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THS4513: Understanding Open loop gain with differential amplifier.

Part Number: THS4513


I am trying to test the Aol for THS4513. It is not behaving as expected.


Vsupply:  ± 2.5v

     +Vin:     30mV

      -Vin:    -30mV

no feedback resistors.

I get  ~± .7v on the ouput; open loop gain of ~24.

Is this right? Where is the 63dB Aol coming from in the datasheet?

Also I currently do not have access to the Texas Instruments website or any of the app-notes or datasheets because my computers security software is malfunctioning. So I will not be able to open any link that is sent. 



  • So the Aol curve needs to be generated with the I/O pins centered between the supplies - you are probably saturated - Most of what we look at is the LG back to the inverting inputs, but - if you take the circuits described in this article and probe at the output pins differential over AC you should be able to see the Aol curve - use the specified differential load
  • And also, this is a sim approach, sounds like you are trying to bench test - that is a whole different level of effort.
  • Hi Tony,

    I see you are working to test the open loop gain for the THS4513.  I built a simulation of my own and have been able to verify the Aol of 63 dB on the datasheet.  Are you doing this test in simulation, or on a board?  I am also a bit unsure of your decision not to use feedback resistors, unless you are just absolutely opening the loop.  If that is the case, the article Michael linked does a great job of explaining how to achieve desired behavior at both DC and AC, using capacitors and inductors.  Fully removing the loop in its entirety would prevent the AC component of the response from being accounted for.  There are also TI reference designs and articles for stability, should you regain access to our materials.

    Also, if you could provide a quick schematic for any resistors, wiring, etc you are using, I can more accurately replicate your test to further assist you.

    Please keep us informed of your situation, as well as if you no longer need our assistance.



  • Hi Tony,

    I thought I would link some additional learning resources, in addition to above comments.  TI has a training series on op-amps, under TI Precision Labs.  Extremely useful and pointed lectures which can act as a refresher or new exposure.  Two areas of relevance here are the series on Bandwidth, namely lecture two:; and the series on Stability, the most crucial lesson being lesson three: