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How to eliminate the self-oscillation of OPA847?

Bob61009
Prodigy 60 points
Other Parts Discussed in Thread: OPA847, OPA846, OPA684

Dear All,

I made an amplification circuit with  OPA847 as the amplifier recently. The designed the working-frequency is 76MHZ. The designed Gain of my circuit is G=10.

When the input signal is strong, the circuit works perfect.

However, when there is no input signal,  a self-oscillation of 420MHZ exists. When the input signal is weak,  self-oscillation of 420MHZ and 38MHZ exists.

How can I eliminate the the self-oscillation?

Any help on this topic would be highly appreciated.

  • 0
    TI__Genius 17905 points

    Bob,

    The OPA847 is stable for gain >= 12V/V. Looking at the first Non-inverting Small-Signal Frequency Response figure in the top left of p.4 of the OPA847 datasheet, there is already about 4dB of peaking in the frequency response with a gain of 12V/V. Frequency response peaking translates to the possibility of ringing or oscillation in the time domain and to inadequate phase margin in terms of stability analysis.

    To use the op amp in lower gains requires compensation. What resistor values are you using in the feedback? Try adding a 1pF capacitor in parallel with your feedback resistor, Rf. You can think of this capacitor as shorting out the Rf at higher frequencies, in effect, reducing the gain (and, thus, peaking) at higher frequencies.

    The reason that there is no oscillation with larger input signal is illustrated by the third figure on p. 4 of the datasheet, Non-inverting Large-Signal Frequency response. You can see that the amplifier has a decreasing bandwidth, which will reduce the peaking, as the output voltage swing gets larger.

    Please let me know if that helps!

  • 0 in reply to Kris F
    TI__Genius 17905 points

    Bob,

    A couple of other part options to consider:

    OPA846 - stable with gain >= 7V/V, slightly higher noise than the OPA847

    OPA684 - current-feedback amplifier which should get similar distortion performance as the OPA847 and will not have an issue with stability at G=10. However, it has higher noise than the OPA846/OPA847.

  • 0 in reply to Kris F
    Prodigy 60 points

    Hi Kristoffer,

    Thank you so much for your valuable advices!

     

    Next, I plan to change the feedback resisitor from 500 ohm to be 1K ohm, so the gain would be 20. I hope the perfomance may be better.

    By the way, do you have some comment on this circuit?

    Again, thanks a lot!

     

     

  • 0 in reply to Kris F
    Prodigy 60 points

    Hi Kristoffer,

    Thank you so much for your good advices! My problem has solved!

     After I change  the gain from 10 to 20, the self-oscillation  disappears!  Now the circuit works in a very good condition!

  • 0 in reply to Bob61009
    TI__Genius 17905 points

    Good to hear, Bob.

    I just want to add to this thread:  if going to a higher gain was not an option here, the recommended way to increase the stability of the circuit for the OPA847 in G <= 12 configuration is to use noise gain compensation. This technique is described starting on p. 14 of the OPA847 datasheet in the section labeled "Non-Inverting Gain Flatness Compensation." It involves adding a resistor across the non-inverting and inverting terminals of the op amp to increase the noise gain of the amplifier to increase phase margin/stability, while maintaining the desired signal gain. The lead compensation technique I mentioned earlier with the feedback capacitor is not the best choice for non-inverting configurations using decompensated amplifiers such as the OPA847 as only a narrow range of capacitor values will stabilize the circuit. Other capacitor values, too high or too low, may even lead to worse oscillation.