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OPA551 maximum output voltage

Other Parts Discussed in Thread: OPA551

I am using an OPA551 running on +/-15V supplies, connected as a unity gain non-inverting amp.  With a 1kHz signal, the output is clipping at about +/-14V with 100k load (as expected), but at only +/- 8.3V into a 33 ohm load (250mA peak).  When I apply a 10 ohm load, it clips at about +/- 6.5V (I would expect it to current limit with this load). 

Is this expected behavior?  Does the output stage run out of headroom at 15V?

The Vout datasheet specs are all at +/-30V supply, so I'm not sure of performance at +/-15V, other than the Isc vs. Vs graph in the datasheet seems to indicate it should not be entering current limit with a 33 ohm load until about 12V peak.

Thanks!

  • Hello Peter,

    I wouldn't expect the swing to the supply rail to be much different for an OPA551 operating with +/-30 V supplies, or +/-15 V supplies, when supplying the same output current level. Something on the order of  VO = (V+) - 3 V, or 27 V for the +/-30 V supply and 12 V for the +/15 V supply, at Iequal to 200 mA, as an example.

    If you are operating the OPA551 as a unity-gain buffer and attempting to swing +/-14 V at the output, then you would have to apply +/-14 V at the non-inverting input. The OPA551 has a common-mode input voltage range, VCMR of (V-)+2.5 V, to (V+)-2.5 V, or +/-12.5 V in the +/-15 V supply case.  The normal linear operation of the amplifier may be falling apart when the input voltage hits the ends of the VCMR range, or when the output is driven into the output rails (output transistors saturate). The amplifier may be less tolerant of a VICR violation at the very high output current levels (33 Ohms) compared to the low current level (100 k).

    You may want to try configuring the amplifier as an inverting amplifier and see if the output swing improves. If the non-inverting input is connected to ground the VCMR will be zero. Try not to drive the output into the output rails when making the measurement.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Thanks for the response.

    I'm pretty sure it's not an input CM issue.  Since it's connected as a follower, IN+ = IN- = VOUT, so they are at +/-8.3, which is 6.7V away from the rails, when the output clips into 33 ohms.

    So, something is not quite adding up.  Unless the output swing vs. current plot looks a lot different for +/-15V supplies...

    Pete

  • Hi Peter,

    I was making the assumption that you hadn't reduced the input swing from the +/- 14 V condition that you first described.

    The OPA551 data sheet doesn't indicate that the output swing to the rails should be more limited at lower supply voltages. In the Applications, Power Supply section it states, "Most behavior remains unchanged throughout the full operating voltage range. Parameters that vary significantly with operating voltage are shown in the Typical Performance Curves." I really don't expect the output swing to the rail to be significantly more limited by the supply voltage level.

    Which package did you select for your OPA551? What kind of assembly did you use for the circuit? Please provide your circuit schematic and information about the power supply. Scope images of the input and output signals under the different load conditions would be helpful too.

    Regards, Thomas

    PA - Linear Applications Engineering