Low current until triggering
• Forward Conduction: High current after triggering (latched)
• Holding Current: Minimum current to maintain conduction
• Latching Current: Minimum current to start latching
• Reverse Blocking: Blocks current in reverse direction

Mnemonic: "Trigger Once, Conducts Forever, Until Current Falls"

Question 3(a OR) [3 marks]

Explain about natural commutation technique of SCR.

Answer: Natural commutation turns off SCR without external circuit when AC current naturally reaches zero.

Process Diagram:

• Principle: Uses natural zero-crossing of AC supply
• Advantage: No additional commutation circuit required
• Application: AC power control circuits, light dimmers
• Limitation: Only works with AC supplies, not DC

Mnemonic: "Natural Commutation: Zero Current, Zero Effort"

Question 3(b OR) [4 marks]

Explain about Opto-couplers.

Answer: Opto-couplers provide electrical isolation using light transmission.

Structure:

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Table: Opto-coupler Types

• CTR: Current Transfer Ratio (output/input current)
• Key Feature: Complete electrical isolation between circuits
• Benefits: Noise immunity, voltage level shifting, safety

Mnemonic: "Light Leaps gaps Electrons Can't"

Question 3(c OR) [7 marks]

Draw symbol and construction of TRIAC. Also draw and explain V-I characteristic of TRIAC.

Answer: TRIAC (Triode for Alternating Current) is a bidirectional three-terminal semiconductor device.

Symbol:

goat

Construction:

V-I Characteristic:

• Bidirectional: Conducts in both directions after triggering
• Quadrant Operation: Four triggering modes based on polarities
• Applications: AC power control, light dimmers, motor control
• Advantage over SCR: Controls both halves of AC cycle

Mnemonic: "TRIAC: Two-way Road In AC Circuits"

Question 4(a) [3 marks]

State characteristics of ideal Op-Amp.

Answer: An ideal Op-Amp has perfect characteristics that real Op-Amps approximate.

Table: Ideal Op-Amp Characteristics

Mnemonic: "Infinite Gain, Impedance, Bandwidth; Zero Offset, Output Z"

Question 4(b) [4 marks]

Draw and explain monostable multivibrator using 555 timer IC.

Answer: Monostable multivibrator produces single pulse of fixed duration when triggered.

Circuit:

• Operation: Negative trigger produces output pulse with duration T = 1.1RC
• Stable State: Output LOW until triggered
• Timing Control: R and C values determine pulse width
• Retriggering: Can be retriggered after timeout

Mnemonic: "One Shot Wonder: Trigger Once, Pulse Once"

Question 4(c) [7 marks]

Draw and explain Inverting amplifier using IC 741. Also draw input and output waveforms.

Answer: Inverting amplifier reverses polarity while amplifying input signal.

Circuit:

Waveforms:

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• Gain Equation: Av = -Rf/Rin (negative sign indicates inversion)
• Input Impedance: Equal to Rin
• Virtual Ground: Inverting input maintained near 0V
• Bandwidth: Depends on gain (higher gain = lower bandwidth)
• Applications: Signal conditioning, audio amplifiers

Mnemonic: "Flips and Multiplies by Rf/Rin"

Question 4(a OR) [3 marks]

Draw symbol and pin diagram of IC 741.

Answer: The 741 is a popular general-purpose operational amplifier.

Symbol:

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8-Pin DIP Package:

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• Pin Functions: Inverting input, non-inverting input, output, power supplies
• Optional Pins: Offset null, no connection
• Power Supply: Typically ±15V or ±12V dual supply

Mnemonic: "Never Invert Plus, Very Output Not Connected"

Question 4(b OR) [4 marks]

Explain term (i) CMRR (II) Slew Rate.

Answer: These parameters define operational amplifier performance limits.

Table: Key Op-Amp Parameters

• CMRR Formula: CMRR = 20 log₁₀(Ad/Acm) dB
• CMRR Importance: Rejects noise common to both inputs
• Slew Rate Formula: SR = dVo/dt (max)
• Slew Rate Limitation: Causes distortion at high frequencies

Mnemonic: "CMRR Crushes Common Noise, Slew Rate Shows Speed"

Question 4(c OR) [7 marks]

Draw and explain Astable multivibrator using 555 timer IC.

Answer: Astable multivibrator generates continuous square waves without external trigger.

Circuit:

Output Waveform:

goat

• Timing: T1 = 0.693(RA+RB)C, T2 = 0.693(RB)C
• Frequency: f = 1.44/((RA+2RB)C)
• Duty Cycle: Can be adjusted by RA and RB
• Applications: Clock generators, LED flashers, tone generators

Mnemonic: "Always Oscillating, Never Stopping"

Question 5(a) [3 marks]

Draw basic block diagram of regulated power supply and explain it.

Answer: A regulated power supply converts AC to stable DC voltage.

Block Diagram:

• Transformer: Steps down AC voltage to required level
• Rectifier: Converts AC to pulsating DC (diode bridge)
• Filter: Smooths pulsating DC (capacitors)
• Regulator: Maintains constant output despite variations
• Output: Stable DC voltage for electronic circuits

Mnemonic: "Transformer Rectifies Filters Regulates"

Question 5(b) [4 marks]

Draw and explain summing amplifier using Op-amp.

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

Circuit:

• Output Equation: Vout = -Rf(V1/R1 + V2/R2 + V3/R3)
• Special Case: When all resistors equal, Vout = -Rf/R × (V1 + V2 + V3)
• Applications: Audio mixing, analog computers, signal averaging
• Variations: Inverting and non-inverting configurations available

Mnemonic: "Multiple Inputs, One Output, Weighted Addition"

Question 5(c) [7 marks]

Draw and explain the circuit diagram of 3 terminal voltage regulator using IC LM317 with adjustable output voltage.

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

Circuit:

• Output Voltage: VOUT = 1.25V(1 + R2/R1)
• Fixed Components: R1 = 240Ω, reference voltage = 1.25V
• Adjustability: Changing R2 sets desired output voltage
• Protection Features: Current limiting, thermal shutdown
• Applications: Variable power supplies, battery chargers
• Advantages: Few external components, robust protection

Mnemonic: "Adjust with R2, Reference Stays at 1.25"

Question 5(a OR) [3 marks]

State full form of SMPS. Also state applications of SMPS.

Answer: SMPS stands for Switch Mode Power Supply, a modern efficient power conversion technology.

Applications Table:

• Key Benefits: High efficiency (80-95%), small size, lightweight
• Drawbacks: EMI generation, more complex circuits

Mnemonic: "Switch Mode Powers Small devices"

Question 5(b OR) [4 marks]

Draw and explain differentiator using Op-amp.

Answer: Differentiator produces output proportional to rate of change of input.

Circuit:

Input/Output Waveforms:

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• Equation: Vout = -RC × d(Vin)/dt
• Function: Converts square wave to spikes, triangle to square
• Practical Issue: High noise sensitivity
• Modification: Small resistor in series with C to limit high-frequency gain
• Applications: Waveshaping, rate-of-change detection

Mnemonic: "Rate of Change Goes In, Amplitude Comes Out"

Question 5(c OR) [7 marks]

Draw and explain the circuit diagram of -12 V regulated dc power supply.

Answer: A -12V regulated supply provides stable negative voltage for analog circuits.

Circuit Diagram:

• Working Principle: Full-wave rectifier creates negative voltage
• Components: Transformer, bridge rectifier, filter capacitors, 7912 regulator
• Regulator IC: 7912 provides fixed -12V output with internal protection
• Filter Capacitors: Input capacitor filters ripple, output capacitor improves transient response
• Applications: Op-amp negative rail, analog circuits, audio equipment

Mnemonic: "Full Bridge, Big Capacitor, 7912 Regulates Negative"