# Very basic op-amp question

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#### Stevecb

##### New Member
Hello
Can you tell me what my output would be with a 748 op amp with 2v in the inverting input and 3v on the non-inverting input and 15v for power in and neg 15v for the other power in.

I don't have a bit map to post.
Thanks for any help
Steve

#### MikeMl

##### Well-Known Member
The amp would be railed to the positive supply. Whenever the amplifier is operating "normally" (as a linear amplifier with negative feedback), the voltage at the two inputs are essentially the same (except for a few mV due to offset).

#### ericgibbs

##### Well-Known Member
Hello
Can you tell me what my output would be with a 748 op amp with 2v in the inverting input and 3v on the non-inverting input and 15v for power in and neg 15v for the other power in.

I don't have a bit map to post.
Thanks for any help
Steve

hi Stebe,
It depends upon the resistors connected to the 748, the Rinput and Rfeedback set the Gain
What are the values.?

#### Stevecb

##### New Member
thanks for the quick response.
There aren't any resistors in the circuit.
Probably not a very useable circuit.

Steve

#### ericgibbs

##### Well-Known Member
thanks for the quick response.
There aren't any resistors in the circuit.
Probably not a very useable circuit.

Steve

hi,
In that case, its as Mike says, hard positive voltage, about +13V I would guess.
What do you want it to do.?

#### crutschow

##### Well-Known Member
An op amp with no feedback acts as a comparator.

If the voltage at the + input is more positive than the - input, the output will be at its positive maximum. If the voltage at the + input is less positive than the - input, the output will be at its negative maximum.

#### Stevecb

##### New Member
If the voltage at the + input is more positive than the - input, the output will be at its positive maximum. If the voltage at the + input is less positive than the - input, the output will be at its negative maximum.

So if the voltage at the non-inv input is -3v and at the inv input is 2V the output would be negative maximum?

Steve

#### SneaKSz

##### Member
If the voltage at the + input is more positive than the - input, the output will be at its positive maximum. If the voltage at the + input is less positive than the - input, the output will be at its negative maximum.

So if the voltage at the non-inv input is -3v and at the inv input is 2V the output would be negative maximum?

Steve

If the opamp is supplied symmetric ( +VCC and -VCC ) , the output will be the " negatieve maximum " or the negative voltage rail.

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