Many times it is necessary to have a dual power supply when all you actually have is one. It is possible to synthesize a pseudo ground using techniques similar to normal power supplies, but with important differences. I'm going to assume a 36V ungrounded power supply for most of my models when it is necessary to have a number. The most basic and simplest technique is a simple voltage divider. Add capacitors as shown and the AC characteristics become very low impedance, though this circuit will have a definite frequency response. The DC stability is atrocious, but for many applications this doesn't matter as much as you might think. ................Figure 1 The capacitors also can be used as bypass caps, since they are fundimentally being used that way for the virtual ground. The 3db point for the RC network is approximately F = 1 / (6.28 R C) or 0.724 Hz Multiply the answer by 10 (as a rule of thumb) and this virtual ground will be good to 7Hz. Figure 2 shows how this ground might be used for an op amp inverting amplifier. Note that the input and output of the op amp circuit can be treated as if they were AC signals, since the power supply is presumed isolated (as a battery would be) the DC values will not interact with the outside world. ..............................Figure 2 Another old technique that works well within the limits of a zener, the current is limited by the resistor, exceed the current and the voltage folds back, yet the other polarity continues to function. ........Figure 3 You design this as you would any zener regulator, design information can be found in the AAC eBook in Vol. III, Chapter 3, Section 11. The fact this works so well might encourage you to make a leap to solid state regulators, to use something similar to these configurations. ............................Figure 4 ...........................Figure 5 This won't work. The reason is simple, if the plus side is loaded and the minus side is floating (no load) where does the excess current go? With the zeners the answer is into the second zener, but solid state regulators are not designed to absorb current, only to be a source. This point can be illustrated with a simple one transistor one zener regulator. .....Figure 6 Note that if the voltage on the emitter exceeds the base voltage the base emitter junction is back biased, and the transistor is turned off. What is needed is more than the series pass transistor, you need a transistor or device that can swing both ways, absorbing as well as sourcing current.