The Instrumentation Amplifier

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:

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 R_gain.

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