Negative voltage

There is also the virtual earth method using an op-amp to derive a split-rail DC supply from a single battery.

 

Voltage is always a difference between two numbers called potentials. Depending on what you (arbitrarily) call zero, a higher potential will be a positive voltage with respect to zero (ground) and a lower potential will be negative.

Look at the battery example a couple of posts up. Let's assume 9V batteries, since that is a good voltage to run op amps at. You could call the bottom zero voltage, the middle nine Volts and the top eighteen volts. Or you can call the middle zero volts, the bottom -9 Volts and the top +9V.

The choice of the zero voltage is often related to the signal you are dealing with. Say it comes out of a microphone. You have two wires. You call one zero, and the other will vary from a negative to positive voltage when a sound is driving the mike. When you attach the zero of the mike to the zero of the op amp's power supply, the amplified signal can also vary from minus to plus. If you connected it to the lowest voltage, the negative half of the signal could not be reproduced by the amp.

Bob