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OPA695: OPA695, OPA2695 unity gain stability

Yasushi Fujiwara
Genius 11675 points
Part Number: OPA695
Other Parts Discussed in Thread: OPA2695, , BUF602, LMH6559

Hi Experts,

My customer would like to use OPA695 or OPA2695 by unity gain. Does these devices can be stable operation at unity gain? I saw datasheet, OPA695 does not show a frequency response of +1V/V condition and OPA2695 shows a frequency response of +1V/V condition but peak is about +4dB. Would you please give your comments whether these devices can be used the unity gain or not. If it can be used, would you please provide a reference circuit?

Best Regards,
Fujiwara

  • 0
    Guru 47415 points

    Morning Yasushi-san

    These very wideband current feedback devices can operate at a gain of +1 if need be. The overall loop gain needs to be adjusted to good phase margin by slightly increasing the feedback resistor. This runs into a secondary problem in that you add a loop pole with that R and the parasitic input C at the V- input. Probably not easy to get flatness due to that effect. They might consider the BUF602 as a unity gain alternate. 

    This may not show up in sim, here is the 2009 model gain of 1 with the 536ohm Rf a bit higher than the OPA2695 shows - I added a 25ohm series input to the V+ node that might be compensating peaking with the parasitic C there. 

    If I add an external 1pF to the inverting node (in case that is not in the model) here is the sim result. Yes, it doesn't take much to give a lot of peaking, 

  • 0
    TI__Mastermind 22250 points

    Hello Yasushi-san,

       As Michael said, this is a current feedback amplifier which does need feedback resistor to function properly, but can be used in a unity gain configuration as Michael has also shown. The amount of peaking at unity gain is characterized in the OPA695 datasheet within the spec table:

                         (click on picture)

                         (click on picture)

       If they are looking for a fixed gain of 1 amplifier, the BUF602 is a good choice. There is also the LMH6559 for a higher speed buffer option. Here is a wider list of products with relatively the same bandwidth as the OPA695 and is stable at a gain of 1.

    Thank you,

    Sima