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Chapter 8: OPERATIONAL AMPLIFIERS

# The Instrumentation Amplifier

As suggested before, it is beneficial to be able to adjust the gain of
the amplifier circuit without having to change more than one resistor
value, as is necessary with the previous design of differential
amplifier. The so-called *instrumentation* builds on the last version of differential amplifier to give us that capability:

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 V_{1}. Likewise, the voltage at point 2 (bottom of R_{gain}) is held to a value equal to V_{2}. This establishes a voltage drop across R_{gain} equal to the voltage difference between V_{1} and V_{2}. 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). 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 V_{1} and V_{2} 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:

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: R_{gain}. 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 R_{gain} completely open (infinite resistance), and that gain value is 1.

**REVIEW:**- An
*instrumentation amplifier*is a differential op-amp circuit providing high input impedances with ease of gain adjustment through the variation of a single resistor.

## Related Content:

Circuits: Instrumentation amplifier.