Differential and Instrumentation Amplifiers

Differential and instrumentation amplifiers are analog circuits designed to amplify the difference between two input voltages while ignoring any signal that is common to both inputs. This ability to reject common-mode signals makes them valuable for measuring small voltages in the presence of electrical noise, which is a frequent challenge in measurement and sensing applications.

A differential amplifier produces an output proportional to the difference between its two inputs. Because interference such as mains hum often appears almost equally on both input wires, it can be treated as a common-mode signal and largely cancelled, leaving only the genuine differential signal amplified. The effectiveness of this cancellation is described by a parameter called the common-mode rejection ratio (CMRR); a higher CMRR means better rejection of unwanted common signals. A basic differential amplifier can be built around a single operational amplifier, but its performance and input impedance are limited.

The instrumentation amplifier is a refined form of differential amplifier, commonly built from three operational amplifiers, that provides very high input impedance on both inputs, a high common-mode rejection ratio and gain that can be set precisely with a single resistor. These properties make it well suited to amplifying the tiny outputs of sensors such as strain gauges, thermocouples and bridge circuits, and to biomedical measurements like electrocardiography. Instrumentation amplifiers are available as dedicated integrated circuits, simplifying the design of accurate measurement systems.

Frequently asked questions

What does a differential amplifier do?

It amplifies the difference between two input voltages while suppressing signals common to both inputs, which helps reject noise picked up equally on both wires.

What is common-mode rejection ratio?

It is a measure of how well an amplifier rejects signals common to both inputs relative to how much it amplifies the differential signal; a higher value indicates better noise rejection.

Why use an instrumentation amplifier instead of a single op-amp?

It offers very high input impedance, high common-mode rejection and easily adjustable gain, making it better suited to amplifying small sensor signals accurately.