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Opa228 distortion at some frequencies

Philip Davies
Prodigy 40 points
Other Parts Discussed in Thread: OPA228, OPA227, TINA-TI

Hi,

I would like some advice regarding the OPA228 op-amp.

I am using the opamp in a preamp project that I am currently working on. It is configured as a non-inverting with a gain of 4.4 by using a 5.1K resistor betwean pin 6 and pin 2 and a 1.5K resistor betwean pin 2 and ground. I also have a 220pf silver mica capacitor connected across the 5.1K resistor.

I have noticed that there is distortion on the output of the OPA228 at some frequencies. I have managed to improve the matter by swaping the 1.5K resistor with a 2.7K resistor but its still there and noticeable with some recordings. Its mostly noticeable at 3khz to roughly 6khz. Could the silver mica capacitor be turning into a short circuit at some frequencies?

Any suggestions would be welcome.

Best Regards,

Phil

  • 0
    TI__Guru**** 189399 points
    Hi, Philip,

    The apps engineers who support this device live over at the Precision Amplifier e2e. I'm going to move your post there.

    -d2
  • 0
    TI__Genius 12325 points
    Phil,
    I think this is most likely a stability issue. It's been my experience that loop stability problems can appear as distortion problems. First, I should point out that the OPA228 is a decompensated op amp which is stable for gains of 5 or greater. Even worse, at high frequencies where the loop gain is approaching unity, your capacitor is decreasing the gain to unity, and making the amplifier less stable. A quick simulation in Tina-TI showed that your circuit configuration has a phase margin of about 20 degrees as-is, and not including any loading on the output. Decreasing the feedback capacitor to 22pF improved the phase margin to 40 degrees. Another option would be to switch to the OPA227 which is stable for all gains.
  • 0 in reply to John Caldwell
    Prodigy 40 points

    Hi.

    Thank you Don and John for your help.

    I did try the circuit without the 220pF capacitor but that also didn't cure the problem which makes sense if the OPA228 is stable with gains of 5 or more where I have it set to 4.4. Since this is a hifi project would It be better if I were to upgrade to a LME49710 opamp instead of the OPA228 or OPA227?

    I forgot to mention in the previous post that i'm supplying the opamp with a dual rail 15v power supply.

    Many thanks again for your help.

    Best Regards,

    Phil