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.
Stare at this for a while. It is not an accident that this configuration subtracts out the offset automatically. Note that R2/R1 = R4/R3 = 60. Further note that R1 = R3 and R2 = R4.
Note that the supplies have to be more than +-12V.
I like that design, but I need to use a potentiometer somewhere at the non inverting terminal. When the 20 kohm pot is at its max, it needs to subtract .5 V DC offset, and when it is at its min, it needs to subtract 2.5 V DC offset. Any ideas?
I like that design, but I need to use a potentiometer somewhere at the non inverting terminal. When the 20 kohm pot is at its max, it needs to subtract .5 V DC offset, and when it is at its min, it needs to subtract 2.5 V DC offset. Any ideas?
Well, I was trying to get you to do some of the work. As the offset goes from 0.5 to 2.5V, what is happening at P1? Hint: It has something to do with R1 and R2.
If you configure a voltage divider consisting of fixed resistors and a pot which is capable of producing the identical voltages that appear at the non-inverting input (Node P1) in the circuit I posted, you will have the magic values that subract the offset to keep the output at the same level...
The voltages I came up with earlier were .4918 V and 2.459 V. I am having trouble with the voltage division and picking the two resistor values. I did:
Ok, one other question? What reference voltage are you starting from to create the two required voltages of 0.5*60/61 = 0.4918V and 2.5*60/61 = 2.459V ?
Do you know how to solve a system of two simultaneous equations in two unknowns?