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Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I hope the original author of the below picture doesn't mind, but I'm using a snippet of his current regulator schematic to illustrate a question of mine.
I discovered recently that my unity gain current regulator feedback (just like this one) path has much too high a frequency response for its application (gas laser negative resistance). Some kind soul suggested a capacitor from the negative input of the last op-amp stage to its output, but I think that they had in mind an Inverting Integrator, which is a hell of a lot simpler.
Anyway, I've got this circuit on a PCB and I need to cut off its gain above, say, 3-5 kHz. I'm looking now at the below circuit for a Non-Inverting Integrator for this application, but it looks my my circuit has a very low non-inverting input impedance as a result of the previous summer stage.
Any ideas how to limit the feedback's frequency response without redoing the PCB?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need to add a resistor in series between the MOSFET source and the opamp's inverting input. Say 10k. Then you can put a capacitor between the opamp output and inverting input.
This will be an integrator as far as the feedback is concerned. The op-amp's contribution to loop gain will go as 1/(sRC). This method works and I've used it many times in commercial designs.
However it doesn't integrate the set point signal. What this means in practice is that noise on the signal will be fed straight through to the mosfet gate. So you may also want to add a matching low-pass filter to the non-inverting input. Then the output will be the integral of the difference between the two inputs.
I've seen circuits for non-inverting integrators, but that's not what you have there, and it's not what you want either. The regulator relies on negative feedback to work, so the integrator must invert.
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Non-Inverting Integrator for Slowing Regulator Feedback
