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Imagine dialing in a gain of 10 to boost a 2Β V2\text{ V} peak audio signal on your bench, but your oscilloscope displays a flat-topped, distorted wave. Even if your math says you should get 20Β V20\text{ V}, your physical operational amplifier cannot produce more voltage or current than its internal silicon and power rails allow.
How do you predict the exact ceiling where your signal clips, and how much current can your op-amp pump into a load before it gives up?

Output Voltage Swing

An operational amplifier powered by supply voltages VCCV_{CC} (positive rail) and VEEV_{EE} (negative rail) cannot swing all the way to those supply rails unless it is specifically designed as a rail-to-rail device.
Standard general-purpose op-amps, such as the classic ΞΌA741\mu\text{A}741 designed by Dave Fullagar in 1968, drop between 1.5Β V1.5\text{ V} and 2.0Β V2.0\text{ V} across their internal output-stage transistors, defining a saturation voltage (VsatV_{sat}) ceiling.
To find the peak-to-peak output swing, subtract the internal headroom drop (VdropV_{drop}) from each rail: Vout(pkβˆ’pk)=(VCCβˆ’Vdrop+)βˆ’(VEE+Vdropβˆ’)V_{out(pk-pk)} = (V_{CC} - V_{drop+}) - (V_{EE} + V_{drop-})
For example, with supplies of +15Β V+15\text{ V} and βˆ’15Β V-15\text{ V} and an internal drop of 1.5Β V1.5\text{ V}, the maximum unclipped peak-to-peak output swing is 13.5Β Vβˆ’(βˆ’13.5Β V)=27.0Β Vpβˆ’p13.5\text{ V} - (-13.5\text{ V}) = 27.0\text{ V}_{p-p}.
Rail-to-Rail Output (RRO) op-amps can swing within millivolts of the supply rails under light loads (Vout,maxβ‰ˆVS+βˆ’VdropV_{out,max} \approx V_S^+ - V_{drop}).
However, this headroom degrades as load current increases due to the voltage drop across the internal channel resistance (RDS(on)R_{DS(on)}) of output MOSFETs or collector resistance of output BJTs.
What happens when your output voltage is well within limits, but your output still distorts?

Load Current and Feedback Demand

The op-amp output pin does not just drive the external load resistor (RLR_L); it must also supply the current flowing through its own feedback resistor (RfR_f).