This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THS4131: Large feedback capacitance causes it to oscillate

Part Number: THS4131
Other Parts Discussed in Thread: OPA1632

In the process of using THS4131, we found that if the feedback capacitor is 4.7nF and a DC input is provided at this time, an oscillation of 280/320MHz will be generated at the output end.The oscillation frequency is related to the positive and negative input, and the oscillation amplitude is related to the input amplitude.

I tried disconnecting the load, but it didn't solve the problem, reducing the feedback capacitor to 470pF the oscillation still exists, but if it is reduced to 100pF, the oscillation disappears.

In my experience, adding feedback capacitance to a CFA can cause oscillations, but according to the data sheet this is not a CFA. It seems to me that adding a feedback capacitor to the VFA improves the phase margin, but here it causes oscillation, which is very confusing to me. Looking forward to your reply.

  • Hi Yue,

    a too large feedback capacitance can even make issues in very slow voltage feedback OPAmp circuits. I have seen this many times. With a too large feedback capacitance the phase margin can decrease and finally result in instability.

    Kai

  • Hi kai

    Thanks for you reply. But why is this, can you talk about it? I tried to do margin simulation with TINA, but the phase margin is always enough.

    4834.THS4131_margin - Copy.TSC

    Yue

  • Yes Yue, setting up the LG phase margin sim a little better, I also get about 23deg which should be fine. 

    I was wondering if maybe a later model update might have incorporated the open loop Zout better - that might be in the OPA1632 model (the same die, newer model) but I have not been able to get that to work yet.

    This sim file, 

    THS4131 feedback pole LG.TSC

    Also, I tried a number of fixes - not much helps, but here I got the nominal phase margin up to 36deg using a very heavy snubber - this loads the output with diff 50ohms at higher F using that 1nF in series, 

  • Thanks for your reply. 

    You saied: I was wondering if maybe a later model update might have incorporated the open loop Zout better - that might be in the OPA1632 model (the same die, newer model) but I have not been able to get that to work yet.

    Do you mean that the open loop Zout of the model affects the simulation results? Therefore, the simulation results do not match the experimental results.

  • Hi Yue,

    even when omitting the feedback capacitance at all, your circuit is not very stable because the feedback resistors are way too big. See table 9-1 of datasheet.

    Kai

  • Hi Kai

    Yes, I have noticed this table, I've always thought that the resistor values recommended here are to limit noise. I did some simulations and found that as the resistor value increases, the system is indeed more unstable and the circuit bandwidth becomes lower. Can you elaborate on why the value of the feedback resistor affects the circuit bandwidth? Thanks very much!

    However, according to the simulation results, the system is still stable, and the experimental results show that there is no oscillation if the feedback capacitor is removed.

    Yue

  • Hello Yue,

      Good observations above. The reason why larger feedback resistor values cause oscillations is due to their interaction with the input internal capacitance of the amplifier. This interaction creates a pole, and the larger the "R" value, the lower in frequency which lowers the overall phase margin at the loop-gain crossover. 

       Here is a blog that covers an intuitive approach to looking at this interaction: https://e2e.ti.com/blogs_/archives/b/thesignal/posts/why-op-amps-oscillate-an-intuitive-look-at-two-frequent-causes

    Thank you,
    Sima 

  • Sima

    Thank you very much for your explanation. I didn't think about the connection between input capacitance and feedback resistance before, this is very enlightening to me.

    Yue