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# Op amp w/ varying DC offset

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#### apw5020

##### New Member
Hello everyone, this is my first post so please take it easy on me! I am desperate right now. I have been trying to figure this problem out for hours to no avail...someone please help!!

So here it is: I need to design a variable level shifting amplifier. The input into the inverting terminal is .2 Vpp and needs to be amplified to a maximum of -12 Vpp. The input signal ALSO has a varying DC offset between .5 and 2.5 V. I need to use a 20 kohm potentiometer (in combination with resistors) at the non inverting terminal to cancel out this DC offset. The extremes of the potentiometer must cancel out .5 and 2.5, respectively.

I know I need to use voltage division somehow, but I can't figure out the resistor values. I know the negative feedback needs a gain of -60. The resistor values I have right now to achieve this gain are 60 kohm and 1 kohm.

Can anyone help me figure this out tonight?

Thanks,
Aaron

PS. This is a repost...sorry I missed the homework specific forum.

Assume that one end of the pot is connected to a DC reference voltage through resistor R1,(you didn't mention the value of this DC voltage), and the other end of the pot is connected to GND through another resistor R2. The wiper on the pot connects to the noninverting op amp input. Call the voltage on the pot wiper Vos. Write the equation for Vout as a function of Vin and Vos. Solve the equation for Vos. Recognizing that Vout will equal zero when you have canceled the offset, plug the Vin extremes (0.5V and 2.5V) into the equation, one at a time. The two answers you get will be the voltages at the ends of the pot. Then all you have to do is solve for the values of R1 and R2.

EDIT: After reading your other thread, I see that you have already gotten to this point. You are basically stuck on Ohm's law.

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