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Keeping AC voltage constant across an impedance

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PeterTr

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Hello,

I am looking for circuit to keep the AC voltage across an impedance constant.
In the attached image the impedance is formed by R3 and C1. Since the impedance can get as low as the output resistance of the driving opamp a significant voltage drop may occur in R1. This is why the driving voltage has to be adjusted to maintain a constant voltage across R3/C1.
The ( not good ) approach I tried so far was to measure the voltage drop over the R3/C1 and feed it back to the driving opamp. This works as long as the capacitance is very small and only a small phase shift is introduced. For larger values of C1 the circuit starts to oscillate.

Is there a smart approach the solve this problem.

Thanks for any advice!

Peter

 

The general answer is, you need to use an amplifier with suitable frequency compensation rather than an operational amplifier with full open loop gain. The compensation should be easy for fixed pure RC loads.

Refer to stability calculations for amplifiers or general control systems.
 

I think you will do better by removing R1, and putting feed back around the first Op Amp to reduce its output impedance to some insignificant value (.1 ohms?). The amplifier will still deliver the same amount of current and can actually provide more voltage with out it.
Frank
 

As noted by the other posts, direct feed back from the op amp output to the negative input generates a low output impedance, which reduces the open loop impedance by the large open loop gain of an op amp. There can be a problem with stability for large capacitive loads though, so a small resistance may be required in series with the output. If you need more current than the op amp can provide, you can add an emitter follower buffer at the op amp output with the feedback taken from the follower emitter.
 

Thank you for all the competent advice to my question!

I will look into them in more detail !

Best regards
Peter
 

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