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# Novice Question

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#### codan

##### New Member
Could someone help me get a better understanding of the basics.
I have attached a circuit & is where i get confused.

It shows four ground symbols a signal input & output & a 15v source, then it has -15v & is what i don't understand when there are ground symbols as well.

I have drawn a 15v source & added the positive & negative signs to it.
Is this correct, so the -15v is just the ground side of the 15v supply?.

If this is correct, are then the four ground symbols just a circuit ground-- 0 trace & there interconnected together & not the 15v supply ground?

Basic, but i get confused with this being a learner?

#### Attachments

• LM111.gif
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#### kchriste

##### New Member
Forum Supporter
I have drawn a 15v source & added the positive & negative signs to it.
Is this correct, so the -15v is just the ground side of the 15v supply?.
No.
The ground symbols are all at zero volts, are connected together, and serve as a common between the input and output. The 15V line is +15 volts above ground. The -15V is minus 15 volts below ground. So between the 15V and the -15V lines you should have a total of 30V.

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#### codan

##### New Member
Thanks kchriste,

Ok, i understand, but how do i get the (-15v) 15v below ground to do it?
I am not sure how to go about it?

#### MikeMl

##### Well-Known Member
Just to add to what Kchriste said, think of the ground symbol from two standpoints; one, to simplify the schematic, it is an implied connection to all other ground symbols without explict lines drawn, and two, think of it as "0V".

btw: lots of older opamps (LM111 is a special type of opamp called a comparator) utilize split power supplies, which are really two supplies sharing a common tie point which is tied to the ground of the opamp circuit, even though the opamp IC does not have a ground pin itself. Newer CMOS (especially rail-to-rail) OpAmps typically only use a single supply.

#### MikeMl

##### Well-Known Member
Thanks kchriste,

Ok, i understand, but how do i get the (-15v) 15v below ground to do it?
I am not sure how to go about it?

Requires two independant 15V "floating" power supplies. Tie the minus of one to the Plus of the other, and call that connection 0V. Now you have three terminals: +15V, 0V, and -15V,

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#### codan

##### New Member
Thanks MikeMl,

I have not been considering two independant power supplies & this has confused the begeesus out of me, i get it now, thank you!

I have attached a picture of LTSpice to make sure i have it correct.

#### Attachments

• 21.gif
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#### MikeMl

##### Well-Known Member
By Jove, I think he's got it

#### codan

##### New Member
Thanks again MikeMl,

This has been a great help, basic, but when you don't know it's confusing, especially when you only have two brain cells & ones lost & the other one is looking for it hahaha!.

Many thanks.

#### killivolt

##### Well-Known Member
Thanks again MikeMl,

This has been a great help, basic, but when you don't know it's confusing, especially when you only have two brain cells & ones lost & the other one is looking for it hahaha!.

Many thanks.

Codan, Good luck with lost cell.

One more step down the long lonely road of enlightenment.

My 2 brain cells hate each other and stopped talking long ago.

kv

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