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MALVINO & BATES

SEVENTH EDITION

Electronic

PRINCIPLES

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Operational Amplifiers

ChapterChapter1818

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Topics Covered in Chapter 18

• Introduction to op amps

• The 741 op amp

• The inverting amplifier

• The noninverting amplifier

• Two op-amp applications

• Linear ICs

• Op amps as surface-mount devices

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Op amp

• Noninverting and inverting input

• Single-ended output

• A perfect amplifier – a voltage-controlled voltage source

• An ideal op amp has:

Infinite open-loop voltage gain

Infinite input resistance

Zero output impedance

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The typical op amp has a differential

input and a single-ended output.

Class B

push-pull

emitter

follower

Diff

amp

More

stages

of gain

Vin

Vout

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Symbol

+VCC

-VEE

Noninverting

input

Inverting

input

Output

Op amp symbol and equivalent circuit

AVOL(v1-v2)

Rout

Rin

v1

v2

vout

Equivalent circuit

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The 741C op amp is an industry standard.

AVOL(v1-v2)

Rout

Rin

v1

v2

vout

Rin = 2 MΩΩΩΩ AVOL = 100,000Rout = 75 ΩΩΩΩ

Iin(bias) = 80 nA Iin(off) = 20 nA Vin(off) = 2 mV

CMRR = 90 dBfunity = 1 MHz

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Bode plot of the 741C op amp

20 dB/decade

rolloff

funity

10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

100 dB

80 dB

60 dB

20 dB

0 dB

40 dB

AVOL

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+VCC

-VEE

RB

RB

10 kΩΩΩΩ

15

3

42

7

6

741C pinout and offset nulling

Adjust

for null

(i.e. 0V at

Pin 6)

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The internal frequency compensation capacitor found in most

op amps also limits the rate at which the output can change.

SR = 0.5 V/µµµµs (for the 741)

When a signal exceeds the slew-rate of an op amp, the

output becomes distorted and amplitude limited.

Slope > SR

Slew rate

distortion

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dvdt

dvdt

>

v

t

dvdt

dvdt

>

v

t

The rate of voltage

change (slope) is directly

related to both amplitude

and frequency:

SS = 2π2π2π2πfVp

The power bandwidth

of an op amp is given by:

fmax =2π2π2π2πfVp

SR

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Inverting op amp

• The most basic op amp circuit

• Uses negative feedback to stabilize the

closed-loop voltage gain

• Closed-loop voltage gain equals feedback

resistance divided by input resistance

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R1

Rf

The inverting amplifier

The negative feedback produces a

virtual ground at the inverting terminal.

A virtual ground is a short for voltage but an open for current.

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R1

Rf

Analyzing the inverting amplifier

vin vout

iin

iin

vin = iinR1 and vout = iinRf

AO(CL) = =R1

Rfvout

vin

zin(CL) = R1

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10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

100 dB

80 dB

60 dB

20 dB

0 dB

Negative feedback increases

the closed-loop bandwidth.

f2(CL) ≅≅≅≅funity

AV(CL)

40 dB

AVOL

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Negative feedback reduces

error• V1err = (RB1 - RB2)Iin(bias)

• V2err = (RB1 + RB2)Iin(off)/2

• V3err = Vin(off)

• Verror = ± AV(CL)(± V1err ± V2err ± V3err)

• V1err eliminated with resistor

compensation

• Use offset nulling in demanding

applications

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R1

Rf

Resistor compensation for V1err

vin vout

RB2 = R1 Rf

RB2 has no effect on the virtual-ground approximation

since no signal current flows through it.

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Noninverting op amp

• A basic op amp circuit

• Uses negative feedback to stabilize the

closed-loop gain

• Closed-loop voltage gain equals the

feedback resistance divided by the input

resistance plus 1

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R1

Rf

The noninverting amplifier

A virtual short is a short for voltage but an open for current.

The negative feedback

produces a virtual short.

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Analyzing the noninverting amplifier

R1

Rf

vin

vout

i1

vin = i1R1 and vout = i1(Rf+R1)

i1

AV(CL) = =R1

Rf+R1vout

vin

=R1

Rf + 1

zin(CL) →→→→ ∞∞∞∞

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Op amp application:

summing amp

• A summing amp has two or more inputs

and one output

• Each input is amplified by its channel

gain

• If all channel gains equal unity, the

output equals the sum of the inputs

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R1 Rf

The summing amplifier

R2

v1

voutv2

R2

Rf v2R1

Rfv1 +vout =

In a mixer, a summing amp can amplify and combine audio signals

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Op amp application:

voltage follower

• Has a closed-loop gain of unity

• Has a bandwidth of funity

• Useful as an interface between a high-

impedance source and a low-impedance

load

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Rhigh

Rlow

The voltage follower

voutvin

The virtual short tells us vout = vin

AV(CL) = 1

zin(CL) →→→→ ∞∞∞∞

zout(CL) →→→→ 0

f2(CL) = funity

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Other than the 741

• BIFET op amps offer extremely low

input currents.

• High-power op amps supply amperes of

output current.

• High-speed op amps slew at tens or

hundreds of volts/µµµµs and some have

hundreds of MHz of bandwidth.

• Precision op amps boast small offset

errors and low temperature drift.

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Other linear ICs

• Audio amps in the mW range optimized

for low noise (preamplifiers)

• Audio amps in the watt range for driving

loudspeakers

• Video amps with wide bandwidths

• RF and IF amps for receiver applications

• Voltage regulators

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Op amps as surface-mount devices

• Pin out is simple for most op amps

• Small outline package (SOP) is the

preferred SM style