Hello,

I have tried all day to make sense of this... calculation after calculation and I really don't get it.

I need some clear and concise help !!!

Please refer to the image attached.

I would like to be able to build and calculate (for my personal learning) an op amp that takes a 3 to 4VDC

ramping voltage and linearly outputs a 5 to 6.5 Vdc ramping signal.

If the inverting input has a gain of 2 (ra=10K and Rf = 10K) and we calculate the inverting inputs branches gain like this:

GnonInv = (6.5 - 5)/(4-3) = 1.5

so: GnonInv = ResG (1+rf/ra)

1.5 = ResG(2)

ResG = 0.75

So I choose:

r1= 2500 ohms

r2 = 7500 ohms

When the input is at 3VDC we get at R1,R2 node 2.25VDC

Therefore 2.25VDC is reflected on the inverting input and since the gain is 2, then the output becomes 4.5V

And when the input is at 4 volts we get at R1,R2 node 3.0VDC and since the gain is 2, then the output becomes 6VDC

So I have a problem. As my output I want 5.0 VDC to 6.5VDC and not 4.5VDC to 6.0VDC

How do you guys solve a problem like this??

Thanks for any feedback... it will be much appreciated ....

Very discouraged!

24v

I have tried all day to make sense of this... calculation after calculation and I really don't get it.

I need some clear and concise help !!!

Please refer to the image attached.

I would like to be able to build and calculate (for my personal learning) an op amp that takes a 3 to 4VDC

ramping voltage and linearly outputs a 5 to 6.5 Vdc ramping signal.

If the inverting input has a gain of 2 (ra=10K and Rf = 10K) and we calculate the inverting inputs branches gain like this:

GnonInv = (6.5 - 5)/(4-3) = 1.5

so: GnonInv = ResG (1+rf/ra)

1.5 = ResG(2)

ResG = 0.75

So I choose:

r1= 2500 ohms

r2 = 7500 ohms

When the input is at 3VDC we get at R1,R2 node 2.25VDC

Therefore 2.25VDC is reflected on the inverting input and since the gain is 2, then the output becomes 4.5V

And when the input is at 4 volts we get at R1,R2 node 3.0VDC and since the gain is 2, then the output becomes 6VDC

So I have a problem. As my output I want 5.0 VDC to 6.5VDC and not 4.5VDC to 6.0VDC

How do you guys solve a problem like this??

Thanks for any feedback... it will be much appreciated ....

Very discouraged!

24v