Operational Amplifiers

Sourabh Kumar www.androroot.comiamskyuvraj@gmail.com

Operational Amplifiers Introduction & History Basic Structure & ckt of Op-Amp Types of Operational Amplifier Comparison between Ideal & Practical

Operational Amplifier Inverting and Non-Inverting OP-amp Other Useful Circuits Op-amp based Daily life Devices with

their circuit diagram.

Operational AmplifierAn active circuit element designed to perform

mathematical operations like addition, subtraction, multiplication, division, differentiation and integration. known as OP-amp.

_

+

Key Points:-Operational amplifiers are among the most widely used building blocks in electronic circuitsAn ideal operational amplifier would have infinite voltage gain, infinite input resistance and zero output resistanceDesigners often make use of cookbook circuitsReal op-amps have several non-ideal characteristics However, if we choose components appropriately this should not affect the operation of our circuitsFeedback allows us to increase bandwidth by trading gain against bandwidthFeedback also allows us to alter other circuit characteristics

HistoryOperational Amplifier is invented

by Karl D. Swartzel Jr. 1941-First Production 1952 -Philbrick Operational

Amplifiers (marketed by George A. Philbrick) were constructed with vacuum tubes

1965-Philbrick Op-amp Used in analog computers.

vacuum tube operational amplifiers were still in use and cost in the range of $75.

Some reports say that Loebe Julie actually

developed the operational amplifier circuitry.

• i(+), i(-) : Currents into the amplifier on the inverting and noninverting lines respectively

• vid : The input voltage from inverting to non-inverting inputs

• +VS , -VS : DC source voltages, usually +15V and –15V• Ri : The input resistance, ideally infinity• A : The gain of the amplifier. Ideally very high, in the

1x1010 range.• RO: The output resistance, ideally zero• vO: The output voltage; vO = AOLvid where AOL is the

open-loop voltage gain

The Operational Amplifier+VS

-VS

vid

Inverting

Noninverting

Output

+

_i(-)

i(+)

vO = Advid

ROARi

Basic Structure of OP-Amp

Input Stage

Intermediate Stage

Level Shifting Stage

Output

Stageinput Outpu

t

Dual input Balanced

output Differential Amplifier

Dual Input unbalanced output

Differential Amplifier

Emitter follower

using constant current Source

Push-pull

Amplifier

Complete circuit of an OP-amp

Types of OP-am

pon the basis of purpose &

Application

General purpose op-amp

Special purpose op-amp

General purpose operational Amplifiers may be used for variety of applications such as

Adding signals Subtracting signals integrating signalsExample:- µA 741 & 351

Special purpose operational Amplifier are used only for specific application such as

Audio Power Application High voltage Application Low Noise ApplicationExample:- LM380 & 1641

The Four Amplifier Types

Description Gain Symbol

Transfer Function

Voltage Amplifieror

Voltage Controlled Voltage Source (VCVS)

Av vo/vin

Current Amplifieror

Current Controlled Current Source (ICIS)

Ai io/iin

Transconductance Amplifieror

Voltage Controlled Current Source (VCIS)

gm(siemens) io/vin

Transresistance Amplifieror

Current Controlled Voltage Source (ICVS)

rm(ohms) vo/iin

IC Product• A single package will often contain several op-amps [DIP 741] [Dual op-amp 1458 device]

Ref:080114HKN Operational Amplifier

Ideal Vs Practical Op-AmpIdeal Practical

Open Loop gain A 105

Bandwidth BW 10-100HzInput Impedance Zin

>1M

Output Impedance Zout

0 10-100

Output Voltage Vout Depends only on Vd = (V+V)Differential mode signal

Depends slightly on average input Vc = (V++V)/2 Common-Mode signal

CMRR 10-100dB

+

~AVin

Vin VoutZout=0

Ideal op-amp

+

AVinVin Vout

Zout~

Zin

Practical op-amp

An Ideal Operational Amplifier An ideal op-amp would be an ideal voltage amplifier and

would have: Av = , Ri = and Ro = 0

Equivalent circuit of an ideal op-amp

Basic Operational Amplifier Circuits

Inverting and non-inverting amplifiers

A non-inverting amplifierAnalysis

Since the gain is assumed infinite, if Vo is finitethen the input voltage must be zero. Hence

Since the input resistance of the op-amp is

and hence, since V– = V+ = Vi

and 21

2RR

RVV o

21

2RR

RVV oi

2

21RRR

VVGi

o

iVVV

An inverting amplifierAnalysis

Since the gain is assumed infinite, if Vo is finite the input voltage must be zero. Hence

Since the input resistance of the op-amp is

its input current must be zero, and hence

Now

0 VV

21 II

1111

0RV

RV

RVVI ooo

2222

0RV

RV

RVVI iii

Analysis (continued)Therefore, since I1 = -I2

or, rearranging

Here V– is held at zero volts by the operation of the circuit, hence the circuit is known as a virtual earth circuit

21 RV

RV io

2

1RR

VVGi

o

Some of the Daily Life Devices Where Op-Amp is used:-

Op-Amps have many applications and can be used when a signal needs to be modified, amplified or compared.

In the Next few Slides Some of the Op-amp based Devices with there circuit Diagram is shown .

+-

1

2

3

4 5

6

7

8

GND Out

+ V

Gain

- IN Bypass

+ IN

+Gain

Maximum RatingsSupply Voltage ±15 VPower Dissipation 660 mWInput Voltage ±.4 VOperating Temperature 0°C to 70°C

CharacteristicsInput Resistance 50 kΩCMMR 70 to 90 dBBandwidth 300 kHzTotal Harmonic Distortion 0.2%

+-

+-

+-

+-

In

R1

R1

R1

R1

R1

R1

R1

R1

R2+V

-V+V + - + -

LED

LED

LED

LED

R1

R1

R1 R1

R1

+

-

Source 1

Source 2

Source 3Out

Operational Amplifier

+

vi

Ra

vov-

v+

Rf

+

vi

Ra

vov-

v+

Rf

R2R1

+

vi vov-

v+

Rf

+

vivov-

v+

Rf

R2R1

Noninverting amplifier

Noninverting input with voltage divider

ia

fo v

RRR

RR

v ))(1(21

2

Voltage follower

io vv

ia

fo v

RR

v )1(

Less than unity gain

io vRR

Rv21

2

In addition to simple amplifiers op-amps can also be used in a range of other circuits.1. Unity gain buffer amplifier2. Current to voltage converter3. Differential amplifier (or subtractor)4. Inverting summing amplifier. etc

Extra But Important Open-Loop Op Amp Characteristics

Device LM741C LF351 OP-07 LH0003 AD549K

Technology BJT BiFET BJT Hybrid BJT BiFET

AOL(typ) 200 k 100 k 400 k 40 k 100 k

Rin 2 M 1012 8 M 100 k 1013 || 1 pF

Ro 50 30 60 50 ~100

SR 0.5 V/s 13 V/s 0.3 V/s 70 V/s 3 V/s

CMRR 90 dB 100 dB 110 dB 90 dB 90 dB

Sources:- Dailey, Denton. Electronic Devices and Circuits, Discrete

and Integrated. Prentice Hall, New Jersey: 2001. (pp 456-509)

1Table 12.1: Selected Op Amps and Their Open Loop Characteristics, pg 457

Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998.

Web Sources www.infoplease.com/ce6/sci/A0803814.html http://www.infoplease.com/ce6/sci/A0836717.html http://people.msoe.edu/~saadat/PSpice230Part3.htm http://en.m.wikipedia.org/wiki/operational_amplifier? http://www.electronics-tutorials.ws/opamp/ http://www.ti.com

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