Chapter 14: Operational Amplifiers. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices

Chapter 14:Operational Amplifiers

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Basic Op-AmpBasic Op-Amp

The electronic amplifier: is an electronic device that increases the power of a signal.

Operational amplifier or op-amp, is a very high gain differential amplifier (that amplifies the difference between two voltages) with a high input impedance (typically a few meg-Ohms) and low output impedance (less than 100 ).

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

22



differential amplifier



Op-Amp GainOp-Amp Gain

Op-Amps have a very high gain. They can be connected open-loop or closed-loop.

• Open-loopOpen-loop refers to a configuration where there is no feedback from output back to the input. In the open-loop configuration the gain can exceed 10,000.

• Closed-loopClosed-loop configuration reduces the gain. In order to control the gain of an op-amp it must have feedback. This feedback is a negative feedback. A negative feedbacknegative feedback reduces the gain and improves many characteristics of the op-amp.

44

















CMRRCMRR

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

1212

















Inverting Op-AmpInverting Op-Amp

• The signal input is applied to the inverting (–) inputinverting (–) input• The non-inverting input (+)non-inverting input (+) is grounded• The resistor Rf is the feedback resistorfeedback resistor. It is connected from the output to

the negative (inverting) input. This is negative feedbacknegative feedback.

2020



Inverting Op-Amp GainInverting Op-Amp GainGain can be determined from external resistors: Rf and R1

Unity gain—voltage gain is 1

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

1

f

i

ov R

R

V

VA

1R

RA

RR

1

fv

1f

Constant Gain—Rf is a multiple of R1

2121



Virtual GroundVirtual GroundAn understanding of the concept of virtual groundvirtual ground provides a better understanding of how an op-amp operates.

The non-inverting input pin is at ground. The inverting input pin is also at 0 V for an AC signal.

The op-amp has such high input impedance that even with a high gain there is no current from inverting input pin, therefore there is no voltage from inverting pin to ground—all of the current is through Rf.

2222



Practical Op-Amp CircuitsPractical Op-Amp Circuits

Inverting amplifierInverting amplifierNoninverting amplifierNoninverting amplifier

Unity followerUnity followerSumming amplifierSumming amplifier

IntegratorIntegratorDifferentiatorDifferentiator

2323



Inverting/Noninverting Op-AmpsInverting/Noninverting Op-Amps

11

fo V

R

RV

Inverting AmplifierInverting Amplifier Noninverting AmplifierNoninverting Amplifier

11

fo V)

R

R1(V

2424



Unity FollowerUnity Follower

1o VV

2525



Summing AmplifierSumming Amplifier

Because the op-amp has a high input impedance, the multiple inputs are treated as separate inputs.

3

3

f2

2

f1

1

fo V

R

RV

R

RV

R

RV

2626



IntegratorIntegrator

The output is the integral of the input. Integration is the operation of summing the area under a waveform or curve over a period of time. This circuit is useful in low-pass filter circuits and sensor conditioning circuits.

(t)dtvRC

1(t)v 1o

2727



DifferentiatorDifferentiator

The differentiator takes the derivative of the input. This circuit is useful in high-pass filter circuits.

dt

(t)dvRC(t)v 1

o

2828



Op-Amp Specifications—DC Offset Op-Amp Specifications—DC Offset ParametersParameters

• Input offset voltage• Input offset current• Input offset voltage and input offset current• Input bias current

Even when the input voltage is zero, there can be an output offsetoffset. The following can cause this offset:

2929



Input Offset Voltage (VInput Offset Voltage (VIOIO))

The specification sheet for an op-amp indicate an input offset voltage (VIO).

The effect of this input offset voltage on the output can be calculated with

1

f1IOo(offset) R

RRVV

3030



Output Offset Voltage Due to Input Offset Output Offset Voltage Due to Input Offset Current (ICurrent (IIOIO))

• The input offset Current (IIO) is specified in the specifications for the op-amp.

• The effect on the output can be calculated using:

fIO)I to dueo(offset RIVIO

If there is a difference between the dc bias currents for the same applied input, then this also causes an output offset voltage:

3131



Total Offset Due to VTotal Offset Due to VIOIO and I and IIOIO

Op-amps may have an output offset voltage due to both factors VIO and IIO. The total output offset voltage will be the sum of the effects of both:

)I to due(offset V)V to due(offset V(offset)V IOoIOoo

3232







Input Bias Current (IInput Bias Current (IIBIB))

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

input bias currentinput bias current (IIB)

The separate input bias currents are:

The total input bias current is the average:

IBI

IBI

2

III IBIB

IB

3535



An op-amp is a wide-bandwidth amplifier. The following affect the bandwidth of the op-amp:

• Gain• Slew rate

Frequency ParametersFrequency Parameters

3636



An op-amp is designed to be a high-gain, and wide-bandwidth amplifier. This

operation tends to be unstable (oscillate) due to positive feedback .

To ensure stable operation, op-amps are built with internal compensation circuitry,

which also causes the very high open-loop gain to reduce with increasing frequency.

This gain reduction is referred to as roll-off.

A number of circuit improvements result from this gain reduction.

1.More stable amplifier voltage gain

2.Incresing the input impedance of the circuit over that of the op-amp alone.

3. Decreasing the circuit output impedance from that of the op-amp alone.

4. Increasing the frequency response of the circuit over that of the op-amp alone.



a measure of the width of a range of frequencies

BandwidthBandwidth

As the frequency of the input signal increases the open-loop gaindrops off until it finally reaches the value of 1 (unity). A frequency at which the gain becomes 1, since the frequency band from 0 Hz to the unity-gain frequency is also a bandwidth.

Op-amp specifications provide a description of the gain versus bandwidth.



Slew Rate (SR)Slew Rate (SR)

Slew rate (SR)Slew rate (SR) is the maximum rate at which an op-amp can change output without distortion.

The SR rating is given in the specification sheets as V/s rating.

s)V/ (in Δt

ΔVSR o

3939



Maximum Signal FrequencyMaximum Signal Frequency

The slew rate determines the highest frequency of the op-amp without distortion.

where VP is the peak voltage

pVπ2

SRf

4040



General Op-Amp SpecificationsGeneral Op-Amp Specifications

Other ratings for op-amp found on specification sheets are:

• Absolute Ratings• Electrical Characteristics• Performance

4141



Absolute RatingsAbsolute Ratings

These are common maximum ratings for the op-amp.

4242



Electrical CharacteristicsElectrical Characteristics

Note: These ratings are for specific circuit conditions, and they often include minimum, maximum and typical values.

4343















What is the range of the voltage-gain adjustment in the circuit:





Op-Amp ApplicationsOp-Amp Applications

Constant-gain multiplierConstant-gain multiplierVoltage summingVoltage summing

Voltage bufferVoltage bufferControlled sourcesControlled sources

5252



Constant-Gain AmplifierConstant-Gain Amplifier

Inverting VersionInverting Version

more…more…

5353



Constant-Gain AmplifierConstant-Gain Amplifier

Noninverting VersionNoninverting Version

5454



Multiple-Stage GainsMultiple-Stage Gains

5555

R3

R

R2

R

R

R1A

AAAA

ff

1

f

321

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

or



Voltage SummingVoltage Summing

[Formula 14.3]

3

3

f2

2

f1

1

fo V

R

RV

R

RV

R

RV

The output is the sum of individual signals times the gain:

5656



Voltage Subtraction



Voltage BufferVoltage Buffer

Realistically these circuits are designed using equal resistors (R1 = Rf) to avoid problems with offset voltages.

Any amplifier with no gain or loss is called a unity gain unity gain amplifieramplifier. The advantages of using a unity gain amplifier:

• Very high input impedance • Very low output impedance

5858



Controlled SourcesControlled Sources

Voltage-controlled voltage sourceVoltage-controlled voltage source Voltage-controlled current sourceVoltage-controlled current source Current-controlled voltage sourceCurrent-controlled voltage source Current-controlled current sourceCurrent-controlled current source

5959



Voltage-Controlled Voltage SourceVoltage-Controlled Voltage Source

The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance characteristics and gain calculations that depend solely on external resistors.

Noninverting Amplifier VersionNoninverting Amplifier Version

more…more…

6060



Voltage-Controlled Voltage SourceVoltage-Controlled Voltage Source

The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance characteristics and gain calculations that depend solely on external resistors.

Inverting Amplifier VersionInverting Amplifier Version

6161



Voltage-Controlled Current SourceVoltage-Controlled Current Source

The output current is:

11

1o kV

R

VI

6262



Current-Controlled Voltage SourceCurrent-Controlled Voltage Source

This is simply another way of applying the op-amp operation. Whether the input is a current determined by Vin/R1 or as I1:

or

in1

fout V

R

RV

6363

L1out RIV



Current-Controlled Current SourceCurrent-Controlled Current Source

This circuit may appear more complicated than the others but it is really the same thing.

in

in

f

out

21

in

f

out

inin

fout

R

V

R

V

R||R

V

R

V

VR

RV

kIR

R1II

R

RR

R

VI

RR

RRVI

R||R

VI

2

1o

2

21

1

ino

21

21ino

21

ino

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Documents

Chapter 14: Operational Amplifiers. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Electronic Devices