Chapter 5portal.unimap.edu.my/portal/page/portal30/Lecture... · Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey

Chapter 5

Operational Amplifiers

Operational Amplifiers

Boylestad

Electronic Devices and Circuit Theory

Electronic Devices and Circuit TheoryBoylestad

© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.10 Summary

The Basic Op-Amp

Operational amplifier (Op-amp): A high gain differential

amplifier with a high input impedance (typically in M) and

low output impedance (less than 100).

Note the op-amp has two inputs and one output.




Ch.10 Summary

Op-Amp Gain

Op-Amps can be connected in open-loop or closed-loop

configurations.

Open-loop: A configuration with no feedback from the op-amp output

back to its input. Op-amp open-loop gain typically exceeds 10,000.

Closed-loop: A configuration that has a negative feedback path from

the op-amp output back to its input. Negative feedback reduces the

gain and improves many characteristics of the op-amp.

• Closed-loop gain is always lower than open-loop gain.




Ch.10 Summary

Inverting Op-Amp

The input signal is applied to

the inverting (–) input

The non-inverting input (+)

is grounded

The feedback resistor (Rf)

is connected from the output

to the negative (inverting)

input; providing negative

feedback.




Ch.10 Summary

Inverting Op-Amp Gain

Gain is set using external resistors: Rf and R1

The negative sign denotes a 180 phase shift between input and output.

1R

R

V

VA f

i

ov

11

1

R

RA

RR

fv

f

Gain can be set to any value

by manipulating the values of

Rf and R1.

Unity gain (Av = 1):




Ch.10 Summary

Common Op-Amp Circuits

Inverting amplifier

Noninverting amplifier

Unity follower

Summing amplifier

Integrator

Differentiator




Ch.10 Summary

Inverting/Noninverting Amplifiers

1

1

VR

RV f

o

Inverting Amplifier Noninverting Amplifier

1

1

)1( VR

RV f

o




Ch.10 Summary

Unity Follower

1VVo




Ch.10 Summary

Summing Amplifier

Because the op-amp

has a high input

impedance, the

multiple inputs are

treated as separate

inputs.

3

3

2

2

1

1

VR

RV

R

RV

R

RV fff

o




Ch.10 Summary

Integrator

The output is the

integral of the input;

i.e., proportional to the

area under the input

waveform. This circuit

is useful in low-pass

filter circuits and sensor

conditioning circuits.

dttvRC

tvo )(1

)( 1




Ch.10 Summary

Differentiator

The differentiator

takes the derivative

of the input. This

circuit is useful in

high-pass filter

circuits.

dt

tdvRCtvo

)()( 1




Ch.10 Summary

DC-Offset Parameters

Input offset voltage

Input offset current

Input offset voltage and input offset current

Input bias current

Even when the input voltage is zero, an op-amp can have an

output offset. The following can cause this offset:




Ch.10 Summary

Input Offset Voltage (VIO)

The specification sheet for an op-amp indicates

an input offset voltage (VIO).

The effect of this input offset voltage on the

output can be calculated with

1

1

R

RRVV f

IOo(offset)




Ch.10 Summary

Input Offset Current (IIO)

The input offset current (IIO) is specified in the specifications for

an op-amp.

The effect of IIO on the output offset voltage can be calculated

using:

)()()( IOIO to Ioffset dueo to Voffset dueooffset o V V V

If there is a difference between the dc bias currents generated by

the same applied input, this also causes an output offset voltage:




Ch.10 Summary

Total Offset Due to VIO and IIO

Op-amps may have an output offset voltage due to VIO

and IIO. The total output offset voltage equals the sum of

the effects of both:

)()()( IOoIOoo to Ioffset dueV to Voffset dueVoffsetV




Ch.10 Summary

Input Bias Current (IIB)

A parameter that is related to input offset current (IIO) is called

input bias current (IIB)

The input bias currents are calculated using:

The total input bias current is the average of the

two:

2IO

IBIB

III

2IO

IBIB

I II

2

IBIB

IB

III




Ch.10 Summary

Frequency Parameters

An op-amp is a wide-bandwidth amplifier. The

following factors affect the bandwidth of the op-

amp:

Gain

Slew rate




Ch.10 Summary

Gain and Bandwidth

The op-amp’s high

frequency response is

limited by its internal

circuitry. The plot shown

is for an open loop gain

(AOL or AVD). This means

that the op-amp is

operating at the highest

possible gain with no

feedback resistor.

In the open loop mode, an op-amp has a narrow bandwidth. The

bandwidth widens in closed-loop mode, but the gain is lower.




Ch.10 Summary

Slew Rate (SR)

Slew rate (SR): The maximum rate at which an

op-amp can change output without distortion.

The SR rating is listed in the specification sheets

as the V/s rating.

s) V/(in Δt

ΔVSR o




Ch.10 Summary

Maximum Signal Frequency

The slew rate determines the highest

frequency of the op-amp without distortion.

where VP is the peak voltage

pπV

SRf

2




Ch.10 Summary

General Op-Amp Specifications

Other op-amp ratings found on specification

sheets are:

Absolute Ratings

Electrical Characteristics

Performance




Ch.10 Summary

Absolute Ratings

These are

common

maximum ratings

for the op-amp.




Ch.10 Summary

Electrical Characteristics

Note: These ratings are for specific circuit conditions, and they often

include minimum, maximum and typical values.




Ch.10 Summary

CMRR

One rating that is unique to op-amps is CMRR or common-mode

rejection ratio.

Because the op-amp has two inputs that are opposite in phase

(inverting input and the non-inverting input) any signal that is

common to both inputs will be cancelled.

Op-amp CMRR is a measure of the ability to cancel out common-

mode signals.




Ch.10 Summary

Op-Amp Performance

The specification sheets

will also include graphs

that indicate the

performance of the op-

amp over a wide range of

conditions.

Op-Amp Applications

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Ch.11 Summary

Common Op-Amp Applications

Constant-gain amplifier

Voltage summing

Voltage buffer

Controlled sources

Instrumentation circuits

Active filters




Ch.11 Summary

Constant-Gain Amplifier

Inverting amplifier




Ch.11 Summary

Constant-Gain Amplifier

Noninverting amplifierNoninverting amplifier




Ch.11 Summary

Multiple-Stage Gains

The total gain (3-stages) is given by:321 AAAA

or:

321

1R

R

R

R

R

RA fff




Ch.11 Summary

Voltage Summing

3

3

2

2

1

1

VR

RV

R

RV

R

RV fff

o

The output is the sum

of individual signals

times the gain:




Ch.11 Summary

Voltage Buffer

The unity gain amplifier shown

is commonly referred to as a

voltage buffer or a voltage

follower.

Any amplifier with no gain or loss is called a unity gain

amplifier.

The advantages of using a unity gain amplifier:

• Very high input impedance

• Very low output impedance




Ch.11 Summary

Instrumentation Circuits

Some examples of instrumentation circuits using op-amps:

Display driver

Instrumentation amplifier

Chapter 5portal.unimap.edu.my/portal/page/portal30/Lecture... · Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey

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