Chapter 10: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
Operational amplifier or op-amp, is a very high gain differential amplifier 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.
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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
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.
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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
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.
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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
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
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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
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.
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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
Practical Op-Amp CircuitsPractical Op-Amp Circuits
Inverting amplifierInverting amplifierNoninverting amplifierNoninverting amplifier
Unity followerUnity followerSumming amplifierSumming amplifier
IntegratorIntegratorDifferentiatorDifferentiator
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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
Inverting/Noninverting Op-AmpsInverting/Noninverting Op-Amps
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fo V
R
RV
Inverting AmplifierInverting Amplifier Noninverting AmplifierNoninverting Amplifier
11
fo V)
R
R1(V
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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
Unity FollowerUnity Follower
1o VV
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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
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
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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
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
1111
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
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
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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
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:
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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
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
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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
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:
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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
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
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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
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:
2
III IO
IBIB
2
II I IO
IBIB
2
III IBIB
IB
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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
An op-amp is a wide-bandwidth amplifier. The following affect the bandwidth of the op-amp:
• Gain• Slew rate
Frequency ParametersFrequency Parameters
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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
Gain and BandwidthGain and Bandwidth
The op-amp’s high frequency response is limited by 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, the op-amp has a narrow bandwidth. The bandwidth widens in closed-loop operation, but then the gain is lower.
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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
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
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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
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
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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
General Op-Amp SpecificationsGeneral Op-Amp Specifications
Other ratings for op-amp found on specification sheets are:
• Absolute Ratings• Electrical Characteristics• Performance
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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
Absolute RatingsAbsolute Ratings
These are common maximum ratings for the op-amp.
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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
Electrical CharacteristicsElectrical Characteristics
Note: These ratings are for specific circuit conditions, and they often include minimum, maximum and typical values.
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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
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.
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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
Op-Amp PerformanceOp-Amp Performance
The specification sheets will also include graphs that indicate the performance of the op-amp over a wide range of conditions.
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