negative supplies

[Elektor]

i have seen at various places negative supply is used.i.e -Vee is used alone.I am not talking about +Vcc and -Vee combinedly used as dual supply.Is there any specific reason to do so?I am specifically talking about Telephone Exchanges.they uses power supply of -48 Volts?
why :?:

 

[Someone Electro]

This is mostly used by Op-amp ICs.Also half H bridge motor drivers need an dual suply.

Its to make our lifes easyer and simpler.

i dont know whats with the telephone thing.

 

[Russlk]

Voltage for the telephone was supplied by batteries in the beginning, they could have grounded the pos or neg but chose the pos arbitarily. Electrolysis of the ground rod could have been a factor, but I doubt it. In the early automobiles, some used pos ground and some used neg but now all use neg ground as far is I know.

 

[lord loh.]

Its to make our lifes easyer and simpler.

It is to give equal swing in the output voltage of the opamp in both directions.