Thank you! Do you happen to have a link to this thread? I am having problem with auto-oscillation with many opamps. I am using OPA187 which is stable with.
Is this below the circuit that matches that measurement? if so, this has pre-regulation.
Is this below the circuit that matches that measurement? if so, this has pre-regulation.
Do you recall at one point a feedback resistor to the opamp was added for improved performance?
The version I created is slightly different from the original. I will try to restore it tonight to the original configuration and start from there with measurements.
The version I created is slightly different from the original. I will try to restore it tonight to the original configuration and start from there with measurements.
Modified the regualtor and this is the results I get.
It seems the noise is approaching the limit of my test setup. High frequency the regulator could be doing better. I will look into that tomorrow.
It seems the noise is approaching the limit of my test setup. High frequency the regulator could be doing better. I will look into that tomorrow.
Look at OLG curves. At 100 kHz, AD825 & AD817 have about 60 dB (for 30V supply). OPA187 has 15 dB.
you are correct. I have had many stability issue with opamps with larger OLG at higher frequency. Apparently, adding a 120-200uF cap at the output is mandatory for stability.
AD825 (JFET) has 45dB OLG at 100KHz (similar to OPA140 I ordered and will try today)
View attachment 1352463
While the AD817 sports a whopping 60dB but the phase rapidly decreases just right around unity gain so I can see it very prone to oscillation
View attachment 1352465
Is there any advantage of JFET input over BJT in this case?
AD825 (JFET) has 45dB OLG at 100KHz (similar to OPA140 I ordered and will try today)
View attachment 1352463
While the AD817 sports a whopping 60dB but the phase rapidly decreases just right around unity gain so I can see it very prone to oscillation
View attachment 1352465
Is there any advantage of JFET input over BJT in this case?
People deeply involved in design and testing of the Jung-Didden super regulator are present and they could clarify. 🙂
AFAIK, only phase margin counts for stability and JFET or BJT input is not important there. You could always add a small compensation capacitor. It would decrease HF performance but stability is more important.
In the super regulator thread, there are many examples of confirmed opamps that work stable. My favorite OPA828 is found to work stable as well.
Thanks. Do you happen to recall, if you have stability with any of these op-amp, without the 220uF capacitor at the output?
People deeply involved in design and testing of the Jung-Didden super regulator are present and they could clarify. 🙂
Thank you and sorry I missed your comment.
Very frustrating night, I cannot get stability with any opamp with decent OLG at high frequency only with OPA187. It must be something with my layout for sure. I thought I had studied carefully GND strategy, apparently not enough.
Below is the captured noise floor of the regulator and next to it the input. Given I have not the ability to change layout right at this point, I can work around using OPA187 and trade off some high frequency noise rejection.
I have started reading the Super regulator's thread, it is quite long, I believe I got to page 20 lol.
Thank you and sorry I missed your comment.
Very frustrating night, I cannot get stability with any opamp with decent OLG at high frequency only with OPA187. It must be something with my layout for sure. I thought I had studied carefully GND strategy, apparently not enough.
Below is the captured noise floor of the regulator and next to it the input. Given I have not the ability to change layout right at this point, I can work around using OPA187 and trade off some high frequency noise rejection.
I have started reading the Super regulator's thread, it is quite long, I believe I got to page 20 lol.
Like any composite opamp circuit the Jung regulator is very sensitive to the exact behavior of the slave amplifier (the current stealing circuit plus the output transistor). Even more so when when we basically short the output with a capacitor which hugely impacts gain and phase of the slave amp.
I think doing spice analysis is definitely required to get a feeling for how delicate the balance is for the different counteracting contributors of the slave amp's total gain, basically the pass transistor fighting against the output capacitor.
Starting with an master op amp that has aggressive higher-order compensation makes things worse, that's why eg the OPA828 is better suited than others, leaving more room for tolerable phase contribution from the slave section.
I think doing spice analysis is definitely required to get a feeling for how delicate the balance is for the different counteracting contributors of the slave amp's total gain, basically the pass transistor fighting against the output capacitor.
Starting with an master op amp that has aggressive higher-order compensation makes things worse, that's why eg the OPA828 is better suited than others, leaving more room for tolerable phase contribution from the slave section.
Are those opamps unity gain stable? JD regulator's error amplifier works at unity gain for AC signal.
"It oscillates, now what?"
You can test the stability of the regulator by running the impedance test but you will need an analyzer with much greater bandwidth than the AP. An AM radio will also let you know if there are problems. My first SR using the Old Colony boards would oscillate at 770kHz when using the AD797, no problem with the AD825.
When you graph noise, assuming you have the DSP analyzer, you should use the AP Macro "FFT Scaling for Noise". This will yield noise per root hertz.
If you are using Analog Analyzer, Sweep use the Bandpass Function ... download the data into Excel, divide by the square root of frequency and the bandpass filter factor -- this will also give you voltage noise per root Hertz. The two methods give similar results.
You can test the stability of the regulator by running the impedance test but you will need an analyzer with much greater bandwidth than the AP. An AM radio will also let you know if there are problems. My first SR using the Old Colony boards would oscillate at 770kHz when using the AD797, no problem with the AD825.
When you graph noise, assuming you have the DSP analyzer, you should use the AP Macro "FFT Scaling for Noise". This will yield noise per root hertz.
If you are using Analog Analyzer, Sweep use the Bandpass Function ... download the data into Excel, divide by the square root of frequency and the bandpass filter factor -- this will also give you voltage noise per root Hertz. The two methods give similar results.
Last edited:
thanks for the comment, the board I designed has STO23-5 footprint, so the SO-8 won't fit there, so I am limited to OP-AMPS with this land pattern.
Could you try to explain why OPA140, for example, should be more prone to oscillation that OPA828?
Also, if you have a recommendation of a more suited OP-AMP than OPA140, OPA187, OPA197 and a bunch of other you want me to try, please let me know and I can try.
My main suspect is that, something wrong with my layout, that's the only explanation since basically reports on this regulators are that it is stable with pretty much every opamp.
yes, these op-amps are all unity gai stable. The main difference I can see is that these op-amps such as OPA825 etc. have the first dominant pole at higher frequency over 100Hz, which lower the gain around DC. Given this is a HF issue, I am assuming it should be irrelevant, however, worth mentioning.
Yes, I have the 2522A so it has DSP. I will use the FFT Scaling Feature and replot this.
I can't see the scaling feature, how do I enable it?
maybe you are referring to a pre-built test. Let me look in the folder for this particular test.
Last edited:
Found the macro, but it is unfortunately for the AP2700 I am running AP2.24. If I want to run AP2700 I need to buy the USB interface. Given I have just bought this AP plus the Interface and also the omicron Line Injector, I would like to work around with AP2.24, at least for now, then maybe later on I can get the AP USB interface since I have the 2522, but I also would like to get SYS-2722 ahahaha,
OPA180 stable Rejection improved up to 20Khz below graph using DSP analyzer
View attachment 1352872
. Below analysis up to 100KHz with the analog analyzer I did not expect the OPA180 to rise given it has higher OLG at 100KHz. However, in the audio band it has higher rejection.
View attachment 1352872
. Below analysis up to 100KHz with the analog analyzer I did not expect the OPA180 to rise given it has higher OLG at 100KHz. However, in the audio band it has higher rejection.
Last edited: