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| Current source feedback capacitor |
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| OM222O:
I've been having a few issues regarding oscillations on the output of op amps that are used for constant current sources. there doesn't seem to be any rhyme or reason behind it either, for the most part I've ordered the op amps and actually tested them in the exact same circuit to see if they are suitable or not, which is obviously less than ideal. there are two suggestions that are always provided: adding a resistor to the output of the op amp, and adding a capacitor between the output and the inverting input. http://tinyurl.com/yxjyktlf I have used 1k resistor (which is the max I can use in order to drive the darlington pair) and a 100nF capacitor which doesn't seem to do much. after adding both the oscillations were better but still far from good enough. I was wondering if a larger capacitor , for example 1uF would be a better choice as it passes more AC and provides a better feedback? I don't have any suitable capacitors to test right now (wrong value or wrong footprint) but I'm not sure if increasing the capacitor is a bad idea because op amps tend to have issues with driving capacitive loads. is there a way to calculate the values rather just trial and error that I'm missing out? if not, what are the general ball park values that I should test? |
| NiHaoMike:
Add a resistor between the shunt and inverting side of the opamp so that the opamp isn't trying to drive a heavily capacitive load. |
| OM222O:
--- Quote from: NiHaoMike on May 16, 2019, 05:11:28 pm ---Add a resistor between the shunt and inverting side of the opamp so that the opamp isn't trying to drive a heavily capacitive load. --- End quote --- wouldn't that defeat the purpose of the capacitor which is to provide quick feed back? Also I think this is way too many extra components to stabilize an op amp and feels like wrong design. |
| Benta:
Which opamps? Are they unity-gain stable? Decoupling (I know, but still...)? You provide far too little specific information. |
| Kalvin:
--- Quote from: OM222O on May 16, 2019, 05:17:17 pm --- --- Quote from: NiHaoMike on May 16, 2019, 05:11:28 pm ---Add a resistor between the shunt and inverting side of the opamp so that the opamp isn't trying to drive a heavily capacitive load. --- End quote --- wouldn't that defeat the purpose of the capacitor which is to provide quick feed back? Also I think this is way too many extra components to stabilize an op amp and feels like wrong design. --- End quote --- Nope. Replace the wire between the 1 ohm sense resistor and the 1 uF capacitor with a 10 kohm resistor (for example). Then you can also reduce the capacitor value from 1 uF down to 10 nF or less in order to speedup the control loop response. |
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