Chapter 31

Applications of Op-Amps

2

Comparators• Op-amp as a Comparator

– No negative feedback– Output saturates with very small + or – input

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

4

Comparators• A comparator circuit: Sine wave in,

square wave out

5

Comparators• Input sine wave• Output square wave

Vout = ±VSAT

• +VSAT (determined by VCC) when sinusoid is +

• –VSAT (determined by VEE) when sinusoid is –

___-

___-

+

- vout

741

VCC

VEE

6

Comparators• Compare input waveform to reference

• Reference can be ground or dc source

• Can compare two waveforms

• Specialized comparator IC’s also available

• Detects when waveform reaches given level

7

Comparators• Zero-Crossing Detector

8

Voltage Summing Amplifier

• Circuit

9

Voltage Summing Amplifier

• Inverse sum321F IIII ++=

ff RIV −=out

i

F

R

R

10

Voltage Summing Amplifier

• Multiplies each input by

⎟⎟⎠

⎞⎜⎜⎝

⎛++−= 3

32

21

1

VR

RV

R

RV

R

RV FFFout

11

Integrators and Differentiators• In general

• Using resistors and capacitors– Integrators– Differentiators

-

+___-

vout

vin

ZF

Z1

in1

Fout v

Z

Zv −=

12

Integrators and Differentiators

• Voltage across capacitor

• Current through capacitor

0

0

1( ) ( )

( )

t

C

CC

v t i t dt VC

dvi t C

dt

= +

=

∫

13

Integrators and Differentiators• Op-amp Integrator

01 0

1 0

1( ) ( )

1( ) ( )

t

C in

t

out in

v t v t dt VRC

v t v t dtRC

= +

=

∫

∫

-

+___-

vout

C

R1vin

0 Vi i = 0

14

+ -

Integrators and Differentiators• Op-amp

differentiator

• Circuit inherently unstable

( ) inout F

dvv t R C

dt=−

-

+___-

vout

Cin

RF

vin

i

i

15

Integrators and Differentiators• Stable op-amp differentiator

16

Instrumentation Amplifiers• Op-amp in differential amplifier

configuration

• Noise suppression

• High CMRR

• Reasonable gain

• IC instrumentation amps

17

Instrumentation Amplifiers• An op-amp instrumentation amp circuit

18

Instrumentation Amplifiers• Measurement of very small voltages

• Transducer– Converts a physical change into an electrical

change

19

Instrumentation Amplifiers• Strain gage

– Converts force into ∆R– ∆R is milliohms– Use bridge circuit

20

Instrumentation Amplifiers• Strain gage example

– Thin metal foil (resistor) on plastic backing– Glued to metal bar– Bar subjected to tension and compression

21

Instrumentation Amplifiers• Strain gage example

– Tension• Resistance of strain gage is R + ∆R

– Compression• Resistance of strain gage is R – ∆R

22

Active Filters

• Basic filter types– Passive elements, gain < 1– Low-pass– High-pass– Bandpass– Band reject

23

Active Filters

• With op-amps/active filters– Gain can be ≥ 1– Filter response closer to ideal

24

Active Filters• Low-pass (RF = R1)

• Add resistor for gain > 1

11

1

1

1

1( )

1

Cout C in in

C

Z j Cv v v v

R Z Rj C

TF jj RC

ω

ω

ωω

= = =+ +

=+

-

+

___-

vout

C

RF = R1

I = 0

0 VR1

vin

25

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

RCTF j

j RC

ω

ωω

= = =+ +

=+

26

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

27

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

28

Active Filters• Narrowband BP

circuit

01

0.11251

0.1591

Rf

RC R

BWRC

= +

=

-

+

___-

vout

2R

R1

C

C

___-

vin

R

29

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

30

Voltage Regulation• Voltage regulator

– Constant voltage to load– Specified current range– Specified input voltage range– Zener diode regulator

• Inefficient• Dissipates power

31

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.

32

Voltage Regulation• Line Regulation

– Small output change with change in input

RL

___-

Regulatedoutput

Voltageregulator

Unregulatedinput

% 100%out

in

vline regulation

v

Δ= ×

Δ

33

Voltage Regulation• Load regulation

– Small output voltage change with smaller RL

– VNL = no-load voltage (open-circuit load)– VFL = full-load voltage (specified by manufacturer)

% 100%NL FL

FL

V Vload regulation

V

−= ×

34

Voltage Regulation

• Circuit to increase efficiency of Zener regulator with an op-amp

+

-

Q1Unregulatedinput RD

R1

R2

vout

+

-___-

vin

+

-

35

Voltage Regulation

• Three-terminal IC regulators– 7800 series,

positive voltage– 7900 series,

negative voltage

36

Voltage Regulation

• 5 V output, 7805

• 12 V output, 7812

• –5 V output, 7905

• –12 V output, 7912RL

___-

Vout=12 V

μA7812

Unregulatedinput

+

-

+

-

___-

IN OUT

COM

37

Voltage Regulation• Ripple

• Greatly reduced by IC regulator

Vr(in) = input ripple voltage

Vr(out) = output ripple voltage

[ ] ( )

( )

20 log r in

dBr out

Vripple rejection

V=