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high speed, high current modified howland

steven suddarth
Prodigy 60 points
Other Parts Discussed in Thread: OPA552, THS6184, THS6132, OPA561

Hi,

I need a fast VCCS that can put out 1.5 A. I was going to use the Cirrus PA09 but for external reasons

I can't use that package/part.  I am thinking about using parallel OPA522 because that part is almost

fast enough but most of the suggestions around this technique use the other amps as slaves off a

single master amp. I don't see why I can't just use ballast resistors on a collection of OPA522 which

are having the same input voltage. I feel like the slave method will slow it down by the propagation

delay of one amp. My desired BW is at least 10 MHz and my desired slew is 10V/uS.

thanks,

steve

  • 0
    Prodigy 60 points

    Oops I meant OPA 552, sorry.

    -steve

  • 0 in reply to steven suddarth
    TI__Expert 4950 points

    Hi Steven,

    You can certainly parallel amplifier together to achieve higher current.  The concept of load sharing is explained in the sboa127 application note:

    http://www.ti.com/lit/an/sboa127/sboa127.pdf

    I see that the PA09 can operate between ±12V to ±40V.  The OPA552 could certainly be used in such a circuit.  To achieve the 1.5A target, you would need 6 amplifiers in parallel but would cover the voltage range to ±30V.

    Other option at lower voltage would be to parallel the 4 amplifiers in the THS6184 package but the maximum supply voltage would be ±16V with a combined maximum current of 1.6A.

    A class G amplifier the THS6132 (2 package for 2A of current capability) can be used as well as long as the supply voltage are less than to ±16V.  Note that lower supply voltage would be required at this point for class G power saving operation.

    To increase output current in an improved Howland current pump, slave amplifiers are following the master amplifier.  I show below an article for this technique.

    http://www.eetimes.com/electronics-news/4168963/Get-increased-output-from-an-improved-Howland-current-pump

    The article above uses 2 OPA561.  As you mentionned the slave OPA561 will create a delay which may not be acceptable for your speed requirement.  Using a higher speed amplifier with a much shorter propagation delay may be an option.

    Depending on how much voltage is needed at the output of the amplifier to satisfy you requirement, the slave amplifier may not have to be operating on the same supply as the master amplifier, which would open up a lot more option for high speed and high output current amplifiers.

    What is your supply voltage requirement?  Considering that you are looking at 1.5A output current, have you dimensionned the RSEN (using the same nomenclature as the article) ?

     

  • 0 in reply to Xavier Ramus
    Prodigy 60 points

    Hi Xavier,

    Thanks for your advice. I have done a circuit in TINA that I would like you to look at. If you

    have time to look at it I could email it to you. I found that the model for the OPA552

    did not current limit when I called for more than 200 mA. I had a few other difficulties with the

    simulation software of a minor nature. One being that if I selected a big block for a copy/paste

    then thereafter I could not copy a component. The big block was stuck in the paste buffer

    and if I tried to copy a component I got a non-fatal memory error. But I could paste the big block

    repeatedly.

    I had previously looked at the ADSL parts as well and might try one out in the simulator.

    -steve

  • 0 in reply to steven suddarth
    TI__Expert 4950 points

    Hi Steve,

    Yes you can send me your schematic at x-ramus2@ti.com.

    Best regards,


    Xavier