Application Operational Amplifiers

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APPLICATION OPERATIONALAMPLIFIERS

Engr. Robert R. Cabagnot


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Summary Of Previews Topics

Non inverting Amplifiers

Inverting Amplifiers

Summing Amplifiers Differentials Amplifiers


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Instrumentation

Conversion of Different Forces or energy intomuch definable and more amplifiablevoltage.


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Introduction

Instrumentation amplifiers are used in manyapplications. from motor control to dataacquisition to automotive. Let us study thefundamentals of what an instrumentationamplifiers is, how it operates, and how andwhere to use it. In addition we will see

several different types of instrumentationamplifiers configuration.


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Not all amplifiers used in instrumentationapplication are instrumentation amplifiers

And further not all instrumentation amplifiersused only in instrumentation application.


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Operational Amplifiers

The Simplest configuration of aninstrumentation is differential amplifier. Thiscircuit amplifiers differential signals whilerejecting those that are common mode.CMRR.

However, it has some limitations, First, the

impedances of the inverting and non-inverting inputs are relatively low andunequal


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It rejects signals which common to the twoinputs and amplify only the difference bet thetwo inputs.

Good CMRR Good Instrumentation !!!!!

70-90 dB

120 dB Common on instrumentationamplifiers


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Operational Amplifiers

IF R1 = R1 = R2 = R2 = 100 K ohms then thedifferential gain is = 1 . Here the inputimpedance to Vin 1 = 100 ohms and Vin2 =200 K ohms . Therefore, when voltage isapplied to one input while grounding theother, different currents will flow depending

on which input receives the applied voltage. This unbalanced in the sources resistance will

degrade the circuits CMRR.


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Tolerance

Further more , this circuit requires a very closeratio match between resistors pairs R1/R2 andR3/R4: otherwise, the gain from each input

would be different-directly affecting commonMode rejection Ratio.

For example , at a gain of 1, with all resistorswqual in value, a 0.1% mismatch in just one of

the resistors will degrade the CMRR to a level 66dB ( 1 part in 2,000).Similarly, a source resistanceimbalance of 100 ohms would degrade CMRR by6dB.


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Problem

In spite of these problems, this type ofinstrumentation amplifier circuit, often calleda difference amplifier or SUBTRACTOR, isused as a building block within higherperformance


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Including a Buffer

High input Impedance for the input sidewould eliminate the difference in the inputside

Note: that an op-amp has a very high inputimpedance


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R5

R6

R7

R8


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The figure shows further improvement, nowthe input buffers are operating with gainwhich provides a circuit with more flexibility.If the value of R5 = R8 and R6 = R7 and, asbefore, R1 = R3 and R2 = R4, then

Vo = (Vin2 Vin1)(1 + R5/R6) (R2/R1)

CMRR also increases


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RG = R7 + R6


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The 3 Op-amp Instrumentation Amp

This Circuit provides further refinement and hasbecome the most popular configuration forinstrumentation amplifier design.

The classic 3-op amp instrumentation ampcircuit is a clever modification of the buffersubtractor circuit. As with previous circuit, op

amps A1 anA2 buffer the input voltage . However, in this configuration , a single gain resistor RG,is connected between the summing junctionsof the two input buffers, replacing R7 and R6.


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The Differential gain may be varied by justchanging RG


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CURRENT BOOSTERS


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Uni Directional Current

Boosters


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B- Feedback fraction


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Bidirectional Current

Boosters


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Standard Z for 741C

Ideal in Op-amps


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Application Operational Amplifiers

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