PERI INSTITUTE OF TECHNOLOGY (AUTONOMOUS)

Mannivakkam, Chennai-48

B.E. DEGREE EXAMINATION APRIL/MAY 2025

DEPARTMENT OF EEE
Regulations2021
Semester: IV
Maximum Marks: 100
SubjectCode:EE3402
Subject Name: Linear Integrated Circuits

ANSWER KEY
PART-A(10 x 2= 20Marks)
Q. No. Answer Spli Tolal
t up Mark
1 1. State the limitations of IC technology.
 Limited power handling capability
 Limited voltage ratings
 Limited component types (inductors and transformers are
Any 4 2
difficult to integrate)
 Poor heat dissipation
 Fixed values of components (not tunable)
 Difficult to repair once damaged
2 2. Discuss the purpose of oxidation process in IC fabrication.
 Purpose of Oxidation:
o To grow a thin SiO₂ (silicon dioxide) layer on the
silicon wafer
o Acts as an insulating layer
Any 2 2
o Provides masking layer during diffusion or ion
implantation
o Serves as a dielectric in MOS devices
o Helps in surface passivation to protect the device
from contaminants
3 Define input bias current.
 The input bias current of an op-amp is the average of the
1
DC currents entering the inverting and non-inverting
terminals of the op-amp.
2

4 How do you construct a voltage follower circuit? List its 1 2

applications.
Construction:
 Connect the output of an op-amp directly to the inverting
input (-)
 Apply the input signal to the non-inverting input (+) 1
Applications:
 Buffering high impedance sources
 Impedance matching
 Preventing loading of circuits
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  Signal isolation
5 List the essential features of an op-amp based instrumentation
amplifier.
 High input impedance
 Low output impedance
Any 3 2
 High common-mode rejection ratio (CMRR)
 High gain accuracy and stability
 Low offset voltage and drift
 Differential input
6 6. State the principle of single slope A/D converter.
A Draw the internal block diagram of a function generator
ICworks by integrating the input voltage and comparing it with
2 2
a linearly increasing (ramp) reference voltage.The time taken
for the ramp voltage to equal the input voltage is measured and
converted to a digital value.
7 7. Why VCO is called voltage to frequency converter?
 A VCO (Voltage-Controlled Oscillator) produces an output
signal whose frequency is directly proportional to the 2 2
applied input control voltage.
 Thus, it converts voltage input → frequency output.
8 8. Draw the pin diagram of IC 555 timer.

2 2

9 9. List the characteristics of three-terminal IC regulators.

 Fixed output voltage (e.g., 5V, 9V, 12V)
 Built-in current limiting
 Thermal shutdown protection Any 4 2
 Short-circuit protection
 Easy to use with only input and output capacitors
 Low ripple and noise
10 2 2
10. Draw the internal block diagram of a function
generator IC

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 PART-B(5 x 13 = 65Marks )
Q. No. Answer Split Tolal
up Mar
k
(a) (i) Epitaxial Growth and Diffusion 1 13
11 Epitaxial Growth:
 Definition:
Epitaxy refers to the controlled growth of a
crystalline silicon layer on a crystalline substrate,
typically a silicon wafer.
 Types:
o Homoepitaxy: Silicon grown on silicon 1
substrate.
o Heteroepitaxy: A different material (e.g., GaAs)
grown on silicon.
 Purpose:
o To provide a high-purity and defect-free active 1
layer for transistor fabrication.
o To improve the electrical characteristics of
devices.

Process:
Performed in a high-temperature furnace, where silicon
gases (e.g., silane – SiH₄) decompose and deposit silicon 1
atoms on the wafer.
Diffusion:
 Definition:
Diffusion is the process of introducing impurities
(dopants) like boron, phosphorus, or arsenic into 1
specific regions of the silicon wafer to alter its
electrical properties.

Subject Code : Page3of18

  Purpose:
o To create n-type or p-type regions in the
semiconductor.
o To form junctions (e.g., base, emitter, collector
in BJTs). 1

 Process:
o The wafer is placed in a furnace at high
temperature (~1000°C).
o A dopant gas (e.g., B₂H₆ for boron) is
introduced, and atoms diffuse into the surface
of the silicon.
(ii) Photolithography 1 7
Definition:
Photolithography is the process of transferring geometric
patterns from a photomask onto the surface of a silicon
wafer using light exposure and a photoresist material.

Steps Involved:
4
1. Wafer Cleaning – Remove dust or particles.
2. Photoresist Coating – A light-sensitive material
is spun onto the wafer.
3. Soft Baking – To evaporate the solvent and
harden the photoresist.

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 4. Mask Alignment and Exposure – The wafer is
aligned with a mask and exposed to UV light.
5. Development – The exposed (or unexposed,
depending on resist type) parts of photoresist
are dissolved.
6. Etching – The unprotected areas of silicon
dioxide or metal are etched away.
7. Photoresist Removal – Remaining resist is
stripped off.
Purpose:
 To create precise patterns for integrated circuit
components like transistors, resistors, and
interconnect.
 Enables miniaturization in IC fabrication.

(b) (i) Explain the different isolation techniques

In Integrated Circuit (IC) fabrication, isolation techniques
are essential to electrically separate components (e.g.,
transistors) on the same chip to prevent interference.
1. PN-Junction Isolation:
 Uses reverse-biased PN junctions to isolate
components.
 A p-type substrate is diffused with an n-type well,
and an n-type region is formed.
 Reverse biasing prevents current flow between
Each
components.
2
 Advantages: Simple and reliable.
 Disadvantages: Consumes more space; leakage
currents can occur. 6
2. Dielectric Isolation:
 Uses an insulating layer (like SiO₂) to isolate devices.
 A device is completely enclosed in an oxide layer and
separated from the substrate.
 Advantages: Excellent electrical isolation, low
leakage.
 Disadvantages: More complex and expensive.
3. Silicon-on-Insulator (SOI):
 A thin silicon layer is placed on an insulating
substrate (SiO₂).
 Devices are built on this silicon layer.
 Offers low parasitic capacitance and high speed.
 Used in high-performance ICs
(ii) Describe in detail about the diffusion process of IC 13
fabrication 1
Diffusion Process:
Diffusion is the process of introducing dopant atoms into
specific regions of the silicon wafer to modify its electrical

Subject Code : Page5of18

 properties.
Steps in Diffusion: 3
1. Pre-Deposition:
o Dopant atoms (e.g., boron for p-type or
phosphorus for n-type) are introduced in high
concentration.
o Typically done in a furnace at high temperature
(~1000°C).
o Dopant gases like B₂H₆ (diborane) or PH₃ 3
(phosphine) are used.
2. Drive-in Diffusion:
o The wafer is heated again (often higher
temperature, longer time) to drive the dopants
deeper into the substrate.
o Achieves desired junction depth and uniform
distribution.
Fick’s Laws of Diffusion:
 Govern the movement of dopants in silicon:

where:
o C is dopant concentration,
o D is diffusion coefficient,
o x is depth,
o t is time.
Applications of Diffusion:
 Forming base, emitter, collector regions in BJTs
 Creating source and drain in MOSFETs
 Tuning resistors and capacitors in ICs

(a) Explain and derive the condition for DC characteristics of 13

12 an operational amplifier
DC Characteristics of Op-Amp:
DC characteristics refer to how the op-amp behaves with
constant (DC) inputs, including: 4
 Input offset voltage (V_os) 4
 Input bias current (I_B) 4
 Input offset current (I_OS) 3
 Thermal drift

(b) Inverting Amplifier and Non-Inverting Amplifier 5 13

The op-amp can be configured as a non-inverting
amplifier or as an inverting amplifier. The circuits
for these two amplifier configuration is shown
below:

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 8
Differential Amplifier

Then differential amplifiers amplify the difference between

two voltages making this type of operational amplifier
circuit a Subtractor unlike a summing amplifier which
adds or sums together the input voltages. This type of
operational amplifier circuit is commonly known as
a Differential Amplifier configuration and is shown
below:

Subject Code : Page7of18

 Differential Amplifier Equation

(a) Log and Anti-Log Amplifiers Using Op-Amp 1 13

13 The same circuit providing antilog of the input can be
obtained by using a transistor instead of a diode .

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 6

(b) i) R-2R Ladder Type D/A Converter (Digital-to-Analog 13

Converter)
Description:
The R-2R ladder DAC is a popular digital-to-analog
converter that uses only two resistor values (R and 2R),
arranged in a ladder network. It converts a binary digital

Subject Code : Page9of18

 input into an analog output voltage.

 Digital inputs (D0 to D3) are connected to switches

(either to Vref or GND).
 Resistor network divides the reference voltage
proportionally based on binary weight.
 Output is taken at the junction of the ladder.
Working Principle:

Each bit controls a switch connecting to either Vref (logic

1) or GND (logic 0). The analog output is the weighted sum
of the binary inputs:
Advantages: 3
 Only two resistor values needed → easier to fabricate
 High accuracy and fast response
SAR (Successive Approximation Register) Type A/D
Converter 2
Description:
The SAR ADC converts an analog signal to digital form
using a binary search algorithm. It is fast and commonly
used in microcontrollers.
Working Principle:
1. SAR sets MSB = 1, others = 0
2. The DAC converts SAR value to analog
3. Comparator compares analog input vs. DAC output: 2

1. Repeat the process for next lower bits until LSB

Example: For 4-bit SAR ADC:
 Initial guess: 1000 (MSB)
 Refines step-by-step to find closest digital equivalent

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 Advantages:
 Faster than integrating ADCs (like single slope)
 Good accuracy
 Low power consumption

(a) Derivation of Voltage to Frequency Conversion Expression: 13

14 Let us consider a typical VCO using op-amp + integrator +
Schmitt trigger configuration (commonly used in ICs like
LM566, LM331, etc.). 1
Assumption:
The VCO consists of:
 A current source controlled by input voltage
VinV_{in}Vin
 A capacitor (C) for integration
 A comparator or Schmitt trigger to provide switching
threshold

Charging Equation:

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 3

Voltage-to-Frequency Conversion Factor (K):

Example: Frequency Modulation (FM)

 In FM transmitters, the modulating signal (audio
voltage) is applied to the input of the VCO.
 The VCO output frequency varies proportional to the
amplitude of the input voltage, producing frequency-
modulated signals.
Other Applications:
 Phase-locked loops (PLL)
 Signal generators

(b) (i) PLL as a Frequency Multiplier

Concept:A Phase-Locked Loop (PLL) locks the output
frequency of a Voltage-Controlled Oscillator (VCO) to 3
the input frequency using feedback. If a frequency
divider (N) is placed in the feedback path, the PLL can
multiply the input frequency.

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 PLL as a Frequency Translator
Concept:
A frequency translator shifts the frequency of a signal 3
from one band to another, typically using mixing. PLL can
generate a stable local oscillator (LO) frequency to
translate an input signal via a mixer.

7
4

(a) 13
15 Functional Block Diagram of IC 723 General-Purpose
Voltage Regulator

Introduction:

IC 723 is a versatile linear voltage regulator IC that can

be configured as either:
3
 A positive voltage regulator (2V to 37V)
 A negative voltage regulator

 Current limiter or current regulator

It can supply output currents up to 150 mA directly or

more with an external pass transistor.

Working Principle:

Subject Code : Page13of1

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  The error amplifier controls the pass transistor
based on the difference between the reference
voltage and the feedback voltage.
 If output voltage drops, the amplifier increases
conduction of the pass transistor → boosts output.
4
 If output rises, conduction decreases → regulates
output.

 Current limiting is done by sensing voltage drop

across an external resistor RscR_{sc}Rsc, triggering
shutdown if exceeded.

(b) Working Principle of Switched Mode Power Supply (SMPS) 13

What is SMPS

A Switched Mode Power Supply (SMPS) is an electronic 3

power supply that converts electrical power efficiently
using high-frequency switching devices, such as transistors
or MOSFETs. It converts an unregulated AC or DC input into
a regulated DC output voltage.

Working Principle Step-by-Step: 5

1. AC Input Conversion (AC to DC):

o The input AC voltage is rectified using a diode
bridge and filtered using capacitors to get
unregulated DC.

2. High-Frequency Switching:

o A MOSFET or transistor rapidly switches ON

and OFF (typically 20 kHz to 1 MHz) under the
control of a Pulse Width Modulator (PWM).

o This converts DC into a high-frequency AC

signal.

3. Transformer Action:

o This high-frequency AC passes through a small

ferrite-core transformer, which can step
up, step down, or isolate the voltage.

o Smaller transformer size is possible due to high

frequency.

4. Secondary Rectification & Filtering:

o The transformer's output is rectified (usually by

Schottky diodes) and filtered with inductors

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 and capacitors to produce smooth DC output.

5. Feedback and Regulation: 5

o A portion of the output is fed back to a

feedback control circuit.

o This feedback adjusts the duty cycle of the

PWM to maintain a constant output voltage,
compensating for load or input changes.

PART-C(1 x 15 = 15Marks)
Q. No. Answer Split TolalM
up ark
(a) Advantages of IC Voltage Regulators 2 15
16
1. Compact size: Integrated on a single chip.

2. High reliability: Less susceptible to noise and

thermal drift.

3. Thermal protection: Built-in overheating

protection.

4. Short-circuit protection: Limits output current

safely.
2
5. Low cost and ease of use: Plug-and-play with
minimal external components.

6. Wide range: Can be used for fixed and adjustable

voltages.

7. Good line and load regulation: Maintains stable

output despite input/load variations.

Subject Code : Page15of1

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 Features of General-Purpose Linear IC 723 Regulator

 Adjustable output voltage from 2V to 37V 4

 Input voltage up to 40V

 Low standby current

 Built-in reference voltage (7.15V)

4
 Current limiting capability

 Can be configured for series or shunt regulation

 Output current up to 150 mA directly or more with an

external pass transistor

Designing a 5V Regulator Using IC 723

(b) (i) Various DC and AC Characteristics of an Op-Amp 6 15

DC Characteristics:
1. Input Offset Voltage (Vos):

o The small differential DC voltage required at

the input terminals to make the output zero.

o Ideally: 0V

Subject Code : Page16of1

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 o Practical: Few millivolts (e.g., 1–5 mV)
2. Input Bias Current (IB):
The average of the DC currents entering both input
terminals.

3. Input Offset Current (IOS):

o The difference between the input bias currents:

4. Input Impedance (Zin):

o Resistance offered at the input terminals.

o Ideal: ∞

o Practical: ≥ 1 MΩ to 10⁶ MΩ
6
5. Output Impedance (Zout):

o Resistance seen at the output.

o Ideal: 0 Ω

AC Characteristics:
1. Gain Bandwidth Product (GBP):
o Product of amplifier gain and bandwidth.
o For unity-gain bandwidth op-amp:
A⋅f=constantA \cdot f = \
text{constant}A⋅f=constant
2. Slew Rate (SR):
o Maximum rate at which the output voltage can
change:
o Affects performance in high-speed applications.

3. Frequency Response:
o Indicates how the gain decreases with
frequency.
o 3dB bandwidth is the frequency at which gain
drops by 3 dB.
4. Phase Margin:
o Measure of stability; difference between phase

o Higher phase margin ⇒ more stable op-amp.

and –180° at unity gain.

(ii) Short Note on Slew Rate 3

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 Definition:
 Slew Rate is defined as the maximum rate of change
of output voltage per unit time in response to a step
input.
 Expressed in V/μs (volts per microsecond).

Example:
If the output changes by 10V in 2μs,

Importance:
 Affects the op-amp’s ability to handle high-frequency
or fast-changing signals.
 Low slew rate causes distortion in large amplitude or
fast signals.
Typical Values:
 LM741: 0.5 V/μs
 TL081: 13 V/μs
 High-speed op-amps: >100 V/μs

Prepared by:

Course Lecturer (or) Coordinator/Question Paper Scrutiny

Name MrsA.Sindhuja
Designation& Assistant Professor
Dept.
Signature

Approved by:

Programme Coordinator/HOD / Board of

Studies

Instruction for Key preparation:

 Use Book Antiqua with font size 12.
 Line spacing: 1.15
 Keep the images at the center and use high quality line diagrams only.

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