I need an opamp which is able to drive unlimited load capacitance with high output current

The load can be thousands of microfarads and I would like to drive 500+ mA into and out of it.

Is there an opamp that can do this?

Or is there a way to increase the output current of a opamp like the AD817 without compromizing it's unlimited cap drive ability?

  • Hi GerdF,

    I think that the ability of an op-amp to control capacitive load depends only on correction method, but not on additional power amplifiers. You can use any additional output stage which can provide required output current. If circuit will be unstable you can add external correction components.

    I did a little experiment, look at the result

    Currents through emitter resistors

    There is a peak current at the beginning of the transition process. I think that this is due to the fact that the capacitor is ideal. I added 10 milliohm resistor in series with capacitor to model the impedance of a real capacitor.

    Currents through emitter resistors:

    You can try it in nature and report about results.

    I hope that this help you.

    Regards,

    Kirill

  • Hi Kirill,

    thanks for your answer.

    I am currently not on my computer with LTspice, but I suspect this circuit would oscillate with a very low frequency if allowed to run for a longer time.

    I think making the sine source a pulse one with a pulse duration of a few milliseconds and a period of say 10 seconds and running the transient simulation for a few seconds would show instabilities.

    Regards,

    Gerd

  • But with small capacity frequency response is bad

    1 uF

    100uF

    I have to admit that I have never dealt with such applications. But it seems to me a very interesting and entertaining problem. Could you tell me more about the required output waveform and the characteristics of the load?

    You said that load have capacitance in thousands uF range and I will simulate circuit at similar conditions. As far as I understand you need an op-amp which not required external compensation for capacitance driving and I would like to design simple circuit which not required compensation and which has simple output stage which can provide high current. If you clarify the requirements, maybe I can help you with something.

    Regards,

    Kirill

  • Hi Kirill,

    First, thank you very much for your efforts.

    Indeed it is a very interesting behavior. I think at very small load capacitances it would be very stable, because no relevant phase shift would be added.

    At very high capacitance loads it seems the output resistance and the load capacitance build the dominant pole and thus provide the loop compensation.

    Only when the load capacitance is in medium ranges, the internal compensation pole and the external rout-cload pole are too close to each other and introduce excessive phase shift while the loop gain is still above 1. Due to the isolation by the output buffer the automatic increase of the internal copensation effect with increased capacitive load capacitance does not work then.

    I think the circuit's behavior gets even more interresting if a certain DC output current flows, putting one of the transistors on by default.

    In the application, there capactitve load can be variable, because the user can switch in either small, medium or very large capacitors. The DC current, however, should be negligible.

    The amplified or buffered signal is mostly DC voltage but the DC value changes from time to time and then the transition should be quite fast, hence the demand for higher output current than the AD817 can provide by itself.

    I wonder if an output current boost like the one in figure 85 of AN87 would work with the AD817.

    If the resistors in the supply line are kept low enough to only activate the transistors when the required current exceeds like half of the AD817's capability, there would be no extra quiescent current and maybe no stability issues once the signal has settled again after a jump of the DC value.