What is the difference between operational, differential, and instrumentation amplifiers?

RS Components lists three categories of similar amplifiers: Operational Amplifiers, Differential Amplifiers, and Instrumentation Amplifiers. I think that there should be a single section called Operational Amplifiers. Why do these three separate sections exist? What is the significance of this difference?

Answer

An operational amplifier is an integrated circuit, and somewhat of a building block. The inputs (V+ and V-) of an op amp are very high impedance, and very little current goes into these inputs because of that. The general formula for the performance of an op amp is Vout=A*(V+ - V-), and A is a very large number. When wired up in feedback modes, the op amp can have many different configurations (even though the "open loop" gain is a very big number.

A differential amplifier, by definition, also functions with the relationship Vout=A*(V+ - V-), but A can be a much smaller number than in an op amp operating in open loop, and the currents into the inputs are not necessarily zero. The input impedance can be relatively low, and the input impedance to each input does not need to be the same. A differential amplifier can be built out of one or more operational amplifiers and some resistors, or it can be made out of more basic parts, like transistors.

An instrumentation amplifier is a special kind of differential amplifier. In general, it is a differential amplifier, but the input impedances on the two inputs are very high (meaning very small input currents), and the same for each input. There is usually a way to change the gain with one resistor. Very often, the instrumentation amplifier has a three op amp configuration (or the equivalent), with two op amps serving as an input stage, and the output stage is a simple one op amp difference amplifier with a reference point that can be used to move the baseline around. Because the resistors on the difference amplifier are usually laser trimmed inside the integrated circuit, common mode rejection is very high. There are also two op amp instrumentation amplifier configurations, but you lose some of the benefits (like setting the gain with one resistor).