Single-Supply Operation
Older textbooks often use +15 V and -15 V supplies. Modern equipment may have only 5 V, 3.3 V, or a battery. AC signals can still be handled by establishing a reference inside the supply range.
Mid-supply reference
On a 5 V system, 2.5 V can serve as the signal's temporary zero. The waveform then swings above and below 2.5 V while remaining inside the real supply rails.
AC coupling
A capacitor blocks a source's unwanted DC level, while a resistor biases the op-amp input to the reference voltage.
Common-mode range
Some older op-amps cannot sense inputs near either rail. LM358-family devices became popular partly because their input common-mode range includes the negative rail on a single supply.
Output swing
Input range and output range are separate. An amplifier may sense ground correctly yet still be unable to drive its output all the way to ground under the required load.
Rail-to-rail amplifiers
Modern CMOS devices may provide rail-to-rail input, output, or both. The guaranteed ranges still depend on load, temperature, and supply voltage.
Headroom
Allow room for DC bias, wanted signal, noise, transient peaks, common-mode restrictions, and output limits. A mathematically correct gain can still clip in a low-voltage circuit.