# Minus Voltage & Ground

Sometimes i come across different notations in Op Amps. Sometimes they are fed by V and ground, sometimes +V and -V. I am confused. Does ground mean minus?

-
Is it just me, or the minus sign (-) is missing in the gain formula for the inverting OpAmp in the 2nd picture? The input and output sine waves seem to be drawn correctly. At least, in general. It would be nice if the ground potential was indicated on the input and output waveforms. –  Nick Alexeev Apr 3 '12 at 4:17
I would like to thank you for the "Williamson labs" website, I found it really useful and answered some of my questions regarding opamps. Good luck in finding an answer to your question. –  Abdella Aug 23 '12 at 23:03

Opamps need to see a voltage accross their power terminals within a certain range. Generally the other voltages need to be limited to this range, and may need to be a bit less than this range to work properly.

Otherwise, the opamp has no idea what you think "ground" is. That is just the net you decided to use as the 0 V reference for other nets. As long as the opamp sees the right relative potentials on its pins, it doesn't care, in fact can't know whether any of them are at, above, or below ground.

-
do you mean that if I use "+V -V" configuration i obtain more current and voltage comparing to "V Ground" configuration. If so, does that mean "+V -V" configuration is equivalent to "2V ground configuration"? –  cmd1024 Apr 2 '12 at 23:37
@cmd1024: What matters is the voltage difference between the positive and negative supply pins. For the opamp to function correctly, this difference must be within the range specified in the datasheet. If a opamp is specced to run on 10 V, for example, then it doesn't matter whether its power is 0 V and 10 V, -5 V and +5 V, or 100 V and 110 V. Of course its inputs are relative to that range too, and it's output can only drive to within that range. Ground is only the level we choose to call 0 V. Actual voltages are only meaningful relative to each other. –  Olin Lathrop Apr 3 '12 at 0:55
thanks, but one more question. pls have a look at this picture: img690.imageshack.us/img690/4619/samplegv.jpg which is 5V to ground config. I have this amplifier, and it works when i feed a speaker. So according to what you say the sound quality must be better if i use +V -V configuration since there will be no clipping of negative part?? And if i want to use +V -V configuration should i use +2,5V -2,5V to obtain an equal current? –  cmd1024 Apr 3 '12 at 7:01
+1 ... and it's about time the ground symbol was banned - unless there is a connection to mains earth or a metal stake in the ground. It causes too much confusion & misunderstanding otherwise. This guy says it well. Rant over. –  MikeJ-UK Apr 3 '12 at 9:19
@MikeJ: Your link is broken. It is unfortunate that what we use the name "ground" for the 0 V reference in circuit, but it makes some sense in the historical context. That is why I use a down-pointing filled triangle, and reserve the tranditional symbols for chassis and earth ground when they come up. Sometimes people still use metal chassis, and earth ground can be very important for radio systems. –  Olin Lathrop Apr 3 '12 at 12:02
show 1 more comment

Op-amps have the ability to drive negative voltages. This allows the output signal to go positive and negative (using -V and +V as the minimum and maximum voltages respectively) as opposed to positive to ground. Look in the datasheet of a Op-amp to see the minimum and maximum voltage ratings of that particular device. As long as you stay in those limits +V and -V can usually be any arbitrary value.

-
do you mean that if I use "+V -V" configuration i obtain more current and voltage comparing to "V Ground" configuration. If so, does that mean "+V -V" configuration is equivalent to "2V ground configuration"? –  cmd1024 Apr 2 '12 at 23:37

It's very difficult to precisely control output at the rails. With bipolar transistor outputs, for example, you can only really get to within a diode drop of either rail. Op-amps in particular are often used in applications where you want accurate output either down to and including 0V, or right through to some negative value as well (think audio or some sensor applications). In order to achieve this you need to have the power supply a little bigger than the range of voltage you want on the output or you run into distortion problems where the output SHOULD be x but you can only get to x+0.7V or something, where x is usually 0V or the positive supply voltage.

There is also a similar problem where your design might end up saturating the output of the op-amp. This means that your input has caused the op-amp's output to slam into one of the supply rails. This does not harm the op-amp in any way, but it usually takes the op-amp a lot longer to recover and this also leads to distortion. By giving the op-amp a power supply that is larger than the output swing you want you can help avoid this (and you can help prevent it in the first place with clamping or limiting circuits on the inputs).

Also if you think about it, if your circuit is designed such that you want your op-amp output to be able to swing both positive and negative, the op-amp needs a power source that can provide this.

There are such things as rail-to-rail op-amps which use various tricks in order to get (within millivolts) of the rails, but you usually pay for that with increased distortion.

-
do you mean that if I use "+V -V" configuration i obtain more current and voltage comparing to "V Ground" configuration. If so, does that mean "+V -V" configuration is equivalent to "2V ground configuration"? –  cmd1024 Apr 2 '12 at 23:37
No, you don't get more current; you get the ability to have your output go below 0V since it has a negative supply to draw from. With your configuration, if you applied +/- 1V at the input you would get a clipped output if you only had +V/gnd since the op-amp can't make the output negative. With a +/-12V supply (for example) you could get to +10V and -10V output. –  akohlsmith Apr 3 '12 at 0:15
thanks, but one more question. pls have a look at thia picture: img690.imageshack.us/img690/4619/samplegv.jpg I have this amplifier, and it works when i feed a speaker. So according to what you say the sound quality must be better if i use +V -V configuration since there will be no clipping of negative part?? And if i want to use +V -V configuration should i use +2,5V -2,5V to obtain an equal current? –  cmd1024 Apr 3 '12 at 6:47
It depends. What's the op-amp? What's the input signal range? What's the output signal range? Generally speaking, if the op-amp isn't being asked to deal with negative voltages and it can drive to the rails you won't need a bipolar supply. For your application: usually the output is connected to a speaker through a capacitor (AC coupling), which I can't see if you're doing. Also, most slow op-amps are plenty fast for driving speakers, and you're probably driving a small speaker so any performance degradation will be masked by the speaker's poor performance. :-) –  akohlsmith Apr 3 '12 at 14:16
mate thanks, i use lm324. input audio is about 300mv, gain is 100. i use "5V ground" to feed the amplifier. i have a problem with adding capacitors to output. what do you think i should use as a capacitor? should i connect it to the output of the amplifier and then to the ground? –  cmd1024 Apr 3 '12 at 15:58