Electronics Devices & Circuits (1323202) - Summer 2024 Solution

Question 1(a) [3 marks]

What is heat sink. lists its types

Answer: A heat sink is a passive device that absorbs and dissipates heat from electronic components to prevent overheating.

Table: Types of Heat Sinks

Mnemonic: "PAPER" (Passive, Active, Pin-fin, Extruded, Radial)

Question 1(b) [4 marks]

Define the Following: 1. Thermal Runaway 2. Thermal Stability

Answer:

Thermal Runaway: The self-accelerating destructive process where increased temperature causes increased current flow, which further increases temperature, potentially destroying the transistor.

Thermal Stability: The ability of a transistor circuit to maintain stable operation despite temperature changes, preventing thermal runaway.

Diagram: Thermal Runaway Process

Mnemonic: "RISE" (Runaway Is Self-Escalating)

Question 1(c) [7 marks]

Explain voltage divider bias in details.

Answer: Voltage divider bias is a common transistor biasing technique that provides stable operation.

Circuit Diagram:

• Voltage divider network: R1 and R2 establish a fixed base voltage
• Stable Q-point: Maintains operating point despite temperature variations
• Better stability: Higher stability factor compared to fixed bias
• Self-adjusting: Base current automatically adjusts to counter temperature changes

Mnemonic: "VSST" (Voltage divider, Stable, Self-adjusting, Temperature resistant)

Question 1(c) OR [7 marks]

Explain D.C. Load Line in details.

Answer: DC Load Line is a graphical method for analyzing transistor bias conditions.

Diagram: DC Load Line on Transistor Characteristic Curves

• Definition: Graphical line showing all possible operating points for a given circuit
• Endpoints: (0, VCC/RC) and (VCC, 0) on IC-VCE plane
• Q-point: Intersection of load line with transistor characteristic curve
• Equation: IC = (VCC - VCE)/RC

Mnemonic: "QECC" (Q-point Exists where Collector Current meets characteristics)

Question 2(a) [3 marks]

Explain how transistor works as a switch.

Answer: A transistor switch operates in either saturation (ON) or cutoff (OFF) regions.

Table: Transistor Switch Operation

Mnemonic: "COS" (Cutoff Off, Saturation on)

Question 2(b) [4 marks]

Draw and explain colpitt oscillator.

Answer: Colpitt oscillator is an LC oscillator using a capacitive voltage divider for feedback.

Circuit Diagram:

• Feedback: Provided by capacitive voltage divider (C1, C2)
• Resonant frequency: f = 1/(2π√(L×C)), where C = (C1×C2)/(C1+C2)
• Oscillation: Maintains through regenerative feedback
• Phase shift: 360° around the loop

Mnemonic: "CFPO" (Capacitive Feedback Produces Oscillations)

Question 2(c) [7 marks]

Explain Frequency Response Two Stage RC Coupled Amplifier with circuit diagram.

Answer: Two-stage RC coupled amplifier combines two amplifier stages with RC coupling.

Circuit Diagram:

Frequency Response:

• Low frequency: Gain drops due to coupling capacitor impedance
• Mid frequency: Maximum flat gain region (bandwidth)
• High frequency: Gain drops due to transistor capacitance effects
• Overall gain: Product of individual stage gains

Mnemonic: "LMH" (Low drops, Mid flat, High drops)

Question 2(a) OR [3 marks]

Draw circuit diagram of Hartley oscillator.

Answer:

Circuit Diagram of Hartley Oscillator:

Mnemonic: "ITLC" (Inductor Tapped for LC Circuit)

Question 2(b) OR [4 marks]

List different types of negative feedback.

Answer:

Table: Types of Negative Feedback

Mnemonic: "VSCS" (Voltage Series, Current Shunt)

Question 2(c) OR [7 marks]

List advantages of Negative feedback amplifier and Explain voltage series negative feedback in details.

Answer:

Advantages of Negative Feedback:

• Stabilizes gain against component variations
• Reduces distortion and noise
• Increases bandwidth
• Modifies input/output impedance
• Improves linearity

Voltage Series Negative Feedback:

• Configuration: Output voltage sampled, fed back in series with input
• Closed-loop gain: ACL = A/(1+Aβ), where A is open-loop gain and β is feedback fraction
• Input impedance: Increases by factor (1+Aβ)
• Output impedance: Decreases by factor (1+Aβ)

Mnemonic: "SIGO" (Stable gain, Increased input impedance, Gain reduction, Output impedance reduction)

Question 3(a) [3 marks]

Draw circuit of SCR using two transistor analogy.

Answer:

Two Transistor Analogy of SCR:

Mnemonic: "PNPNPN" (PNP and NPN structure)

Question 3(b) [4 marks]

Draw and explain Natural Commutation of SCR.

Answer: Natural commutation occurs when the SCR current naturally falls below the holding current.

Circuit Diagram:

Current Waveform:

       ┌───┐     ┌───┐
       │   │     │   │
───────┘   └─────┘   └─────
  SCR OFF    SCR OFF
       SCR ON    SCR ON

• Definition: SCR turns off automatically when current falls below holding current
• AC circuit: Occurs naturally at end of each positive half-cycle
• Zero crossing: SCR turns off when AC voltage crosses zero
• No external circuit: No additional components needed for turn-off

Mnemonic: "NAZC" (Natural At Zero Crossing)

Question 3(c) [7 marks]

Explain how TRIAC can be used as fan regulator and on-off control for ac power.

Answer: TRIAC is a bidirectional device ideal for AC power control applications.

TRIAC Fan Regulator Circuit:

TRIAC On-Off Control:

• Fan Regulation: Phase control technique varies power to fan
• Potentiometer: Adjusts firing angle of TRIAC
• On-Off Control: Simple switch triggers TRIAC gate
• Bidirectional: Controls current in both half-cycles

Mnemonic: "FPOB" (Fan Power is controlled by Phase angle in both directions)

Question 3(a) OR [3 marks]

Draw symbol of SCR, DIAC and TRIAC.

Answer:

Symbols of Thyristors:

goat

Mnemonic: "SDT" (SCR has gate on one side, DIAC has none, TRIAC has gate in middle)

Question 3(b) OR [4 marks]

Draw and explain Gate triggering of SCR.

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

Circuit Diagram:

• Principle: Applying positive voltage between gate and cathode
• Current requirement: Small gate current triggers much larger anode current
• Latching: Once triggered, SCR remains ON even if gate signal is removed
• Turn-off: Requires reducing anode current below holding current

Mnemonic: "GPLT" (Gate Pulse Latches Thyristor)

Question 3(c) OR [7 marks]

Draw Construction and Voltage Vs Current characteristic of SCR and explain V-I characteristic.

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

SCR Construction:

V-I Characteristic:

          I
          ↑
          │        ON State
          │       ┌────────
          │       │
          │       │
  Holding │       │
  current ├───────┤
          │       │
          │Forward│
          │breakover
          │voltage│
          │       │
          └───────┴──────→ V
                   Reverse
                   breakdown
                   voltage

• Forward blocking region: SCR conducts minimal current until breakover voltage
• Forward conduction region: Low resistance state after triggering
• Reverse blocking region: Blocks current in reverse direction
• Gate triggering: Reduces breakover voltage, facilitating turn-on

Mnemonic: "FBRH" (Forward Blocking, Reverse blocking, Holding current)

Question 4(a) [3 marks]

Explain OP-AMP as a summing amplifier.

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

Circuit Diagram:

• Function: Outputs weighted sum of input voltages
• Output equation: Vout = -(V1×Rf/R1 + V2×Rf/R2 + V3×Rf/R3)
• Equal weights: When R1 = R2 = R3, output is simple sum multiplied by -Rf/R
• Virtual ground: Inverting input maintains 0V potential

Mnemonic: "SWAP" (Sum Weighted And Proportional)

Question 4(b) [4 marks]

Define the following OP-AMP parameters: 1. input bias current 2. CMRR

Answer:

Input Bias Current: The average of the currents flowing into the two input terminals of an op-amp when the output is at zero.

CMRR (Common Mode Rejection Ratio): The ratio of differential gain to common-mode gain, indicating how well an op-amp rejects signals common to both inputs.

Table: Op-Amp Parameters

Mnemonic: "BIC-CMR" (Bias Is Current, Common Mode Rejection)

Question 4(c) [7 marks]

Draw and explain monostable multivibrator using 555 Timer.

Answer: Monostable multivibrator generates a single pulse of predetermined duration when triggered.

Circuit Diagram:

Output Waveform:

Trigger  ___┐      ____________
             │______│
             
Output   ____┌──────┐__________
              │      │
              T = 1.1RC

• Operation: Single stable state (output LOW), temporarily HIGH when triggered
• Pulse width: T = 1.1 × R × C (seconds)
• Triggering: Falling edge on TRIG pin (pin 2)
• Timing components: R and C determine pulse duration

Mnemonic: "POST" (Pulse Output, Single Trigger)

Question 4(a) OR [3 marks]

Draw the circuit diagram of OP-AMP as a inverting amplifier.

Answer:

Inverting Amplifier Circuit:

Mnemonic: "IRON" (Inverting Requires One Negative input)

Question 4(b) OR [4 marks]

Define the following OP-AMP parameters: 1. input offset current 2. slew rate

Answer:

Input Offset Current: The difference between the currents flowing into the two input terminals of an op-amp.

Slew Rate: The maximum rate of change of output voltage per unit of time, typically measured in V/μs.

Table: Op-Amp Parameters

Mnemonic: "IOSR" (Input Offset and Slew Rate)

Question 4(c) OR [7 marks]

Explain op-amp as Inverting amplifier and obtain equation of its Voltage gain.

Answer: Inverting amplifier produces an output signal that is inverted and amplified.

Circuit Diagram:

Voltage Gain Derivation:

At node N (inverting input):
I1 + If = 0  (By Kirchhoff's Current Law)
(Vin - VN)/R1 + (Vout - VN)/Rf = 0

Since VN ≈ 0 (virtual ground):
Vin/R1 + Vout/Rf = 0
Vout/Vin = -Rf/R1

• Gain equation: Vout/Vin = -Rf/R1
• Virtual ground: Inverting terminal maintained at 0V
• Input impedance: Equal to R1
• Negative feedback: Provides stability and linearity

Mnemonic: "GIVN" (Gain Is Negative, Virtual ground)

Question 5(a) [3 marks]

Draw the block diagram of IC 555.

Answer:

Block Diagram of IC 555:

Mnemonic: "CVOT" (Comparators, Voltage divider, Output stage, Timer)

Question 5(b) [4 marks]

Draw the circuit diagram of OP-AMP as a wein bridge oscillator.

Answer:

Wein Bridge Oscillator Circuit:

Mnemonic: "WPRC" (Wein Produces Resonant Circuit)

Question 5(c) [7 marks]

Explain working of different types of Fixed and variable voltage regulator IC.

Answer: Voltage regulator ICs maintain stable output voltage despite input or load variations.

Fixed Voltage Regulators:

Variable Voltage Regulator:

• Fixed regulators: 78XX (positive) and 79XX (negative) series provide specific voltages
• Variable regulators: LM317 (positive) and LM337 (negative) allow adjustable output
• Three-terminal design: Input, output, and ground/adjust terminals
• Output equation for LM317: Vout = 1.25V × (1 + R2/R1)
• Protection features: Short circuit, thermal overload, and safe area protection

Mnemonic: "FAVOR" (Fixed And Variable Output Regulators)

Question 5(a) OR [3 marks]

Draw the block diagram of astable multivibrator using 555 timer.

Answer:

Astable Multivibrator Block Diagram:

Mnemonic: "FOFT" (Free-running Oscillator From Timer)

Question 5(b) OR [4 marks]

Draw and explain solar based battery charger circuits.

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

Circuit Diagram:

• Solar panel: Converts sunlight to DC electricity
• Blocking diode: Prevents battery discharge through panel at night
• Regulator IC: Controls charging voltage and current
• Charge indicator: Shows charging status
• Protection: Overcharge and reverse polarity protection

Mnemonic: "SBRCP" (Solar, Blocking diode, Regulator, Charging, Protection)

Question 5(c) OR [7 marks]

Draw and explain the block diagram of SMPS.

Answer: SMPS (Switch Mode Power Supply) converts electrical power efficiently using switching regulators.

Block Diagram:

• EMI filter: Removes noise from AC input
• Rectifier: Converts AC to unregulated DC
• Switching circuit: Chops DC at high frequency (20-100 kHz)
• Transformer: Provides isolation and voltage conversion
• Output rectifier: Converts high-frequency AC back to DC
• Output filter: Smooths DC output
• Feedback circuit: Monitors output for regulation
• Control circuit: Adjusts switching based on feedback

Mnemonic: "ERST-FOFC" (EMI filter, Rectifier, Switching, Transformer, Feedback, Output rectifier, Filter, Control)