EE380: EC Lab

Exp 2: Measurement Of OpampExp.2: Measurement Of OpampParameters & Design of an

IntegratorIntegrator

B. MazhariDept. of EE, IIT Kanpur

G-NumberB. Mazhari, IITK1

Objective

Measure parameters of 741 opamp

Design an opamp Integrator

Specs: %51001 OVSpecs: %5100 1. O

O

VHz100frequencyinput2

V5 ageinput volt 3.

Hz100 frequency input 2.

tageoutput volpeak of%5offset dc 4.

G-NumberB. Mazhari, IITK2

Design Process

SPECS.

Design ProcessSimplified Device

Model

CircuitSchematic

CircuitSimulation

DeviceModel

PredictedResults

No Specs.Satisfied?

Yes

G-NumberB. Mazhari, IITK3

SPECSSPECS.

Ideal DeviceModelDesign Process

C

+VCC

V

CircuitSchematic -VCC

VOR

VIN

Circuit Improved DeviceCircuitAnalysis

Improved DeviceModel

Design Refinement

RF

I d Ci it

Design Refinement

VR C

Improved CircuitSchematic

VINVO

G-NumberB. Mazhari, IITK4

Ideal opamp model

Voltage Gain : ( )VA Voltage Gain : ( )Bandwidth : ( )

V

T

Af

Input resistance : ( )O i ( ) 0

inR Output resistance : ( ) 0OR

Infinite CMRR, no limts on i/p and o/p voltages etc

But no opamp is ideal !G-NumberB. Mazhari, IITK

5

But no opamp is ideal !

Models range from simple to complex

AccuracyAccuracy

Complexity

We use simple models in the initial stages to aid in

G-NumberB. Mazhari, IITK6

p gdesign conceptualization

With negative feedback, ideal opamp exhibits Virtual G d

RF

Ground

F

RR

VIN

Simultaneously short and open circuit !G-NumberB. Mazhari, IITK

7

Simultaneously short and open circuit !

Many interesting circuits can be designed using thisPrinciple including an integrator

C

Principle including an integrator

+VCC

VIN

CC

VOR

-VCC

VO

-VCC

dtVtVdVCVinO

Oin 1)(

G-NumberB. Mazhari, IITK8

RCdtR inO )(

Once a design is conceptualized, it can be refined Using a more accurate device model

Some important opamp non-idealitiesp p p

Input offset Voltage Input offset Voltage Input bias current Bandwidth Bandwidth Slew rate

G-NumberB. Mazhari, IITK9

Input Offset Voltage

VO = ?VO ?

Ideally VO should be zero but in practice it is ~ ± 11Vy O p

These are due to mismatches between components

G-NumberB. Mazhari, IITK10

These are due to mismatches between componentswithin Opamp.

Inside opamp

VCC

RC RC

V

Q1 Q2

Vo

+ -1 2+

RE

VEE

G-NumberB. Mazhari, IITK11

Input Offset Voltage

No mismatches hereVIO

G-NumberB. Mazhari, IITK12

Input Bias Current

IB-

IB+

RC RC

VCC

C C

Q1 Q2+ -

Vo

IB IB

R

+

G-NumberB. Mazhari, IITK13

RE

VEE

741 opamp

G-NumberB. Mazhari, IITK14

Should we bother about ~1mV offset voltage and~30nA bias current?

C 12

R 6

8

10

V)

VOR

2

4

6

V O(V

VOS0 10 20 30 40 50

0

Time(s)

G-NumberB. Mazhari, IITK15

C

R IB-

VO

12

6

8

10

)

2

4

6

V O(V

)

G-NumberB. Mazhari, IITK16

0 50 100 150 200

0

Time(S)

Modified Integrator

RC

RF

VOR

VIN

CVOS

VOR

R

C

IB-

VO

Basic idea: provide a path besides capacitor for dc

G-NumberB. Mazhari, IITK17

current to flow

But Integrator becomes non-ideal !

RF

R C

RF

VIN

R

VO

C

O

V dV V RIN O O

F

V dV VCR dt R

( ) {1 exp( )}FO IN o

RV t V tR

1

G-NumberB. Mazhari, IITK18

CRFo

1

6

2

4

0

tage

(V)

-4

-2

Volt

-6

0 5 10 15 20-8

Tim e (m S)

G-NumberB. Mazhari, IITK19

Measurement of Opamp Parameters

Why measure when manufacturer has alreadyWhy measure when manufacturer has already given the values?

G-NumberB. Mazhari, IITK20

741 opamp

G-NumberB. Mazhari, IITK21

Values vary from opamp to opamp. Manufacturer gives onlytypical and worst case valuestypical and worst case values.

Measurement of these parameters would Measurement of these parameters wouldinculcate greater appreciation for conditionsunder which these parameters becomeunder which these parameters becomeimportant.

G-NumberB. Mazhari, IITK22

Offset Voltage

R2IR1

V

IB-

VO

VVIO

2 1 2(1 )O IO BV R R V I R

Minimize the contribution of the second term by

G-NumberB. Mazhari, IITK23

ySuitably selecting R2 and R1 values

Bias Currents

IB-

R

RIVV VO

V

RIVV BIOO 1

VIO

VOI

RIVV BIOO 2IB2

R

G-NumberB. Mazhari, IITK24

VIO

Other Relevant Non-idealities

fFinite Bandwidth: Unity gain frequency Tf

AOL(f)O

100 dB

f101 10610 10

(0)( ) OLOL

AA f f

G-NumberB. Mazhari, IITK25

( )1 (0)

OL

OLT

A f fj Af

R2

R

R2

VO

R1

VIN

+-

VO

VS

+Vin

R1

AOL(f)xVININ S -

(0)( ) OLOL

AA f ffRRfAV

)( 12

1 (0)OLT

fj Af

ffj1

2Rff T

G-NumberB. Mazhari, IITK26

1

21RR

Estimation of UGF

R2

R1

VOVIN

2 1( ) oV

v R RA f f

( )1

Vin

A f fv jf

G-NumberB. Mazhari, IITK27

Measure 3dB frequency

40

30

40

B)

20

ain

(dB

ff T10G

a

21RR

f

102 103 104 105 106

0 1R

10 10 10 10 10Frequency (Hz)

G-NumberB. Mazhari, IITK28

Slew Rate

VO

R

CVIN

1( )H f RC22( )1

H fj RC

RC2.2

G-NumberB. Mazhari, IITK29

Opamp

VO = ?

10

VIN0

ffj

fH

1

1)(

f35.0

Tf Tf

For fT = 1MHz, rise time is 0.35s

G-NumberB. Mazhari, IITK30

For fT 1MHz, rise time is 0.35s

However, output may take more than 10 s !

Slew Rate (measurement)

VO

8

VIN0

Adjust Input frequency such that the requiredoutput is abtained

8

output is abtained

For Measurement of Slew Rate

G-NumberB. Mazhari, IITK31

Slew rateThere is an upper limit on how fast the outputVoltage can rise or fall

)/(dVO )/(7.0 sVSRdt

dVO

According to small signal opamp model

)2exp(2

tff

Vdt

dVT

OO

)exp()/(63

2tsV

fdt T

G-NumberB. Mazhari, IITK

32

)16.0

exp()/(63s

sV

For fT = 1MHz

741 opamp

G-NumberB. Mazhari, IITK33