This article discusses rail-to-rail op amps, and explains why some inputs and outputs can be less than, nearly, or slightly more than the power rails.

For some, rail travel conjures up nostalgic images. They look at an old train engine and it represents a time and lifestyle from a bygone era. It suggests a simpler, slower time. That image is nothing like today's commute or even air travel. Most business trips today are done on the run. Sadly, waiting in airport security lines is about the only time that we are not racing around.

The flanged wheels on rail equipment guide the wheels on the rails. Steam, diesel, electric, even bullet trains run on rail-to-rail, flanged wheels every day. Around corners and through switches the flanges steer the heavy loads. Do the wheel flanges span rail to rail? Yes, but not quite. In order to turn, the wheels need a little clearance. The manufacturing tolerances of wheels, axles, tracks, and thermal expansion need to be understood to make a safe operating railroad. Maxim's operational amplifiers' (op amps) use of rail to rail has nothing to do with railroads.

Nonetheless, railroads make an excellent mental image to describe the phenomenon. Just as a train's rails place limits on the lateral movement of a train, op-amp power rails place limits on the voltage swing at the input and output. As in a railroad, for op amps "rail to rail" can mean "almost rail to rail." As we will see, the term can mean different things for inputs and outputs. It could also mean more than just rail to rail.

Early op amps had output drives that ran out of current as the signal approached the power-supply rails. When the signal was a few volts peak to peak and the power supplies were plus and minus 12V or 15V, there was no problem. However, as the power supplies reduce, rail-to-rail signal swings become very desirable. Circuit configurations have thus changed with the time. Both rail-to-rail and Beyond the Rails™ inputs and rail-to-rail outputs are common on modern op amps.