rec101 unit iii operational amplifier

Operational Amplifiers

Operational Amplifiers : Introduction and Block diagram of Op Amp, Ideal &Practical characteristics of Op Amp, Differential amplifier circuits, PracticalOpAmp Circuits (Inverting Amplifier, Non inverting Amplifier, Unity GainAmplifier, Summing Amplifier, Integrator, Differentiator). OPAMPParameters: Input offset voltage, Output offset voltage, Input biasedcurrent, Input offset current Differential and Common-Mode Operation

9/15/2017 1REC 101 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

Introduction and Block diagram of Op-Amp


• Op-Amp or operational amplifiers, is a very high gain differentialamplifier with high input impedance and low output impedance.

• Op-Amp find common application in mathematical operations likeaddition, subtraction, integration etc. thus term operational is given

• Nowadays op-amps are IC based (Integrated circuit) that requirerelatively low power, reliable and inexpensive.

• Op-Amp’s originated around 1947

+

-

+VCC

-VEE

Non Inverting input

Inverting input

OutputAV

Op-Amp741

1 2 3 4

8 7 6 5

Off

set

Nu

ll

Inve

rtin

g

No

n In

vert

ing

-VEE

VC

C

Ou

tpu

t

Off

set

Nu

ll

Op

en

+

-

Non Inverting input

Inverting input

OutputAV

Introduction and Block diagram of Op-Amp


• A typical op-amp is made up of three stages; • differential amplifier stage• voltage amplifier • push-pull amplifier stage

• Input stage of op-amp is differential amplifier. It provides amplification of the difference voltage between the two inputs.

• The second stage is a class A amplifier that provides additional gain. Some op-amps may have more voltage amplifier stages

• A push-pull class B amplifier is typically used for the output stage.

Differential amplifier Input

stage

Voltage amplifier gain

stage

Push-pull amplifier

output stage

+

-

Vd output

Op Amp Modes of operation


Single Differential mode+

-

AV

Vi

Vo = AV Vi+

-

AV

Vi

Vo = -AV Vi

double Differential mode+

-

AV

V1

Vo = AV(V1–V2)

V2

+

-

AVVi

Vo = AVVi

Differential Mode: In Differential mode, output of op-amp

input; aldifferenti gain, voltagealdifferenti where

VVVVA

VAV

ddv

dvo

Op-Amp ParametersOpen loop Voltage gain (Av): The open-loop gain of an Op-Amp is the gain obtainedwith respect to differential input when op-amp is used in open loop (no feedback)

Output offset voltage VOO: In practical op-amp, a small dc voltage called output offsetvoltage VOO, appears at the output when nodifferential input voltage is applied.

Input offset voltage VOS: is the differentialdc voltage required between the inputs toforce the output to zero volts. Typicalvalues of input offset voltage are upto 2 mV.Ideally VOS is 0 V.


+

-

AVVd

Vo = AVVd

+

-

AVVOS

VO =0

+

-

AVVOO

Op-Amp ParametersInput biased current (IBias): is the average ofcurrents at both inputs of Op-amp (required tooperate properly amplifier first stage).

Input offset current (IOS): Ideally, the twoinput bias currents are equal, but practically isnot. IOS is the absolute difference of the inputbias currents. (with magnitudes at least tentimes less than the bias current.

Slew Rate: is maximum rate of change of theoutput voltage in response to a step input.Slew rate depends upon the high-frequencyresponse of the amplifier stages of op-amp.

Slew rate =maximum rate at which Op-Ampoutput can change (V/s)


+

-

AVVO

V1

V2

I2

I1

2

21 IIIBias

21 IIIOS

+

-

Vo

Vi

t

Vi

tV0

Op-Amp ParametersInput biased current (IBias): is the average of currents at both inputs ofOp-amp (required to operate properly amplifier first stage).Input offset current (IOS): Ideally, the two input bias currents areequal, but practically is not. IOS is the absolute difference of the inputbias currents. (with magnitudes at least ten times less than IBias)


+

-AV

VOV1

V2

I2

I1

2

21 IIIBias

21 IIIOS

+

-

Vo

Vi

t

Vi

Slew Rate: is maximum rate of change of the output voltage inresponse to a step input expressed in V/s. Slew rate depends uponthe high-frequency response of the amplifier stages of op-amp

tV0 t

V t

V

rate Slew

Op Amp Modes of operation


+

-

AV

Vi

Vo = 0

Vi

Common mode operation: In common mode common input isapplied on both inputs of op-amp. The difference applied is zero sothe output is zero. This action is called common mode rejection

Common mode rejection ratio (CMRR): Practical op-amps providesmall common mode gain with very large differential mode gain.CMRR is defined as

It means that unwanted signal appearing on bothinputs will be rejected and will not distort output

cm

OL

A

AdBCMRR

CMRR

log20)(

)(Again voltagemodecommon

)(Again voltagealdifferenti loopOpen

cm

OL

Ideal & Practical characteristics of Op Amp


ideal practical

Differential Voltage gain 106

bandwidth MHz

Input impedance M

output impedance 0 100’s

Slew rate 0.5 V/s

CMRR 90 dB

Input offset voltage 0 1 mV

Ideal and practical characteristics of op-amp is given in table below

Differential amplifier circuits


The differential amplifier circuit is an extremely popular circuit used.It has two separate inputs and two separate outputs which arerelated to difference of inputs.

Differential amplifier works in differential & common mode• If an input signal is applied to either input with the other input connected to

ground, the operation is referred to as “single-ended differential mode.”• If two different input signals are applied, the operation is referred to as “double-

ended differential mode.”• If the same input is applied to both inputs, the operation is called common-mode

Main feature of differential amplifier is very large gain for differentialinput as compared to the very small gain for common inputs. Theratio of this difference gain to the common gain is called common-mode rejection.

Differential amplifier circuits


Q1 Q2

RE

RCRC

VCC

-VEE

VO2VO1

Vi1 Vi2

Vo1

Vi2 Vo2

Vi1

Differential amplifier circuit using BJTDifferential amplifier Symbolic notation

Op-Amp


Virtual ground: When op-amp is used in feedback, potentialdifference between the inputs to nearly zero (due to very high openloop gain) and no current flows to the input of op-amp.

Steps to solve op-amp circuits :

Open loop configuration: • Find the potential between non-inverting to inverting input (V+-V-) • Output is open loop gain (Av) times the difference input, Vo=Av(V+-V-)

Feedback configuration: • Due to virtual ground, voltage at both input terminals of op-amp are

equal & no current flow to the inputs of op-amp ( input impedance)• Solve the circuits for unknowns using Kirchhoff law’s• If Op-Amp saturates for any input (output rises close to bias voltage

magnitude), output is equal to saturation voltage

Inverting Amplifier

Applying KCL at inverting junction


+

-

Vo

RF

Vi

R V=0

I

IF

iF

O

F

Oi

VR

RV

R

oV

R

oV

or

or

-II F

R

R

V

V F

i

O Again voltage

With negative feedback through a resister, input be applied oninverting terminal through another resister, while non invertingterminal is grounded, Op-Amp acts as an inverting amplifier

Non inverting Amplifier


Applying KCL at inverting junction+

-

Vo

RF

Vi R Vi

I

IFi

FiO

F

iOi

VR

RVV

R

VV

R

V

or

0or

-II F

R

R

V

V F

i

O 1Again voltage

With negative feedback through a resister, input be applied on non-inverting terminal, while inverting terminal is grounded throughanother resister, Op-Amp acts as an non-inverting amplifier

Unity Gain (Buffer) Amplifier


+

-

Vo =- Vi

R

Vi

R V=0

I

IF

In non-inverting amplifier configuration, if RF=R, then voltage gain isunity (in inverting mode). The output follows input (with 1800

phase shift), therefore it is called unity follower configuration.

unity follower configuration of op-amp is achieved by connectingoutput of op-amp & input waveform to inverting input of op-amp.

Connecting wire’s small resistance serve the purpose of resistances.

+

-

Vo =- Vi

Vi

V=0

I

IF

Summing AmplifierA popular application of an op-amp is as a summing amplifier. Various input voltages can be summed (each multiplied by a different weight) as shown in below circuit


+

-

Vo

RF

R2 V=0

I2

IF

V2

R1

I1

V1

R3

I3

V3


F

O

R

oV

R

oV

R

oV

R

oV

3

3

2

2

1

1

F321

or

-IIII

321321

3

3

2

2

1

1

then If VVVVRRRR

VR

RV

R

RV

R

RV

OF

FFFO

IntegratorOp-amp can be used as a integrator with feedback capacitorconnected at inverting terminal and applying input through aresistor at inverting terminal



00

or

-II F

Oi V

dt

dC

R

V

+

-

Vo

C

Vi

R V=0

I

IF

t

iO dVRC

V0

1

Differentiator


Op-amp can be used as a differentiator by with applying inputthrough a capacitor inverting terminal and negative feedback


R

VV

dt

dC i

00or

-II

0

F

+

-

Vo

Vi R

V=0

I

IF

C

iO Vdt

dRCV

rec101 unit iii operational amplifier

Engineering