# help me please

Thread Starter

#### ksalatawi

Joined Nov 29, 2004
21
I have project about op-amp and i want you to help me please i but the question in attachment with the message

#### pebe

Joined Oct 11, 2004
626
Originally posted by ksalatawi@Nov 29 2004, 06:23 PM
I have project about op-amp and i want you to help me please i but the question in attachment with the message
[post=3850]Quoted post[/post]​
I may have missed something, but I think the circuit cannot work.

The amp needs a gain of 2 to amplify a 5v swing on the input to a 10v swing at the output. This gives a value of 10k for Rf. With a bias of 2.5v on the +input, the amp output would invert the input change and swing between -2.5v and +7.5v With the 2.5v offset provided by the battery at the output, Vo would then swing from -5v to +5v. So far, so good.

The trouble is the supply+ rail is only +6v and so the output cannot possibly swing to +7.5v.

It would work with a +-9v supply, though.

Thread Starter

#### ksalatawi

Joined Nov 29, 2004
21
thank you
but still the graph how ican draw it

#### pebe

Joined Oct 11, 2004
626
Graph for Vg/Vo

Let x be the input voltage and y the output voltage.

Scale the graph as x = 0 to 5, and y = -5 to +5

Draw a straight line from 0x/+5y to 5x/-5y

That is how the graph *should* look except for the supply rail problem.

With the limitation of the 6v positive supply rail, the slope cuts off at approximately +3.5y, so draw a line from 0x/+3.5y to 0.75x/+3.5y

Thread Starter

#### ksalatawi

Joined Nov 29, 2004
21
thank you very much

Thread Starter

#### ksalatawi

Joined Nov 29, 2004
21
what is the equation of the diagram

#### pebe

Joined Oct 11, 2004
626
Vo=5-2(Vg)

Thread Starter

#### ksalatawi

Joined Nov 29, 2004
21
thank you very much

#### dragan733

Joined Dec 12, 2004
152
Originally posted by ksalatawi@Nov 29 2004, 06:23 PM
I have project about op-amp and i want you to help me please i but the question in attachment with the message
[post=3850]Quoted post[/post]​
The schematic is Ok. On the output of the opamp is 7,5V as a result of the sum of signals Vg on the - , and the voltage 2,5V of the + of the opamp.
Then:
Vo= - (Rf/5)Vg + (1+Rf/5)2,5 - 2,5
Vo=(Rf/10)(5-2Vg)
For: Vg=5V, Vo=-5V, then: Rf=10K
or, for: Vg=0V, Vo=5V, then: Rf=10K,
then:
Vo=5-2Vg

#### dragan733

Joined Dec 12, 2004
152
Yes, the power supply is not good
Or to explain better the schematic.
The Output voltage is:
Vo=5-2Vg
The Output voltage of the op amp is:
Vop=7.5-2Vg
For the range of the voltage: Vg=(0-0.75V) the Output voltage of the op amp is alvays 6V, then Vo=4.5V

In the attachment one can see the results graphic