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100mA 100ns current pulse to 20nF capacitor

David Blake
Prodigy 90 points
Other Parts Discussed in Thread: OPA615

Hi

I would like to make a 100mA, 100ns current pulse (or somewhere in this range and a bit below, maybe 50mA 100ns is OK) to a small capacitor to test its ESR accurately.

I understand that I cannot use a function generator to do this and I need to use some kind of amplifier circuit. There are voltage controlled current source examples in a few datasheets

http://www.ti.com/lit/ds/symlink/ina128.pdf

http://www.ti.com/lit/ds/symlink/ina114.pdf

This style has also been presented elsewhere outside of TI.

However these are using amplifiers with slew rates of below 10V/us. There is no way they can make the 100ns current pulse I need?

Would I use similar designs to this and simply select faster opamps? However in simulation it seems that if the opamps are too fast then this also causes problems...

I am not familiar with selecting opamps and I can see that generating repeatable current pulses of this magnitude does not appear to be trivial

Any help appreciated

  • 0
    TI__Prodigy 320 points

    Hi David,

    You might want to look into the OPA615. This IC contains 2 transconductance amplifiers (voltage to current converters) and an integrated switch. The switch is very fast with a control delay time of approx. 2.5ns. See the OPA615 block diagram below:

    As you can see, the first transconductance amplifier is a comparator, essentially a differential pair input followed by a switch.  Note that this comparator output is a current source.  The comparator and the switch form a sampling comparator (SC) that is of interest here.  The operational transconductance amplifier (OTA) block is ignored here.

    The SOTA has a bandwidth of rougly 500MHz (see OPA615 datasheet) and can provide an output current of approx. +/-3mA. The switch has a control propagation delay of 2.5ns.To increase the output current you can use current mirrors to provide the desired current amplification.

    To avoid voltage compliance limitations from the current source due to the loading, make sure to pay attention in regard to the maximum collector-emitter voltage of the selected transistors. Also, you should make sure that you can adjust the bias of your current mirror independently from the +/-5V supply for the OPA615.

    Consider the following schematic: (I also attached the TINA schematic file)

    Using additional resistors in series to the emitters should help increase the sharpness of your pulses, you could use 2 Ohm resistors, for example.

    Since you want to source current, you also have to make sure that your final current mirror stage is a PNP stage. When using multiple stages, you need to alternate between NPN and PNP (see schematic, first stage is a NPN, second a PNP). The current mirror in the above schematic gives you a total amplification of 11*11=121.

    The limiting factor will probably be the speed of the transistors used in the current mirror(s). 100ns corresponds to 10MHz, so finding appropriate transistors will probably be a key factor.

    Hope I could help!

    If you have additional questions, please don't hesitate to ask!

    --Pavol

    opa615_current_source_schem.TSC