Dual-supply op-amp LDO does not work, but single-supply does

Most if not all op-amps can be used in either configuration. Voltage is relative, and "ground" is just an arbitrary potential that you assign the value of 0 volts. There are op-amps "optimized" for one configuration or the other, but they can be used in either. This op-amp, for instance, is "optimized for single supply operation", and they emphasize features like:

• Input Voltage Range Extends to Ground
• Output Swings to Ground while Sinking Current

but "Specifications at ±15V are also provided."

Op-amps meant for single-supply often have rail-to-rail outputs, which will usually be featured on the front page of the datasheet. To find the "drop-out", look for a graph like "Maximum Output Voltage Swing vs Load Resistance" in this datasheet.

All op amps have two power pins. In most cases, they are labeled VCC+ and VCC-, but sometimes they are labeled VCC and GND. This is an attempt on the part of the data sheet author to categorize the part as a split-supply or single-supply part. However, it does not mean that the op amp has to be operated that way— it may or may not be able to operate from different voltage rails. Consult the data sheet for the op amp, especially the absolute maximum ratings and voltage-swing specifications, before operating at anything other than the recommended power-supply voltage(s).

A Single Supply Op Amp Circuit Collection

From a purely theoretical POV an opamp is nothing more than a (extremely large-gain) differential amplifier, i.e. an amplifier whose output is an amplified replica of the difference between its two inputs.

The problem with this view, usually found in introductory material/courses on opamps, is that it doesn't take into consideration how power is to be provided to the opamp. Input and output voltages are simply referred to ground, without showing how the component is supposed to know where the ground is (almost as if it were psychic)!

Of course you know that opamps have two power supply terminals, but neither one is labeled "ground", so how the opamp is supposed to know where ground level is? Well, it doesn't know! It simply assumes it is half-way between its two power supplies. That's why you power them up with a symmetric supply. If you didn't you wouldn't get 0V between output and ground when your inputs are shorted together (0 V differential input, and I'm neglecting common mode input to simplify things). The reason for this design choice is twofold:

• opamps are meant to be used also as DC coupled amplifiers, therefore to amplify a differential signal, which can have either polarity, the output must be able to go negative, and for this to be possible you need a negative supply also.

• For a single-supply DC coupled amplifier it is difficult to reach a 0V output level without special circuitry. Older opamps or opamps not meant to be "rail-to-rail" cannot force their output too near the rails, when the negative supply is grounded the output cannot reach 0V (usually the best you can have is 1V above ground, or there about).

Rail-to-rail opamps are designed with more sophisticated circuitry that can bring the output very near the rails (tens of millivolts, maybe less) so they can bring the output really near 0V when used with a single supply, especially if designed to be operated in single-supply mode.

As an aside, although theoretically it is possible to power an opamp with two different positive supplies (e.g. 5V and 25V), because the "difference is 20V", the problem is the other parts of the external circuit. If in such a setup you put a voltage of 0V across one input and ground, actually you are providing a -5V voltage relative to the "lower" 5V supply, which usually means frying something inside the opamp!