In many standard applications, operational amplifiers are used to amplify periodical signals around a certain center value. As positive and negative phases of most analogous signals are typically at a same level (more or less symmetric to a center line), a reference voltage must be provided to the OP to allow the amplification of both, positive as well as negative phases.

This reference voltage is usually half of the entire supply voltage and is supplied to the non-inverting input. If this input is already used as a signal-input (non-inverting amplifiers), the reference voltage should be provided via high impedance resistors.

The easiest way to provide an operational amplifier with the necessary voltage arrangement is to use a split supply. Although the design of a symmetric voltage supply circuit is a bit more complicated than a single one, the amplifier circuits themselves will be reduced to a few components.

In single supplies you will have to create this third voltage source. There are several possibilities to obtain a middle reference voltage from one single supply. Most often, a simple resistance divider is used. The disadvantage of this circuit is that both resistances (Ra,Rb) reduce the global input impedance (for a non-inverting amplifier). A better solution is provided with a third resistance between the positive input and the divider. Another advantage is (but only if the condenser is used), that some other OP's may use the same reference point.

There is also a possibility to split up the supply voltage with an OP. As the output impedance of the operational amplifier is very low, in some applications this virtual center is used as a new (virtual) Ground.