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OPA656: Bigger output current OPAMP for TIA

kent gu
Intellectual 685 points
Part Number: OPA656
Other Parts Discussed in Thread: OPA354, BUF634A, OPA657, OPA698, OPA820

Hi,

Now we are using OPA656 for TIA, SOT23-5 package, output current is 50mA(min), 70mA(type), now I find the output current is not enough for our application. My question is that 

-70mA is pluse current, CW current?

-Do you have bigger output current FET AMP, which is similar OPA656? Such as bandwidth, package, rated voltage.

Best Regards,

Kent

  • 0
    TI__Genius 16735 points
    Hi Kent,

    70mA is the typ current the device can source or sink. Meaning if you a large load that needs to sink or source more than 70mA then this device will not work.

    What are your requirements? Do you need it to be unity gain stable? Do you want the same bandwidth? What voltage range do you need?
    There are devices with higher output current drive like the OPA354 that also as FET AMP input.
    Another alternative is to add the BUF634A to the end of your amplifier to get a boost in current.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Guru 48395 points

    Hello Kent, I wrote the OPA656 and 657 data sheets along with all the spec setting with design and test.

    70mA is a lot in a TIA, but in any case - I don't think there is a current limit in the device, so you could scale that output by increasing the supplies to +/-6V. this was the kind of plot we were doing back then in fig. 25 - you can see at lower output voltage required, it goes beyond 70mA - the output current is combined with voltage for a limit - we did not do a "claw" curve in those days, but that would be useful here.

    Usually, for TIA, we use a decomp part like the OPA657 to get lower input noise, let me know if you need a design - source capacitance, desired gain, Bandwidth.

    incidentally, here is a pretty good TIA design application note, 

  • 0 in reply to Michael Steffes
    Intellectual 685 points

    Hi Michael,

    Thank you a lot for your reply, your reply is very helpful. But I still be a little confused.

    In Fig. 25, base on what test circuit? Such as OPA656 is powered by +/-5V, RF, RL connect as below circuit?

    First circuit is our current application circuit with +/-5V, our PD output current is over 70mA, you mentioned that I can increase power supply, to improve output current? My understanding is that Vs = R * I，R should be Rf, I should be chip inside push-pull output current？I feel TIA OPA656 should be enough drive capability for second OP-AMP, our circuit limit is TIA OP656 is limit by output current.

  • 0 in reply to kent gu
    Guru 48395 points
    Oh well, if your PD puts out 70mA you really don't need a JFET device for the transimpedance, could use a bipolar with much more Iout. We normally use JFET inputs for >100kohm transimpedance needs. If you have something built and need a drop in, what package? Screening to >70mA output, you might look at the OPA820 or if you need output limiting the OPA698.



    you would still need to do compensation for stability, what is the PD capacitance? So, the other thing we run into with this is the PD current is unipolar giving a unipolar FDA output swing, do you need to balance that with a midscale bias on the other FDA input side.


    Also, I notice int he FDA stage, you show different Rf, mistake I am guessing, that should be balanced.