Operational Operational Amplifiers in Amplifiers in

Chemical Chemical InstrumentationInstrumentation

By By

Naaimat MuhammedNaaimat Muhammed

►Most modern analog signal-Most modern analog signal-conditioning circuits owe their success conditioning circuits owe their success to the class of integrated circuits to the class of integrated circuits known as operational amplifiers , known as operational amplifiers , which can be referred to as op amps .which can be referred to as op amps .

► If you open any instrument or piece of If you open any instrument or piece of electronic equipment, it would be electronic equipment, it would be likely to find one or more op amps .likely to find one or more op amps .

Op Amps: Systems-on-a-chipOp Amps: Systems-on-a-chip

►Op amps are key analog building blocks that Op amps are key analog building blocks that condition signals throughout a system.condition signals throughout a system.

►Operational amplifiers derive their name Operational amplifiers derive their name from their original applications in analog from their original applications in analog computers, where they were used to computers, where they were used to perform mathematical operations. perform mathematical operations.

►Op amps also find general application in the Op amps also find general application in the precise measurement of voltage, current, precise measurement of voltage, current, and resistance, which are measured and resistance, which are measured variables in transducers that are used in variables in transducers that are used in chemical instruments. chemical instruments.

Symbols for Operational Symbols for Operational AmplifiersAmplifiers

► An equivalent An equivalent circuit circuit representation of an representation of an op amp.op amp.

ContinuedContinued

►the input potentials are represented by the input potentials are represented by v+v+ and and v-v-. .

►The input difference voltage The input difference voltage vsvs is the is the difference between these two difference between these two potentials. potentials.

►The power supply connections are The power supply connections are labeled +labeled +PSPS and – and –PSPS and usually have and usually have values of +15 and –15 V dc. values of +15 and –15 V dc.

►Circuit common is also referred to as Circuit common is also referred to as groundground. .

Operational Amplifier Operational Amplifier CircuitsCircuits

►Operational amplifiers are used in Operational amplifiers are used in circuit networks that contain various circuit networks that contain various combinations of capacitors, resistors, combinations of capacitors, resistors, and other electrical components.and other electrical components.

►The output of the amplifier is The output of the amplifier is determined entirely by the nature of determined entirely by the nature of the network and its components and is the network and its components and is independent of the operational independent of the operational amplifier itself. amplifier itself.

CONTINUEDCONTINUED

Feedback CircuitsFeedback Circuits

► It is often advantageous to return the It is often advantageous to return the output signal or some fraction of the output signal or some fraction of the output signal of an op amp to one of output signal of an op amp to one of the two inputs. the two inputs.

►When an output signal of an op amp is When an output signal of an op amp is connected to one of its inputs, the connected to one of its inputs, the signal is called feedback.signal is called feedback.

Feedback CircutiFeedback Circuti

► The feedback resistor The feedback resistor RfRf that is connected to the that is connected to the output output SS, which is called , which is called the summing point. the summing point.

► the feedback signal is the feedback signal is opposite in sense to the opposite in sense to the input signal input signal vivi as a result as a result of the characteristics of of the characteristics of the inverting input and is the inverting input and is called negative called negative feedback. feedback.

Applications of Operational Applications of Operational AmplifiersAmplifiers

► Op amps are easily used to generate constant-potential or Op amps are easily used to generate constant-potential or constant-current signals. constant-current signals.

► Constant-voltage sources include several instrumental Constant-voltage sources include several instrumental methods that require a dc power source whose potential is methods that require a dc power source whose potential is precisely known and from which reasonable currents can be precisely known and from which reasonable currents can be

obtained without alteration of this potential.obtained without alteration of this potential. ► A circuit, which satisfies such qualifications, is a potentiostatA circuit, which satisfies such qualifications, is a potentiostat. . ► Constant-current dc sources, called amperostats, find Constant-current dc sources, called amperostats, find

applications in several analytical instruments.applications in several analytical instruments. ► These devices may be used to maintain a constant These devices may be used to maintain a constant

current through an electrochemical cellcurrent through an electrochemical cell . .

Bandwidth of Amplifiers with Bandwidth of Amplifiers with FeedbackFeedback

► An operational amplifier is useful over a certain range An operational amplifier is useful over a certain range of frequencies, called its bandwidth .of frequencies, called its bandwidth .

► The bandwidth is dependent on the particular OA The bandwidth is dependent on the particular OA

used as well as the open loop and closed loop gains.used as well as the open loop and closed loop gains. ► The bandwidth is specified as the frequency limits The bandwidth is specified as the frequency limits

where the voltage gain of the OA is decreased to where the voltage gain of the OA is decreased to

(2)1/2/2 or 0.707 that of the midrange gain.(2)1/2/2 or 0.707 that of the midrange gain. ► The voltage gain can be expressed in decibel units as:The voltage gain can be expressed in decibel units as:

► dB = 20 log AdB = 20 log A

Active Low and High Pass Active Low and High Pass FilterFilter

► The passive RC filters are affected by changes in The passive RC filters are affected by changes in the impedance of circuits following them unless the the impedance of circuits following them unless the

impedances are very highimpedances are very high. . ► If the input impedance of the subsequent circuit is If the input impedance of the subsequent circuit is

not high, loading affects both the efficiency and not high, loading affects both the efficiency and cutoff frequency of the filter.cutoff frequency of the filter.

► The use of an operational amplifier as part of an The use of an operational amplifier as part of an

ACTIVE filter dramatically reduces the problem.ACTIVE filter dramatically reduces the problem. ► Active filters may be high pass, low pass, selective Active filters may be high pass, low pass, selective

(amplify a specific frequency) or notch (reject a (amplify a specific frequency) or notch (reject a specific frequency). specific frequency).

Low pass filterLow pass filter

High Pass filterHigh Pass filter

High Order FilterHigh Order Filter

On the Business EndOn the Business End

► Operational amplifiers, one of the oldest types of Operational amplifiers, one of the oldest types of semiconductors, have outlived their technological semiconductors, have outlived their technological

contemporaries and remain in strong demand.contemporaries and remain in strong demand. ► The more complicated digital functions become, the The more complicated digital functions become, the

more designers demand op ampsmore designers demand op amps..► Analysts project that 4.7 billion op amps will be Analysts project that 4.7 billion op amps will be

shipped this year, translating into about $1.5 billion shipped this year, translating into about $1.5 billion

in revenue.in revenue.

ReferencesReferences

► ““Operational Amplifiers.”Operational Amplifiers.”► http://192.215.107.101/ebn/942/tech/techfocus/http://192.215.107.101/ebn/942/tech/techfocus/

1071main.html1071main.html► ““Operational Amplifiers.”Operational Amplifiers.”► http://bolongo.ee.queensu.ca:8000/www/dept/http://bolongo.ee.queensu.ca:8000/www/dept/

courses/elec221/opamps.htmcourses/elec221/opamps.htm► ““Operational Amplifiers.”Operational Amplifiers.”► http://www.chem.usu.edu/~sbialk/Classes/565/http://www.chem.usu.edu/~sbialk/Classes/565/

opamps/opamps.htmlopamps/opamps.html► Skoog, Holler, and Neiman. Skoog, Holler, and Neiman. Principles of Principles of

Instrumental AnalysisInstrumental Analysis. 5th ed. Orlando: Harcourt . 5th ed. Orlando: Harcourt Brace & Co., 1998. Brace & Co., 1998.

► http://www.unc.edu/courses/pre2000fall/chem142/http://www.unc.edu/courses/pre2000fall/chem142/Experiment/EXPERIMENT6.htmExperiment/EXPERIMENT6.htm