(SEM IV) THEORY EXAMINATION 2022-23 ANALOG CIRCUITS

This document is a B.Tech (Semester IV) Theory Examination Question Paper for the subject KEC402 – Analog Circuits, academic session 2022–2023.
It is a 3-hour, 100-mark exam that evaluates students’ understanding of diodes, BJTs, MOSFETs, op-amps, feedback circuits, oscillators, current mirrors, filters, biasing methods, and amplifier design.

The exam contains three major sections—A, B, and C—covering short conceptual questions, analytical derivations, numerical problems, and circuit-based design questions.

SECTION A – Short Answer Questions (20 Marks, 10 × 2)

This section tests fundamental analog electronics concepts:

V–I characteristics of ideal diode & piecewise-linear model

Knee voltage & breakdown voltage (definitions + formulas)

Trans-resistance amplifier equivalent model

Why common-emitter (CE) configuration is preferred in amplifiers

Unity gain amplifier (circuit + applications)

High-frequency parameters of hybrid-π model

Effect of 6% negative feedback on gain and bandwidth (numerical)

Minimum sustainable voltage & maximum usable load in current mirror circuits

Ideal op-amp characteristics

Barkhausen Criterion for oscillations

SECTION B – Medium-Length Analytical Questions (30 Marks, Attempt Any 3)

a. MOSFET Circuit Analysis

Calculate ID and output voltage V₀ for the MOSFET circuit of Fig.1.

b. BJT DC Bias Point

Determine Q-point (operating point) for the BJT circuit in Fig.2.

c. RC Phase Shift Oscillator

Derive the expression for frequency of oscillation, and explain salient features.

d. Differential Amplifier Using BJTs

Explain structure, balanced operation, CMRR, and small-signal outputs.

e. Super Diode & Full-Wave Precision Rectifier

Explain super diode concept and full-wave precision rectifier operation with circuit diagram.

SECTION C – Long Answer / Circuit-Based & Design Questions (50 Marks, 5 × 10)

Each question has two choices; attempt one from each.

3. Frequency Response / Common Source Amplifier (10 Marks)

(a) Sketch & describe frequency response of single-stage RC-coupled BJT amplifier, including lower cutoff, midband, and upper cutoff behavior.

(b) Derive expressions for voltage gain, input resistance, output resistance of a common-source MOSFET amplifier.

4. BJT Amplifier Analysis / Current Mirror Theory (10 Marks)

(a) For the BJT in Fig.3 (β = 100):

Find DC collector current and DC collector voltage

Draw the small-signal T-model

Determine voltage gain (v₀ / vᵢ)

(b) Show that for the current mirror of Fig.4, when Q₂ has m times the emitter-base area of Q₁,

I0=m⋅IREFI_0 = m \cdot I_{REF}I0​=m⋅IREF​ 

5. Op-Amp Design / Astable Multivibrator (10 Marks)

(a) Describe design stages of an operational amplifier—input differential pair, intermediate gain stage, output stage.

(b) Design an op-amp-based astable multivibrator with 5 kHz oscillation frequency.

6. Negative Feedback / Wide Bandpass Filter (10 Marks)

(a) Explain the operation of negative feedback, block diagram, and its properties: gain stabilization, noise reduction, bandwidth extension.

(b) Design a wide bandpass active filter with:

Lower cutoff = 3 kHz

Upper cutoff = 30 kHz

Passband gain = 12 dB

7. Class-B Amplifier / Op-Amp Applications (10 Marks)

(a) Explain operation of Class-B amplifier, its drawback (crossover distortion), and method to reduce it (Class-AB biasing).

(b) Design circuits for:

Unity gain amplifier

Op-amp integrator

Overall Purpose of the Document

This exam paper assesses student mastery in:

Analog Device Fundamentals

Diodes, BJTs, MOSFET parameters, breakdown behavior.

Amplifier Design & Analysis

CE/CS amplifiers, hybrid-π model, frequency response.

Oscillators & Feedback

RC phase shift oscillator, Barkhausen criterion, negative feedback effects.

Operational Amplifier Circuits

Unity gain, inverting, precision rectifier, astable multivibrator, integrator.

Signal Processing & Filters

Active bandpass filter design.

Current Mirrors & Biasing Circuits

Operation, current transfer ratios, sustainable voltage.

The paper includes numerical problems, derivations, circuit diagrams, frequency calculations, and design-oriented questions, providing a complete assessment of analog electronics.