ETE 3321 Linear

Integrated Circuits

and Applications

Integrated Circuit Packaging Technology

UNIT I.

Introduction Integrated circuits - Classification IC Manufacturing process

o Thin and thick film techniques o Monolithic techniques

Epitaxial Photolithography Metallization Diffusion systems Surface passivation and isolation

Unit 1Introduction

In the early 1960s, a new field of microelectronics was born primarily to meet the

requirements of the Military which wanted to reduce the size of its electronic

equipment to approximately one-tenth of its then existing volume. This drive for

extreme reduction in the size of electronic circuits has led to the development of

microelectronic circuits called integrated circuits (ICs) which are so small that

their actual construction is done by technicians using high powered microscopes.

1.1 What is an Integrated Circuit?

To put it very briefly, an integrated circuit (IC) is just a packaged electronic circuit.

An IC is a complete electronic circuit in which both the active and passive

components are fabricated on a tiny single chip of silicon. Active components are

those which have the ability to produce gain.

Examples are : transistors and FETs.

Passive components or devices are those which do not have this ability. Examples

are : resistors, capacitors and inductors.

ICs are produced by the same processes as are used for manufacturing individual

transistors and diodes etc. In such circuits, different components are isolated from

each other by isolation diffusion within the crystal chip and are interconnected by

an aluminum layer that serves as wires.

A discrete circuit, on the other hand, is one that is built by connecting separate

components. In this case, each component is produced separately and then all are

assembled together to make the electronic circuit

J.S. Kilby of Texas Instruments was the first person to develop (in 1959) an

integrated circuit a single monolithic silicon chip in which active and passive

components were fabricated by successive deposition, etching and diffusions. He

was soon followed by Robert Noyce of Fairchild who successfully fabricated a

complete IC including the interconnections on a single silicon chip. Since then the

evolution of this technology is fast-paced.

1.2 Advantages of ICs

As compared to standard printed circuits which use discrete components, ICs

have the following advantages :

1 Extremely small physical size

Often the size is thousands of times smaller than a discrete circuit. The various

components and their interconnections are distinguishable only under a powerful

microscope.

2. Very small weight

Since many circuit functions can be packed into a small space, complex electronic

equipment can be employed in many applications where weight and space are

critical, such as in aircraft or space-vehicles.

3. Reduced cost

The reduction in cost per unit is due to the fact that many identical circuits can be

built simultaneously on a single wafer this process is called batch fabrication.

Although the processing steps for the wafer are complex and expensive, the large

number of resulting integrated circuits make the ultimate cost of each IC fairly low.

4. Extremely high reliability

It is perhaps the most important advantage of an IC and is due to many factors.

Most significant factor is the absence of soldered connections. Another is the need

for fewer interconnections the major cause of circuit failures. Small temperature

rise due to low power consumptions of ICs also improves their reliability. In fact,

an IC logic gate has been found to be 100,000 times more reliable than a vacuum

tube logic gate and 100 times more reliable than a transistor logic gate.

Obviously, higher reliability means that ICs will work for longer periods without

giving any trouble something most desirable from both military and consumer

application point of view.

5. Increased response time and speed

Since various components of an IC are located close to each other in or on a silicon

wafer, the time delay of signals is reduced. Moreover, because of the short

distances, the chance of stray electrical pickup (called parasitic capacitance) is

practically nil. Hence it makes them very suitable for small signal operation and

high frequency operation. As a result, the response time or the operating speed of

the system is improved.

6. Low power consumption

Because of their small size, ICs are more suitable for low power operation than

bulky discrete circuits.

7. Easy replacement

ICs are hardly ever repaired because in case of failure, it is more economical to

replace them than to repair them.

8. Higher yield

The yield is the percentage of usable devices. Because of the batch fabrication, the

yield is very high. Faulty devices usually occur because of some defect in the

silicon wafer or in the fabrication steps. Defects in silicon wafer can occur because

of lattice imperfection and strains introduced in crystal growth, cutting and

handling of the wafers. Usually such defects are extremely small, but their

presence can ruin devices built on or around. Reducing the size of each device

greatly increases the chance for a given device to be free of such defects. The same

is true for fabrication defects such as the presence of a dust particle on the

photolithographic mask.

1.3 Drawbacks of ICs

The integrated circuits suffer from the following drawbacks :

1. ICs function at fairly low voltages,

2. They handle only limited amount of power,

3. They are quite delicate and cannot withstand rough handling or excessive heat.

However, the advantages of ICs far outweigh their disadvantages or drawbacks.

1.4 Level of Integration

Level of integration in ICs has been increasing ever since they were developed The

number of electronic circuits or components that can be fitted into a standard size

IC has been dramatically increasing with each passing year. In fact, whole

electronic systems rather than just a circuit are incorporated in one package.

1.4.1 Classification of ICs by Structure

Structurally speaking, ICs can be classified into the following three types :

1.4.1.1 Monolithic Integrated Circuits

The word ‘monolithic’ means ‘single stone’ or more appropriately ‘a single-solid

structure’. In this IC, all circuit components (both active and passive) are

fabricated inseparably within a single continuous piece of silicon crystalline

material called wafer (or substrate). All components are atomically part of the

same chip. Transistors, diodes and other passive components are fabricated at

appropriate spots in the substrate using epitaxial diffusion technique.

Component interconnections are provided on the surface of the structure and

external connecting wires are taken out to the terminals. It is a complete circuit

requiring no ‘add ons’.

Despite some of its distinct disadvantages, monolithic ICs are in wide use because

for mass production, monolithic process has been found to be the most economical.

1.4.1.2 Thick and Thin-Film ICs

The essential difference between thick-film and thin-film ICs is not their relative

thickness but the method of depositing the film. Both have similar appearance,

properties and general characteristics though they both differ in many respects

from monolithic ICs.

These ICs are not formed within a silicon wafer but on the surface of an insulating

substrate such as glass or a ceramic material.

Moreover, only passive components (resistors, capacitors) are formed through

thick or thin-film techniques on the insulating surface. The active elements

(transistors, diodes) are added externally as discrete elements to complete a

functional circuit. These discrete active components are frequently produced by

using the monolithic process.

As stated above, the primary difference between the thick and thin film techniques

is the process used for forming passive components and the metallic conduction

pattern.

( a ) Thin-film ICs

Such circuits are constructed by depositing films (typically 0.1 to 0.5 µm) of

conducting material through a mask on the surface of a substrate made of glass or

ceramic.

Resistors and conductors are formed by varying the width and thickness of the film

and by using materials of different resistivity.

Capacitors are produced by sandwiching an insulating oxide film between two

conducting films. Small inductors can be made by depositing a spiral formation of

film.

The active components like transistors and diodes etc. are externally added and

interconnected by wire bonds

Following two methods are used to produce thin films :

( i ) vacuum evaporation

In this method, the vaporized material is deposited through a set of masks on the

glass or ceramic substrate contained in vacuum.

( ii ) cathode sputtering

In this method, atoms from a cathode made of the desired film material are

deposited on the substrate which is located between the cathode and the anode.

( b ) Thick-film ICs

Such type of integrated circuits are sometimes referred to as printed thin-film

circuits. They are so called because silk-screen printing techniques are employed

to create the desired circuit pattern on the surface of the substrate. The screens are

made of fine stainless steel wire mesh and the ‘inks’ are pastes (of pulverized glass

and aluminium) which have conductive, resistive or dielectric properties.

After printing, the circuits are high-temperature fired in a furnace to fuse the films

to the insulating substrate. As with thin-film ICs, active elements are added

externally as discrete components.

1.4.1.3 Hybrid or Multichip ICs

As the name implies, such circuits are formed either by inter-connecting a number

of individual chips or by a combination of film and monolithic IC techniques. In

such ICs , active components are first formed within a silicon wafer (using

monolithic technique) which is subsequently covered with an insulating layer such

as SiO2 Film techniques are then employed to form passive components on the

SiO2 surface.

Connections are made from the film to the monolithic structure through ‘windows’

cut in the SiO2 layer

1.4.2 Classification of ICs By Function

The earlier classification of ICs was based on their method of construction.

However, the integrated circuits can also be classified according to their general

function. The two most important categories are :

1. linear and

2. Digital

Classification Based on Circuits Function

Integrated Circuits

Linear / Analogue ICs

Integrated circuits that operate with analogue signals at the input and output.

Digital ICs

Integrated circuits that operate with digital signals at the input and output.

Examples:i. Op-Ampii. Power Amplifieriii. Multiplieriv. Comparatorv. Voltage Regulator

Examples:i. Logic gatesii. Flip-flopiii. Counteriv. Calculator chipsv. Memoryvi. Microprocessor

1.4.2.1 Linear Integrated Circuits (LICs)

LICs are also referred to as analog ICs because their inputs and outputs can take

on a continuous range of values and the outputs are generally proportional to the

inputs. As compared to digital ICs, LICs are used much less. But LICs are quickly

displacing their discrete circuit counterparts in many applications as their cost

becomes competitive.

They also possess much higher reliability because so many external connections

(major source of circuit failure) are eliminated. LICs find wide use in military and

industrial applications as well as in consumer products.

They are frequently used in

1. Operational amplifiers, 2. Small-signal amplifiers,

3. Power amplifiers, 4. RF and IF amplifiers,

5. Microwave amplifiers, 6. Multipliers,

7. Voltage comparators, 8.voltage regulators etc.

Operational amplifier is by far the most versatile form for an LIC and is discussed

separately.

Examples of linear ICs are :

1. BEL CA-3020—used as multipurpose wide-band power amplifier.

2. BEL CA-3065—it is a monolithic IC which combines a multistage IF amplifier,

limiter, an FM detector, an electronic attenuator, a Zener diode regulated power

supply and an audio amplifier. In fact, this IC provides a high performance

multistage sub-system of a TV receiver. It is available in 14-pin dual-in-line

package.

1.4.2.2 Digital Integrated Circuits

About 80 per cent of the IC market has been captured by digital ICs which are

mostly utilized by the computer industry. Digital ICs lend themselves easily to

monolithic integration because a computer uses a large number of identical circuits.

Moreover, such circuits employ relatively few capacitors and values of resistances,

voltages and currents are low.

Digital ICs contain circuits whose input and output voltages are limited to two

possible levels low or high.

It is so because digital signals are usually binary. Sometimes, digital circuits are

referred to as switching circuits. Digital ICs include circuits such as:

1. Logic gates 2. Flip-flops 3. Counters 4. Clock-chips

5. Calculator chips 6. Memory chips 7. Microprocessors (µP) etc.

Main Applications of IC

Most electronic equipment today use integrated circuit, for

example:

Computer / Server / workstation

TV / Radio / Video

Cell Phones

Digital Clock

Robotic Systems

Telecommunication System

Automotive

Medical Equipment

Aerospace

Children's Toys

Military Field

Missile System

etc.

1.5 How ICs are Made ?

ICs are manufactured in four distinct stages

These are (1) material preparation, (2) crystal growing and wafer preparation, (3)

wafer fabrication and (4) testing, bonding and packaging.

1.5.1 Material Preparation

Silicon, as an element is not found in nature. However, it is found abundantly in

nature in the form of silicon dioxide, which constitutes about 20% of earth’s crust.

Silicon is commonly found as quartz or sand. A number of processes are required

to convert sand into pure silicon with a polycrystalline structure.

1.5.2 Crystal Growing and Wafer Preparation

To grow crystals, the polycrystalline silicon is placed in the crucible. The furnace

is heated to a temperature of 1690 K which is slightly greater than the melting

point (1685 K) of silicon. A precisely controlled amount of dopant (boron or

phosphorus) is added to the melt to make the silicon as P-type or N-type

1.5.3 Wafer Fabrication

Following is the category of the processes that are used in the fabrication of ICs :

i) Oxidation ii) Etching iii) Diffusion iv) Ion implantation

v) Photolithography vi) Epitaxy vii) Metallization and interconnections.

1.5.3 i) Oxidation

The process of oxidation consists of growing a thin film of silicon dioxide (SiO2)

on the surface of a silicon wafer. Silicon dioxide has several uses:

1. To serve as a mask against implant or diffusion of dopant into silicon,

2. To provide surface passivation, (oxide layer serves as a protection for the

semiconductor surface.

3. To isolate one device from another,

4. To act as a component in MOS structures.

Oxide layer grown on the surface of silicon substrate

Formation of Insulating Films

The favored insulator is pure silicon dioxide (SiO2). A SiO2 film can be formed by

one of two methods:

1. Oxidation of Si at high temperature in O2 or steam ambient

2. Deposition of a silicon dioxide film

Several techniques have been developed for forming oxide layers. Some of these

are:

1) Thermal oxidation,

Temperature range: 700oC to 1100

oC

Process: O2 or H2O diffuses through SiO2 and reacts with Si at the interface to

form more SiO2

2) Chemical vapor deposition (CVD) of (SiO2)

Gases dissociate on surfaces at high temperature, typically done at low pressure

(LPCVD) rather than atmospheric (APCVD)

• LPCVD pressures around 300mT (0.05% atmosphere)

Temperature range: Moderate Temperatures (450 SiO2)

Very dangerous gases

Silane: SiH4

Arsine, phosphine, diborane: AsH3, PH3, B2H6

3) Plasma oxidation (Physical Vapor Deposition (“Sputtering”)).

It is used to deposit Al films

Highly energetic argon ions hit the surface of a metal target, knocking atoms loose,

which then land on the surface of the wafer

1.5.3 ii) Etching

Etching is the process of selective removal of regions of a semiconductor, metal or

silicon dioxide. There are two types of etching: wet and dry.

In wet etching, the wafers are immersed in a chemical solution at a predetermined

temperature.

In dry (or plasma) etching, the wafers are immersed in gaseous plasma created by a

radio-frequency electric field applied to a gas such as argon

1.5.3 iii) Diffusion

This process consists of the introduction of impurities into selected regions of a

wafer to form junctions. Diffusion occurs in two steps : the pre-deposition and

the drive-in diffusion. In the pre-deposition step, a high concentration of dopant

atoms is introduced at the silicon surface by a vapour that contains the dopant at a

temperature of 1000ºC. More recently, a more accurate method of pre-deposition

known as ion implantation is used.

Pre-deposition tends to produce, a superficial but heavily doped layer, near the

silicon surface. Drive-in is used to drive the impurity atoms deeper into the surface,

without adding any more impurities.

Impurity concentration versus depth into the substrate) during the pre-deposition

and the drive-in steps of diffusion.

1.5.3 .iv) Ion Implantation

This is a process of introducing dopants into selected areas of the surface of the

wafer by bombarding the surface with high-energy ions of the particular dopant.

The ions are then accelerated in an electric field so that they acquire energy of

about 20 keV and are passed through a strong magnetic field. The ions are further

accelerated so that their energy reaches several hundred keV or MeV, at which

point they are focused on and strike the surface of the silicon wafer.

As is the case with diffusion, the ion beam is made to penetrate only into selected

regions of the wafer by a process of masking (discussed later). On entering the

wafer, the ions collide with silicon atoms and lose their energy. The depth of

penetration of ions in ion implantation is about 0.1 to 1 µm.

The higher the energy of ions and the smaller their mass, the greater is the depth of

penetration

Some of the advantage of ion implantation over diffusion

1- Doping levels can be precisely controlled since the incident ion beam can be

accurately measured as an electric current.

2- The depth of the dopant can be easily regulated by control of the incident ion

velocity. It is capable of very shallow penetrations.

1.5.3 .v) Photomask Generation

The whole process of IC fabrication consists of identifying selected regions of each

circuit of the wafer surface into which identical dopant or metallic interconnections

are made, while protecting other regions of the wafer surface. To carry out one of

the many fabrication processes, a separate mask is required for each operation

whose function is to expose the selected regions and protect the others. There may

be hundreds of identical dies (or ICs ) on a wafer with each circuit containing

hundreds of thousands or millions of devices. Identical steps are carried out

simultaneously for each process. For each process, separate mask is needed.

The mask production starts with a drawing using a computer-assisted graphics

system with all the information about the drawing stored in digital form.

A logic circuit diagram is drawn to determine the electronic circuit required for the

requested function.

Once the logic circuit diagram is complete, simulations are performed multiple

times to test the circuit’s operation.

1.5.3.vi) Photolithography is a process in which the geometrical pattern on the

glass plate (called reticle) is transferred to the surface of the wafer. This is done to

open identical windows to enable the selective impurity diffusion or ion

implantation into silicon wafer and which enables to achieve a definite pattern of

Aluminum interconnections according to the circuit requirement

The process may take place in all identical regions of the same IC and for all ICs

on the wafer.

1.5.3.vii) Epitaxy or Epitaxial growth is the process of the controlled growth of

a crystalline doped layer of silicon on a single crystal substrate..

1.5.3.viii) Metallization and Interconnections

After all the fabrication steps of an IC are completed, it becomes necessary to

provide metallic interconnections for the IC and for external connections to the IC.

The requirement that must be met by the interconnections is that they have low

resistance to minimize both the voltage drops on the lines as well as the

capacitances between the lines so as to reduce delay times

1.5.3.ix) Inspection & Testing

Each IC on the completed wafer is electronically tested by the tester.

After this inspection, the front-end processing is complete

1.5.3.x) Dicing

In back end processing, a wafer completed in front end processing is cut into

individual IC chips and encapsulated into packages

1.5.3.xi) Mounting

After the IC chips are cut apart, they are sealed into packages. The IC chips must

first be attached to a platform called the "lead frame“.

1.5.3.xii) Wire bonding

The mounted IC chips are connected to the lead frames.

1.5.3.xiii) Packaging

The final step in IC fabrication is packaging the device in a suitable medium that

can protect it from environment of its intended application. In most cases this

means the surface of the device must be isolated from the moisture and

contaminants and the bonds and other elements must be protected from corrosion

and mechanical shocks. (The IC chips and the lead frame islands are encapsulated

with molding resin for protection).

Unit 2 Operational Amplifiers

UNIT II. Operational amplifiers

Basic concepts differential amplifiers

ideal op-amp o parameters of op-amp o Basic op-amp applications

Scale changer Inverting and non-inverting amplifier summer and subtractor multiplier and divider differentiator integrator instrumentation amplifier

2.1 Introduction

An operational amplifier, in general, is a three-stage circuit, and is fabricated as an

integrated circuit. The first stage is a differential amplifier, the second stage

provides additional voltage gain, and the third stage provides current gain and low

output impedance

Operational amplifier (Op-amp) is a very high-gain, high- rin directly-coupled

negative-feedback amplifier which can amplify signals having frequency ranging

from 0 Hz to a little beyond 1 MHz. They are made with different internal

configurations in linear ICs. An OP-AMP is so named because it was originally

designed to perform mathematical operations like summation, subtraction,

multiplication, differentiation and integration etc. in analog computers. Present

day usage is much wider in scope but the popular name OP-AMP continues.

Typical uses of OP-AMP are : scale changing, analog computer operations, in

instrumentation and control systems and a great variety of phase-shift and

oscillator circuits.

Equivalent circuit (internal architecture) of 741 IC Op-amp

741 IC Op-amp pin configuration

(Often called single-ended amplifiers) The +V (+Vcc) and -V(+Vee) connections

denote the positive and negative sides of the DC power supply, respectively. The

input and output voltage connections are shown as single conductors, because it is

assumed that all signal voltages are referenced to a common connection in the

circuit called ground. Often (but not always!), one pole of the DC power supply,

either positive or negative, is that ground reference point. A practical amplifier

circuit (showing the input voltage source, load resistance, and power supply) might

look like as below:

Circuit function: to take an input signal (Vin), amplify it, and drive a load resistance

(Rload).

If it is necessary for an amplifier to be able to output true AC voltage (reversing

polarity) to the load, a split DC power supply may be used, whereby the ground

point is electrically "centered" between +V and -V. Sometimes the split power

supply configuration is referred to as a dual power supply

The output voltage across the load resistor can now swing from a theoretical

maximum of +15 volts to -15 volts, instead of +30 volts to 0 volts. This is an easy

way to get true alternating current (AC) output from an amplifier without resorting

to capacitive or inductive (transformer) coupling on the output. The peak-to-peak

amplitude of this amplifier's output between cutoff and saturation remains

unchanged.

Unlike normal amplifiers, which amplify a single input signal (often called single-

ended amplifiers), differential amplifiers amplify the voltage difference between

two input signals.

Notice: The two input leads can be seen on the left-hand side of the triangular

amplifier symbol, the output lead on the right-hand side, and the +V and -V power

supply leads on top and bottom.

Notice that one input lead is marked with a (-) Inverting and the other is marked

with a (+) Non-inverting terminal.

The voltage output of a differential amplifier is determined by the following

equation: Vout = AV(Vnoninv - Vinv) or Vout = AV(V1 – V2)

Consider the following table of input/output voltages for a differential amplifier

with a voltage gain of 4:

Same as VO = AOLVd = AOL(V1 – V2)

Most amplifiers have one input and one output. Differential amplifiers have two

inputs and one output, the output signal being proportional to the difference in

signals between the two inputs.

Since the ideal op-amp responds only to the difference between the two input

signals V1 and V2 , the ideal op-amp maintains a zero output signal for V1 = V2

When V1 = V2 ≠ 0, there is what is called a common-mode input signal. For the

ideal op-amp, the common-mode output signal is zero. This characteristic is

referred to as common-mode rejection because if used properly you can reject

signals you don’t want while simultaneously amplifying signals you do want. This

process is called Common Mode Rejection Ratio or CMRR and it is used to

eliminate noise and hum which can be common to a signal

2.1.2 Ideal Operational Amplifier

When an OP- AMP is operated without connecting any resistor or capacitor from

its output to any one of its inputs (i.e without feedback), it is said to be in the open-

loop condition. The word ‘open loop’ means that feedback path or loop is open.

The specifications of OP- AMP under such condition are called open-loop

specifications.

An ideal OP- AMP

It has the following characteristics :

1- Infinite voltage gain AOL

2- Infinite input impedance Rin

This ensure no current can flow into ideal opamp

3- Zero output impedance Ro

This ensures Vo remains no matter load resistance

4- Zero offset voltage

Offset voltage is that small Vo even though V1 = V2

This ensures Vo = 0 when Vi = 0

5- Infinite Bandwidth

BW is range of frequency opamp performs satisfactorily

Ideal opamp operating frequency range = 0 – inf

It can amplify dc (0 freq.) as well as ac signals

6- Infinite CMRR

CMRR is the ratio of diff gain and common mode gain

Infinite to ensure zero common mode gain in ideal opamp

7- Infinite slew rate

Is the maximum rate of change of the output voltage

Infinite to ensure that when Vi changes, immediately Vo changes too

8- No effect to temperature

Ideal opamp characteristics do not change with temperature

2.1.2 Practical op-amp characteristics

1. Open Loop Gain

Voltage gain when no feedback is applied is Practically in thousand

In case of typical practical op amp like IC 741 it is of the order of 105

i.e. about

100 dB

2. Input Impedance

Typically greater than 1MΩ

In case of IC 741 it is 2 MΩ

3. Output Resistance

Typically a few hundred ohms but can be reduced to 1 or 2Ω using

negative feedback

In case of IC 741 it is 75 Ω

4. Bandwidth

Very small in open loop configuration

Apply negative feedback to increase to desired value

5. Output Offset Voltage

The output dc offset voltage is the measured open-loop output voltage when the

input voltage is zero. This configuration is shown in Figure (a).

a. Under common mode operation, output voltage should be zero, but

due to mismatch in devices it is non-zero

b. Voos is the voltage at the output when both input terminals are

grounded

c. (Can be corrected by applying voltage between Pins 1 and 5 of IC 741)

6. Input Offset Voltage

With V1 and V2 grounded, Vo should be zero, BUT IT IS NOT

So a small dc voltage needed to be applied that must be applied to the open-loop

op-amp at one of the inputs with the other grounded so as to get a zero output

voltage is the input offset voltage , Vios

7. Input Bias Current

The input currents to an ideal op-amp are zero

No current should flow through the input terminals but small currents of 10E-

6A to 10E-14A do exist practically.

In actual operational amplifiers, however, the input bias currents are not zero (IB1

and IB2),the input bias current is then defined as the average of the two input

currents flowing into each of the two input terminals.

IB = |IB1+IB2|/2

8. Input Offset Current

The difference between the two input currents is called the input offset current

Iios = |IB1-IB2|

Following additional points are worth noting :

1. Infinite input resistance means that input current i = 0 . It means that an ideal

OP- AMP is a voltage-controlled device.

2. R0 = 0 means that V0 is not dependent on the load resistance connected across

the output.

3. Though for an ideal OP- AMP AOL = ∞ , for an actual one, it is extremely high

i.e., about 106

However, it does not mean that 1 V signal will be amplified to 106 V at the output.

Actually, the maximum value of V0 is limited by the basis supply voltage,

typically ± 15V. With AOL = 106 and V0 = 15V the maximum value of input

voltage is limited to

= 15µV. Though 1µV in the OP- AMP, can certainly

become 1 V.

Ideal Voltage Transfer Curve

• Plot of Vo and Vd

• Output proportional to difference between input voltages up to saturation

voltages of the opamp specified by manufacturer then stays constant

• Opamp Vo <= ±Vsat slightly less than Vcc & Vee

Derivation of Practical op-amp Output Voltage

• CMRR is the ability of a differential amplifier to reject the signal (of

same voltage) which is present at its both inputs simultaneously i.e. the

common mode signal

• CMRR = ρ = Ad / AC

• Ideally Ac is zero so Ideally CMRR is infinite

• Practically Ad is large & Ac small hence CMRR is also very large

• Vo = Ad Vd [1 + 1/(CMRR).(Vc/Vd)]

• So with CMRR large Vo ~ Ad Vd

• The common mode component is largely rejected

Exercise: Determine the output voltage of a differential amplifier for the

input voltages 300uV and 240uV, given that the differential gain is 5000,

and the CMRR is (i) 100

(ii) 100000

Ideal Op Amp with Negative Feedback

2.1.3 Golden Rules of Op Amps:

1. The output attempts to do whatever is necessary to make the voltage

difference between the inputs zero

2. The inputs draw no current

These two conditions give rise to VIRTUAL GROUND, where the voltages

at both the inputs are maintained at exactly same level.

- Virtual ground means that the differential input voltage is essentially zero

- VO = AOLVd = AOL(V1 – V2)

- Vd = VO /AOL = VO / inf = 0

- V1 = V2

Voltage at one terminal can be assumed same as that at another

- A virtual short-circuit therefore exists between the inputs

+

-ov

v

v

Network

- No current flows from the terminals to the ground

Principle of virtual Ground states that:

If the non-inverting terminal of an op-amp is grounded then the inverting one is

also at ground even if there is no physical connection between them

To achieve this condition, a feedback circuit between the output and the inverting

input terminal of the op amp is necessary.

This results into many applications of op amp, which qualify it to be

OPERATIONAL: adder, subtractor, multiplier, divider etc.

Closed Loop Advantages

• It transforms the open-loop gain into a controllable closed loop gain via the

negative feedback resistor

• Reduces distortion possibilities

• Increases bandwidth or frequency range

• Increases input resistance

• Decreases output resistance

• Decreases effects of temperature and power supply on the gain of the circuit

2.1.4 Op-amp Applications

Although an OP-AMP is a complete amplifier, it is so designed that external

components (resistors, capacitors etc.) can be connected to its terminals to change

its external characteristics. Hence, it is relatively easy to tailor this amplifier to fit

a particular application and it is, in fact, due to this versatility that OP-AMPs have

become so popular in industry.

An OP-AMP IC may contain two dozen transistors, a dozen resistors and one or

two capacitor

Typical uses of OP-AMP are :

Scale changing, analog computer operations, in instrumentation and control

systems and a great variety of phase-shift and oscillator circuits. The OP-AMP is

available in three different packages ( i) standard dual-in-line package ( DIL) ( ii )

TO-5 case and ( iii ) the flat-pack

Polarity Conventions

The input terminals have been marked with minus (−) and plus (+) signs. These

are meant to indicate the inverting and non-inverting terminals only.

It simply means that a signal applied at negative input terminal will appear

amplified but phase-inverted at the output terminal. Similarly, signal applied at the

positive input terminal will appear amplified and in phase at the output. Obviously,

these plus and minus polarities indicate phase reversal only.

Additionally, it also does not imply that a positive input voltage has to be

connected to the plus-marked non inverting terminal and negative input voltage to

the negative-marked inverting terminal.

In fact, the amplifier can be used ‘either way up’ so to speak. It may also be noted

that all input and output voltages are referred to a common reference usually the

ground.

2.1.4.1 Inverting Amplifier or Negative Scale

Non-inverting terminal has been grounded, whereas R1connects the input signal ν1

to the inverting input. A feed-back resistor Rf has been connected from the output

to the inverting input

Gain of inverting amplifier

It is seen from above, that closed-loop gain of the inverting amplifier depends on

the ratio of the two external resistors R1 and Rf and is independent of the

amplifier parameters.

It is also seen that the OP- AMP works as a negative scaler. It scales the input i.e ,

it multiplies the input by a minus constant factor K .

If Rf > Rin →multiplier Rf < Rin→ divider

2.1.4.2 Non-inverting Amplifier or Positive Scaler

Such a positive scaler circuit which uses negative feedback but provides an output

that equals the input multiplied by a positive constant is shown in Figure above

Since input voltage ν2 is applied to the non-inverting terminal, the circuits is also

called non-inverting amplifier. Here, polarity of ν0 is the same as that ν2 i.e .,

both are positive.

Gain of non-inverting amplifier

>>Applying KCL to junction A , we have

2.1.4.3 Unit follower

It provides a gain of unity without any phase reversal. Its gain is very much close

to being exactly unity.

This circuit is useful as a buffer or isolation amplifier because it allows, input

voltage νin to be transferred as output voltage ν0 while at the same time preventing

load resistance RL from loading down the input source. It is due to the fact that its

Ri = ∞ and R0 = 0.

In fact, its circuit can be obtained from that of non-inverter by putting R1=Rf =0

2.1.4.4 Adder / Summer

The adder circuit provides an output voltage proportional to or equal to the

algebraic sum of two or more input voltages each multiplied by a constant gain

factor. It is basically similar to a scaler(-) except that it has more than one input,

the output voltage is phase-inverted

Calculation

2.1.4.5 Op amp integrator

Iin= Vin / Ri n= If = – Cf ( dVout / dt )

Vout = – (1 / Rin Cf ) ∫ Vindt

2.1.4.6 Opamp Differentator

• For a capacitor, Q = CV, so Icap = dQ/dt = C·dV/dt

– Thus Vout = IcapR = RC·dV/dt

• So we have a differentiator, or high-pass filter

– if signal is V0sint, Vout = V0RCcost