What are there special negative voltage supplies, why not just use the comon GND?

Question

Some VRegs have the option to put a miriad of different negative voltages. Why would a circuit need this? Cant you just connect all negative and GND terminals (even thoughs that originated from a different +V source before its component(ie, LED) to a common GND ?

Answer 1

You, in common with your second correspondent, are confused.

You are probably in the common and incorrect habit of thinking of the negative terminal of a supply as "ground".

Think of a dry cell, a "C" cell for example. One pole (terminal) of it is the positive one, the other is the negative one. Neither of them is "ground".

Your second correspondent says "However now the whole circuit "floats" at 6v above the ground level of the power supply."

No it doesn't - unless for some reason you connect one pole of it to ground.

A power supply, just like the dry cell, has a positive pole and a corresponding negative pole. Neither of them is "ground." Unless or until you connect one of them to ground.

Many circuits require voltage supplies of several different values with respect to the circuit "common" - which is often called, out of habit, "ground", whether it is ground or not.

This circuit common will be the negative pole of one or more supplies and will also be the positive pole of one or more negative supplies.

In other words, there will be one or more supply voltages which are positive with respect to the circuit common and one or more supply voltages which are negative with respect to the circuit common.

Consider two strings of eight dry cells each, each making up 12V. There is no connection to ground.

Connect the positive pole of one of the strings as the positive voltage supply. Its negative pole will be connected to the circuit common (the point which is so often called "ground.")

Now connect the negative pole of the other string as the circuits negative supply. Its positive pole goes to the circuit's common connection.

There's still no connection to ground. Yet people still unthinkingly, and probably not really understanding what they are saying, talk about "ground."

Have look at this http://www.davidbridgen.com/earth.htm

Answer 2

This is done in some cases: for example a 12v supply can be divided by a resistor network into two 6v levels, and the circuit "ground" connected to the lower 6v level. However now the whole circuit "floats" at 6v above the ground level of the power supply. If the circuit is (DC) isolated from other external componenets, this is not a problem. But unless everything else the circuit connects to also has a "floating ground" at 6v, you have a problem.

Edit: using resistors to do this is just an example. In practice, the current through the resistor divider would have to be several times the current draw of the circuit in order to prevent variations in voltage. In actual practice, the intermediate voltages are provided by zener diodes or voltage regulators.

Note: "Ground" is used in electrical engineering to refer to the "common potential" between circuits; it is not necessarily the same as earth ground. You will see the the ground symbol used in circuit diagrams even if there is no connection to earth ground.

Answer 3

Good and efficient amplifiers use negative voltage sources. That way AC signals can be dealt with around zero volts istead of having a DC offset. Some of the circuitry would be pulling significant amperage all the time, even with zero input signal, if they did not operate near ground. That would cause heating and non-linear behavior.
You can sometimes trick an amplifier by operating it with a single sided supply and the reference set by a resistor acting as a voltage divider. But then your circuit is subject to variations in the resistances.