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OPA548 ringing right after cross-over but to negative voltage output only

Other Parts Discussed in Thread: OPA548, OPA549

Hello,

my problem is similiar to this:

https://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/t/167338

But in my case it already occurs at 10kHz (and very low output amplitude).

At 100kHz it's worse (so also GBW seems to play a role).

My latest flattened circuitry now is noninverting with unity gain at +/-12V symmetric supply with recommended blocking capacitors. The load is 2 ohm, the peak current therefore +/-50mA only. The 40dB 'safety margin' is fullfilled at 10kHz, unity gain as Thomas recommended in the other thread. To me it looks like a too late conducting element which results in a larger voltage difference amplified by Aol, too late oposite regulation thus creating overshoot/ringing. It's strange that the distortion always occurs in negative output only. Even at 100kHz the positive half looks well. I can't explain it by myself...

Regards

Andreas

  • Hi Andreas,

    Crossover distortion becomes more evident with a low-level vs a high-level output swing because at low levels the crossover region is a larger percentage of the total waveform. It occurs in a region near the output swing midpoint where both output transistors are be conducting minimum operating current. Crossover distortion does benefit from the application off negative feedback and the more loop gain available the more it will be reduced.

    The OPA548 uses an all NPN output stage, which is quite different than the more conventional complementary NPN/PNP output stage. The upper NPN is an emiiter follower, and the lower NPN is a common-emitter amplifier. Looking into the output pin a low-impedance is presented when the output is in the positive portion of the swing, and a high impedance is presented when the output is in the negative portion of the swing. The 2-ohm external load is driven by two very different sources and in those cases and electrical characteristics will be different depending on which transistor is more "on" or "off" than the other.

    The vertical spike in the crossover region occurs when the amplifier stage output slews between one transistor and the other. There is some overshoot on the ends which produces the ringing.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hello Thomas,


    thanks for your quick answer and great information of the NPN-NPN output topology. Unfortunately in the datasheet there is no detailed schematics of the 'inside' and so I didn't think of that possibility. Instead I acted on the assumtion that the OPA548 has a 'standard' complementary 'symmetrical' output (and therefore more 'symmetrical' behavior as many other OPA have). Now everything is clear and I know that I have to live with that.

    Does the more powerful OPA549 has the same NPN-NPN output stage and behavior? (seems so because of very similiar structure)

    Regards


    Andreas

  • Hello Andreas,

    In parallel to Thomas's suggestions and comments I wonder if you've implemented the snubber network recommended in the OPA548/OPA549 datasheet.  There was a customer we worked with a few years ago that had a similar (but not exactly the same) looking output waveforms and adding the output snubber  (10Ohms + 0.01uF) improved the output response.  The cross-over distortion remained but the apparent overshoot and ringing after the distortion glitch was improved.

    Let us know if you see similar improvements in your circuit with the addition of the snubber.

  • Hello Collin,

    thanks for your suggestion. Sorry that I forgot to tell that I also tried a snubber. My more strange version with 220n/4Ohm had no significant effect at that very low amplitudes. At 100kHz maybe one can see a marginal improvement in comparison with the screenshot above:

    At 10kHz the poor resolution of the scope screen revented seeing a noticeable effect.

    For the sake of completeness I just did the same at 100kHz with ~6Vpp (~3A peak @ 1 Ohm load). Now the improvement with snubber can be seen clearly: 

       

        

    Regards

    Andreas

  • Thanks for sending the additional information Andreas. As Thomas mentioned this effect is common with the output structure commonly used in power op amp products.