the output voltage swing
The output voltage swing is how close to the supply rails the op-amp's output can actually reach. Picture a car that cannot quite touch the garage walls — there is always a little gap left. An op-amp's output transistors drop some voltage themselves, so the output saturates a bit short of the positive and negative supplies and can never deliver the full rail-to-rail range.
On a classic op-amp the output can swing only to within about 1 to 2 V of each rail, because the output stage transistors need that headroom to stay in their active region. Example: running on plus and minus 15 V, the usable output might be only plus and minus 13 V. On a single 5 V supply, an old-style op-amp output might top out around 3.5 V and bottom out around 1.5 V — a painful loss when you only had 5 V to begin with. The swing also shrinks as you draw more output current, because the transistors drop more voltage under load.
Why this matters: output swing decides whether your signal fits inside the available voltage, and it is critical in low-voltage and single-supply designs where every volt counts — driving an ADC's full input range, or running from a battery. Rail-to-rail output op-amps were invented to close most of this gap. The honest reality: even a rail-to-rail part does not reach exactly the rail; it leaves tens to a few hundred millivolts, and that residue grows with load current.
On a single 3.3 V supply, a classic op-amp may only swing from 1.2 V to 2.1 V — useless for driving a 0 to 3.3 V ADC. A rail-to-rail-output part on the same supply might swing 0.05 V to 3.25 V, recovering almost the whole range.
Classic op-amps waste a volt or two at each rail; rail-to-rail parts recover most of it.
Output swing is specified at a stated load. A part that nearly reaches the rails into a 100 kΩ load may fall well short into 600 Ω — always read the swing spec together with its load condition.