# op amp rail voltages

#### jrevard

Joined Jan 6, 2014
3
hi, how can I create the rail voltages for an op amp? the (+) voltage seems simple enough but I'm not sure how to supply the (-) voltage... I am seeing that for working with a DC signal the (-) terminal can be connected to ground but what about for AC signals?

thx

#### shteii01

Joined Feb 19, 2010
4,644
Depends on AC signal.

If AC is centered on zero, then your AC signal is from -X to +X volts. So you need to provide a negative rail.

If AC is centered on some positive value and the range of AC is, for example, from +1 to +5, then you don't need negative rail, just connect it to ground.

#### #12

Joined Nov 30, 2010
18,223
I'm not sure how to supply the (-) voltage.
Build a power supply to create a negative voltage and connect that to the (-) supply terminal of the op-amp.

#### tubeguy

Joined Nov 3, 2012
1,157
+1
Using only a positive supply, if your AC signal is centered around 0 volts you can also deal with that by biasing the op amp's input at 1/2 the supply voltage using 2 resistors and coupling the input signal thru a capacitor.

#### shteii01

Joined Feb 19, 2010
4,644
If you don't need anything outrageous, you can use computer power supply, they usually have a couple negative voltages.

#### #12

Joined Nov 30, 2010
18,223
or a 9 volt battery if you're just experimenting for a while.

#### Brownout

Joined Jan 10, 2012
2,390
If you have two power supplies, you can connect them in series and use the wire between the two as 0V, or ground. They you'll have rails for + and -.

#### shteii01

Joined Feb 19, 2010
4,644
or a 9 volt battery if you're just experimenting for a while.
Someone posted a pic of such setup a couple of months? ago. I tried to find it, but no luck.

#### shteii01

Joined Feb 19, 2010
4,644
It was posted on these forums, but I found it with Google.

Connect -9 volts from the battery to the negative rail of the op amp. Connect +9 volts from the battery to the positive rail of the op amp. Connect the ground from the batteries to the circuit ground.

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#### WBahn

Joined Mar 31, 2012
26,398
hi, how can I create the rail voltages for an op amp? the (+) voltage seems simple enough but I'm not sure how to supply the (-) voltage... I am seeing that for working with a DC signal the (-) terminal can be connected to ground but what about for AC signals?

thx
The claim that you can connect the negative rail to ground if you are working with DC signals isn't a valid statement. As with AC signals, if the input and output signals are compatible with grounding the negative rail, then you can ground the negative rail. If they aren't compatible with a grounded negative rail, then you can't.