The Instrumentation Amplifier | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

What Is an Instrumentation Amplifier?

An instrumentation amplifier allows an engineer to adjust the gain of an amplifier circuit without having to change more than one resistor value. Compare this to the differential amplifier, which we covered previously, which requires the adjustment of multiple resistor values.

The so-called instrumentation amplifier builds on the last version of the differential amplifier to give us that capability:

The Instrumentation Amplifier | Analog and Digital Electronics - Electrical Engineering (EE)

Understanding the Instrumentation Amplifier Circuit

This intimidating circuit is constructed from a buffered differential amplifier stage with three new resistors linking the two buffer circuits together. Consider all resistors to be of equal value except for Rgain.

The negative feedback of the upper-left op-amp causes the voltage at point 1 (top of Rgain) to be equal to V1. Likewise, the voltage at point 2 (bottom of Rgain) is held to a value equal to V2. This establishes a voltage drop across Rgain equal to the voltage difference between V1 and V2. That voltage drop causes a current through Rgain, and since the feedback loops of the two input op-amps draw no current, that same amount of current through Rgain must be going through the two “R” resistors above and below it.

This produces a voltage drop between points 3 and 4 equal to:

The Instrumentation Amplifier | Analog and Digital Electronics - Electrical Engineering (EE)

The regular differential amplifier on the right-hand side of the circuit then takes this voltage drop between points 3 and 4 and amplifies it by a gain of 1 (assuming again that all “R” resistors are of equal value).

Advantages of the Instrumentation Amplifier

Though this looks like a cumbersome way to build a differential amplifier, it has the distinct advantages of possessing extremely high input impedances on the V1 and V2 inputs (because they connect straight into the noninverting inputs of their respective op-amps), and adjustable gain that can be set by a single resistor.

Manipulating the above formula a bit, we have a general expression for overall voltage gain in the instrumentation amplifier:

The Instrumentation Amplifier | Analog and Digital Electronics - Electrical Engineering (EE)

Though it may not be obvious by looking at the schematic, we can change the differential gain of the instrumentation amplifier simply by changing the value of one resistor: Rgain.

Yes, we could still change the overall gain by changing the values of some of the other resistors, but this would necessitate balanced resistor value changes for the circuit to remain symmetrical. Please note that the lowest gain possible with the above circuit is obtained with Rgain completely open (infinite resistance), and that gain value is 1.

The document The Instrumentation Amplifier | Analog and Digital Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Analog and Digital Electronics.
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FAQs on The Instrumentation Amplifier - Analog and Digital Electronics - Electrical Engineering (EE)

1. What is the purpose of an instrumentation amplifier in electrical engineering?
Ans. An instrumentation amplifier is used to amplify small differential signals while rejecting common-mode noise, making it ideal for applications where precise measurements are needed.
2. How does an instrumentation amplifier circuit work?
Ans. An instrumentation amplifier typically consists of three operational amplifiers and provides high input impedance, high common-mode rejection ratio, and low output impedance, making it suitable for use in instrumentation and sensor applications.
3. What are the advantages of using an instrumentation amplifier?
Ans. Some advantages of instrumentation amplifiers include high common-mode rejection ratio, low drift, high input impedance, and the ability to accurately amplify small differential signals without introducing additional noise.
4. What are some common applications of instrumentation amplifiers?
Ans. Instrumentation amplifiers are commonly used in medical instrumentation, strain gauge measurements, temperature sensors, and other precision measurement applications where accurate signal amplification is required.
5. How can one design an instrumentation amplifier circuit for a specific application?
Ans. Designing an instrumentation amplifier involves selecting appropriate resistor values, operational amplifiers, and feedback configurations to meet the requirements of the specific application, such as input signal range, gain, and noise specifications.
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