Constant current source using current feedback op amp

Hi everyone

I have a weird design challenge. I have to make high bandwidth constant current AC source. Usual constant current sources include voltage feedback op amps that are strait forward in regards to their design, but since the bandwidth has to be so large (1Mhz) i need to use an op amp with current feedback. I have been struggling to figure out the solution to this problem since the output current depends on output load (which is not good) in every circuit i came up with.

If you have solution to this problem in forms of schematics, or if you can offer any help i would be very grateful

Best regards

  • 0
    •  Analog Employees 
    on Jul 4, 2020 2:54 PM 6 months ago

    Hello HelterSkelter,

    Constant and AC sort of mean different things - are you trying to hold a steady current in response to a varying load, or are you trying to drive an AC current signal into a constant load (or one that changes slowly?)

    What is the range of currents you're trying to produce? (zero to 1mA?, zero to 1A? -10A to +10A?)

    Is the current unidirectional or bidirectional (source / sink)

    What is the range of load impedances?

    What is the compliance voltage range? That is, what range of voltages are expected at the load, either imposed by the load (like if you're charging a battery), or due to the current flowing into / out of the load impedance?

    Is the load grounded, or could you insert a current sense resistor in the ground return path? (which would greatly simplify and increase the options for current sense circuitry.)

    I know that's a lot of questions, but one circuit that came to mind is Application Note 47, Figure 142 (and associated text on page 63, "High Power Voltage Controlled Current Source". This circuit demonstrates quite a few concepts - controlling the load current by actively measuring it, accommodating a grounded load by using a current sense amplifier that can tolerate a large common mode voltage, and it's bipolar.

    Don't forget to read the "Frequency Compensation Without Tears" section.

    But if all you need to do is source a constant current into a load, then the LT3092 and family might be an option. There's an output impedance vs. frequency graph on page 7 of the LT3092 datasheet.

    -Mark

  • Hi Mark,

    First of all thank you for your answer and suggestion.

    I'm trying to build source that outputs constant current in response to varying load. Current demand is low, i would like to get 20mA RMS at the output (so current swing from around +30mA to -30mA). And from that the source has to be capable of both sinking and sourcing current. The major problem of this project is that the impedance ranges are quite large from 5K to 10R. From that impedance range we can deduce that the Op Amp voltage supply has to be around +- 100V RMS (I'm planning to use central tapped 1:1 transformer and power it from EU grid). I would like it to be grounded, but if it is not possible it is no problem to use CS resistor. 

    Your suggestion is quite nice, but the problem is that it can't tolerate the voltage swings that i need. I dont think that there are any Op amps that can do what i want, the closest one is LTC6090-5 (+-70V) that is way too under powered. My idea is bootstraping some wide frequency range op amp (maybe even LTC6090-5) ( https://m.eet.com/media/1152270/24127-45890.pdf ).

    Do you think that there are any better solutions than bootstraping for this high voltage problem?

    Thank you in advance

  • 0
    •  Analog Employees 
    on Jul 6, 2020 2:48 PM 6 months ago in reply to HelterSkelter

    Hi HelterSkelter,

    You have a challenging problem - First advice: BE CAREFUL! If you are starting from scratch, then the best option would be to buy an off-the-shelf high-voltage amplifier module of some sort. Analog devices does not make these directly. There are various application circuits, but you will need to thoroughly test with your circuit, and implementation can be a challenge, you will need to lay out a proper circuit board for any of them.

    But once you've got the high-voltage stage figured out, then using a grounded sense resistor will greatly simplify the rest of your circuit - it can operate from a low voltage auxiliary supply.

    So the best advice we can give in this forum is to find a SAFE, agency approved (UL, CE, etc.), voltage-output power stage, then add a current sense circuit so that the only circuitry you're actually building is low-voltage.

    -Mark