EC-405 Analog Circuits - Jun 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain how negative feedback is implemented in amplifier circuits. Discuss the advantages of negative feedback in amplifiers.
   How to answer

   • Implementation of negative feedback by sampling output and feeding back out-of-phase to input
   • Feedback network and loop gain concept
   • Advantages: stabilized gain, increased bandwidth, reduced distortion and noise
   • Improved input/output impedance control and stability
2. 7 Marks
   bDiscuss about common feedback topologies such as voltage-series, current-series, voltage-shunt and current-shunt.
   How to answer

   • Concept of sampling and mixing in feedback topologies
   • Voltage-series, current-series configurations and effect
   • Voltage-shunt, current-shunt configurations and effect
   • Input/output impedance and gain type for each topology
3. 7 Marks
   aDescribe the working principle of a Wien Bridge oscillator. What are its advantages and limitations?
   How to answer

   • Wien bridge RC network as lead-lag feedback network
   • Barkhausen condition and amplifier with non-inverting gain ~3
   • Frequency of oscillation formula $f = 1/(2\pi RC)$
   • Advantages: stable low-frequency sine wave; Limitations: amplitude stabilization and high-frequency limits
4. 7 Marks
   bDefine the Barkhausen criterion and explain its importance in determining the conditions for sustained oscillations in an oscillator circuit.
   How to answer

   • Definition: loop gain $|A\beta|=1$ and zero phase shift
   • Magnitude and phase conditions for sustained oscillation
   • Role in start-up vs steady-state oscillation
   • Application to oscillator design and stability
5. 7 Marks
   aExplain the importance of datasheets for integrated circuits. What information do datasheets typically provide?
   How to answer

   • Purpose of datasheet for correct selection and design
   • Electrical characteristics, absolute maximum ratings
   • Pinout, package, timing and application circuits
   • Reliability and replacement guidance
6. 7 Marks
   bIn an RC phase shift oscillator, if the resistor $R = 5\,k\Omega$ and the capacitors $C_{1}, C_{2}$ and $C_{3}$ are each $0.01\,\mu F$, calculate the frequency of oscillation.
   How to answer

   • RC phase-shift oscillator frequency formula $f = 1/(2\pi\sqrt{6}RC)$
   • Substitute $R = 5\,k\Omega$, $C = 0.01\,\mu F$
   • Calculate denominator and frequency in Hz
   • Result ~1.3 kHz
7. 7 Marks
   aDescribe the structure and operation of a single-input balanced output differential amplifier.
   How to answer

   • Differential pair with single-ended input and dual balanced output
   • Tail current source and emitter-coupled operation
   • Differential vs common-mode operation
   • Voltage gain and balanced output advantage
8. 7 Marks
   bFor a differential amplifier circuit with resistor values $R_{1} = 10\,k\Omega$, $R_{2} = 10\,k\Omega$ and $R_{f} = 20 k\Omega$, calculate the output voltage $(V_{out})$ if the input voltage difference $(V^{+}_{in} - V^{-}_{in})$ is 2 volts.
   How to answer

   • Differential gain formula $A_v = -R_f/R_1$
   • Substitute $R_1 = 10\,k\Omega$, $R_f = 20\,k\Omega$
   • Multiply by differential input $2\,V$
   • Output magnitude and sign ~ -4 V
9. 7 Marks
   aExplain the concept of input offset voltage, offset current and input bias current in op-amps. How does it affect the accuracy of the amplifier?
   How to answer

   • Definitions of input offset voltage, offset current, bias current
   • Causes due to transistor mismatch
   • Effect as output error and DC inaccuracy
   • Compensation and nulling methods
10. 7 Marks
    bAn op-amp has an input bias current of 100 nA. If the op-amp is connected to a feedback resistor of $10\,k\Omega$, calculate the input offset voltage due to bias current.
    How to answer

    • Relation $V_{os} = I_b \times R_f$
    • Substitute $I_b = 100\,nA$, $R_f = 10\,k\Omega$
    • Compute $V_{os} = 1\,mV$
    • Interpretation as DC output error
11. 7 Marks
    aDescribe the voltage-series feedback amplifier configuration and its characteristics.
    How to answer

    • Voltage-series topology: voltage sampling, series mixing
    • Circuit configuration with feedback network
    • Characteristics: stabilized voltage gain, high input and low output impedance
    • Bandwidth and distortion improvement
12. 7 Marks
    bExplain the magnitude and frequency response of a filter. How do these characteristics vary with the type of filter?
    How to answer

    • Definition of magnitude response $|H(j\omega)|$ and frequency response
    • Passband, stopband, cutoff and roll-off concepts
    • Variation for low-pass, high-pass, band-pass, band-stop filters
    • Order and ideal vs practical response
13. 7 Marks
    aDescribe the block diagram and pin configuration of the 555 timer IC. Explain the function of each pin.
    How to answer

    • Internal block diagram: comparators, flip-flop, discharge transistor
    • 8-pin configuration diagram
    • Function of VCC, GND, Trigger, Threshold, Output, Reset, Control, Discharge
    • Typical supply and timing operation
14. 7 Marks
    bDiscuss the operation of a peak detector circuit. How does it capture and hold the peak value of an input signal?
    How to answer

    • Peak detector circuit using op-amp, diode and hold capacitor
    • Charging to peak and diode blocking discharge
    • Holding peak value and buffering output
    • Reset/leakage and application in measurement
15. 7 Marks
    aDiscuss the differences between fixed and adjustable voltage regulators. What are their respective advantages and limitations?
    How to answer

    • Fixed regulator: preset output vs adjustable with external resistors
    • Circuit simplicity vs flexibility comparison
    • Advantages: stability/simplicity for fixed; versatility for adjustable
    • Limitations: lack of adjustability vs extra components and design care
16. 7 Marks
    bDescribe the features and specifications of the LM317 voltage regulator IC. What are its typical applications?
    How to answer

    • LM317 as adjustable positive voltage regulator overview
    • Key features: 1.2-37V range, current limit, thermal shutdown
    • Electrical specifications and pinout
    • Typical applications: lab supplies, battery chargers, constant current source