ckt analysis with devices

7/29/2019 Ckt Analysis With Devices

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Chapter 31

Applications of Op-Amps


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2

Comparators Op-amp as a Comparator

No negative feedback

Output saturates with very small + or input


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3

Comparators Comparator

Non-linear device

vout has two discrete values,VSAT vout = +VSAT if + input is greater than input

vout =VSAT if input is greater than + input


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Comparators A comparator circuit: Sine wave in,

square wave out


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Comparators Input sine wave

Output square

waveV

out =V

SAT +VSAT (determined

by VCC) whensinusoid is +

VSAT (determinedby VEE) whensinusoid is

___-

___-

+

-vout

741

VCC

VEE


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Comparators Compare input waveform to reference

Reference can be ground or dc source

Can compare two waveforms

Specialized comparator ICs also available

Detects when waveform reaches givenlevel


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Comparators Zero-Crossing Detector


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Voltage Summing Amplifier Circuit


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Voltage Summing Amplifier Inverse sum

321F IIII

ffRIV out

i

F

R

R


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Voltage Summing Amplifier Multiplies each input by

3

3

2

2

1

1

VR

RV

R

RV

R

RV FFFout


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Integrators and Differentiators In general

Using resistors andcapacitors Integrators

Differentiators

-

+

___

-

vout

vin

ZF

Z1

in

1

Fout v

Z

Zv


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Integrators and Differentiators Voltage across capacitor

Current through capacitor

0

0

1( ) ( )

( )

t

C

CC

v t i t dt V C

dvi t C

dt


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Integrators and Differentiators Op-amp Integrator

0

1 0

1 0

1( ) ( )

1( ) ( )

t

C in

t

out in

v t v t dt V R C

v t v t dt

R C

-

+

___

-

vout

C

R1vin

0 Vi i= 0


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+ -

Integrators and Differentiators Op-amp

differentiator

Circuit inherentlyunstable

( ) inout F dvv t R C dt

-

+

___-

vout

CinRF

vin

i

i


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Integrators and Differentiators Stable op-amp differentiator


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Instrumentation Amplifiers Op-amp in differential amplifier

configuration

Noise suppression

High CMRR

Reasonable gain

IC instrumentation amps


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Instrumentation Amplifiers An op-amp instrumentation amp circuit


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Instrumentation Amplifiers Measurement of very small voltages

Transducer

Converts a physical change into an electrical

change


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Instrumentation Amplifiers Strain gage

Converts force into R

R is milliohms

Use bridge circuit


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Instrumentation Amplifiers Strain gage example

Thin metal foil (resistor) on plastic backing

Glued to metal bar

Bar subjected to tension and compression


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Instrumentation Amplifiers Strain gage example

Tension

Resistance of strain gage is R + R

Compression

Resistance of strain gage is RR


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Active Filters Basic filter types

Passive elements, gain < 1

Low-pass

High-pass

Bandpass

Band reject


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Active Filters With op-amps/active filters

Gain can be 1

Filter response closer to ideal


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Active Filters Low-pass (RF= R1)

Add resistor for gain > 1

11

1

1

1

1( )

1

C

out C in inC

Z j C

v v v vR ZR

j C

TF jj R C

-

+

___-

vout

C

RF = R1

I = 0

0 VR1

vin


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Active Filters High-pass (RF= R1)

Add resistor for gain > 1

-

+

___-

vout

RF = R1

I = 0

0 V

R1

vin

C

1 1

11

1

1

1

( )1

out R in inC

R Rv v v v

R Z Rj C

R CTF j

j R C


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Active Filters dc gain

Easily achieved

Not used much due to gain-bandwidth product

Example

GBWP = 106, Gain = 10

Cutoff for filter (HP or LP) only 105


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Active Filters Bandpass

Wideband

Cascade HP and LP active filters LP must have higher cutoff frequency

HP and LP cutoff frequencies far apart

Narrowband Can use single op-amp


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Active Filters Narrowband BP

circuit

0

1

0.11251

0.1591

Rf

RC R

BWRC

-

+

___-

vout

2R

R1

C

C

___-

vin

R


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Active Filters Active notch filter

Cascade narrowband BP filter

Adder circuit

Result is 1 (frequency response of BP filter)

Frequency at resonant frequency of BP filter

will be eliminated


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Voltage Regulation Voltage regulator

Constant voltage to load

Specified current range

Specified input voltage range

Zener diode regulator

Inefficient Dissipates power


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Voltage Regulation Types of regulators

Fixed voltage regulator

Variable voltage regulator

Switching regulator

Specialized IC regulators

For different voltages, e.g. +5 V,5 V, +12 V,12 V, +15 V,15 V, etc.


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Voltage Regulation Line Regulation

Small output

change with

change in input

RL

___-

Regulated

outputVoltageregulatorUnregulatedinput

% 100%out

in

vline regulation

v


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Voltage Regulation Load regulation

Small output voltage change with smallerRL

VNL = no-load voltage (open-circuit load)

VFL = full-load voltage (specified bymanufacturer)

% 100%NL FL

FL

V Vload regulation

V


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Voltage Regulation

Circuit toincrease

efficiency of

Zener regulator

with an op-amp

+

-

Q1Unregulated

inputRD

R1

R2

vout

+

-___

-

vin

+

-


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VoltageRegulation

Three-terminal IC

regulators 7800 series,

positive voltage

7900 series,negative voltage


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Voltage Regulation

5 V output, 7805

12 V output, 7812

5 V output, 7905

12 V output, 7912

RL

___-

Vout=12 V

A7812

Unregulated

input

+

-

+

-

___-

IN OUT

COM


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Voltage Regulation Ripple

Greatly reduced by IC regulator

Vr(in) = input ripple voltage

Vr(out) = output ripple voltage

( )

( )

20 logr in

dBr out

Vripple rejectionV

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