Opamp,Howto set the DC offset as the signal dynamically changes in gain? - Page 1
Forum section: electronics / beginners

author: stian
author_rank: Contributor
timestamp: June 10, 2018, 11:11:14 pm
content: Hello people.I am not sure how to explain my problem but i'll try as best as i can. topic vice dont know how to do better either.Anywho i need some help solving my problem.I have a input signal in range, 0 - 5V max. This signal can be offset on the 0-12V railhowever i want to have a keep my "Virtual Offset point (V1 /2)" as gain changes.Im not at davecad level yet.. so i made up some notes on a A4 papersheet instead* On the Left is what the signal might be at any time.* In the middle you can see what happens when the gain reduces to half, 5 to 2,5* On the right you can see how i want the signal to be.Im not even sure if the formula is correct. but it works out when RF=R1.In short, how do i adjust V2 to its correct values as V1 changes..What is the best solution to this problem?Regards.Stian

author: Zero999
author_rank: Super Contributor
timestamp: June 10, 2018, 11:37:28 pm
content: In future please do some image processing when you scan/photograph drawings, i.e. contrast, brightness, to make it easier to read.I'm not sure what you're trying to achieve. An ordinary op-amp circuit, with the usual 0V node, connected to the offset, will result in the signal amplitude increasing, around the same centre point. For example a 2V peak to peak signal, on 6V, i.e. 7V high and 5V low, will result in a 4V peak-to-peak signal, i.e. 4V low and 8V high, when passed through a non-inverting amplifier with a gain of 2 and the usual 0V node connected to 6V.If the signal is symmetrical, has no DC component i.e average. 50% duty cycle and no gain is required, only a change in offset can be achieved using a potential divider and an AC coupling capacitor.

author: forrestc
author_rank: Unknown
timestamp: June 11, 2018, 12:42:15 am
content: I'm also not clear on what you are looking for.Here's a suggestion:Using Hero999cad, draw the following:A set of possible input signal waveforms, and for each of those inputs, show what you'd like the output to be. That way we can see the desired output for each class of input signal.I'm also confused by what you mean by as 'gain changes'. Is this just an incorrect term for the input signal level changes, or are you saying you're changing the gain of the amplifier? If you're changing the gain of the amplifier, then I'd recommend adding another set of waveform pairs labeled with the desired amplifier gain.

author: stian
author_rank: Contributor
timestamp: June 12, 2018, 02:19:54 am
content: Hello again.Yeah i realize that i didnt explain myself verry well.English isnt my native language so yeah.But i will try again.I want to work with DC "signal" which can be offset to a wanted level without AC coupling.in my drawing i use Vgnd as 1/2 Vcc. but this refrence point can be changed aswell.So if "Vgnd" is changed to lets say 5V instead of 6V.The output from GND(0v) and Vout should Change, but not the output between Vgnd and Vout.But lets stick with Vgnd to 1/2 Vcc for now.Hopefully this new drawing is more clear for what i want to do.And i would like to know what is the best way to go about doing something like this without ac coupling.!Since we have a inverting amp.. how about a inverted picture. Hero999 seems to like contrast :pMy problem here is *4. how do i make V2 auto adjust itself when V1 changes.I was thinking about some summer circuit or something. since i know what V2 must be given Vgnd/2 + V1/4.But im not sure.Edit. after some messing around onhttp://www.falstad.com/circuit/circuitjs.htmli came up with something like this.Still not sure this is the best way to go about doing it.Regards.Stian

author: Zero999
author_rank: Super Contributor
timestamp: June 12, 2018, 09:49:54 am
content: I'm still not 100% sure of what you want. It seems like you're after a differential amplifier. The output will be equal to the difference between the inputs, multiplied by the ratio of R2 to R1 and added to the reference voltage.https://www.eetimes.com/document.asp?doc_id=1272328

author: mikerj
author_rank: Super Contributor
timestamp: June 12, 2018, 10:18:08 am
content: Seems fairly clear to me; the OP has a signal of amplitude Va and a DC level of Vdc. He wants to be able to apply gain to the signal Va but without changing Vdc. Any DC gain applied will obviously change both.This is not a simple problem to solve without AC coupling. You would need to extract the original DC level by integrating (low pass filter). You could then subtract the DC level from your signal (giving an average of 0v), apply your variable gain, and then sum the original DC offset back in after the gain block. If maintaining the precise level is critical, you could have a second integrator on the output, and then have a loop that compares the input and output DC levels and adjusts a DC offset to match them.This implies the system will have a time constant, so it will be a trade-off between settling time and the frequency range of your input signal. This can only work if DC level shifts on the input are at a significantly lower frequency than the signal of interest.

author: b_force
author_rank: Super Contributor
timestamp: June 12, 2018, 11:01:52 am
content: I am also missing more essential information about the circuit.For example, what do you need as an output?Ar you always working with the same output signal? (say from 0 to 5V/3.3V or something)Or does that also has to change with this signal?