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OPA659: how to reduce the input capacitances via power supply bootstrapping?

Part Number: OPA659
Other Parts Discussed in Thread: TL081

Hello

I have studied some papers. I found some useful topics and would like to share it below. But I couldn't manage this technique with OPA659 unity gain! could you please help me?

the power supply is +-5V

I calculated the Ft of the unity gain Amp (I think Ft is the unity gain frequency, frequency where the open loop gain is equal to one) and depends on the situations it was approximately between 100MHz to 200Mhz.

so I choose Rb=7.5K and Cb=10pF. but due to using +-5V power supply I couldn't calculate Rv and Cv!

anyhow I need an effective way to reduce the input capacitances.

Regrds

  • Hi,

    Can you please try to elaborate why using +/-5V power supply was not possible to calculate Rv and Cv? Also, it would be helpful if you could attach a schematic of your simulation circuit that does not work.

    Looking at the bootstrapping technique mentioned in your post, it seems like it can improve the amplifier common-mode input capacitance from 5-10pF to a fraction of pF. The OPA659 common-mode input capacitance seems to be around 2.5pF, which should be low enough for receiving 100 to 200MHz input frequencies. So, I am not sure how much added benefit it would bring including an external bootstrapping circuit for these frequencies. Also, have you looked at the approach mentioned in this article for reducing the op-amp input capacitance: 2055.5706.boostrapping techniques to improve the bandwidth of transimpedance amp.pdf

    Best Regards,

    Rohit

  • OPA659 has a CM input capacitance of 2.5 pF. There is not enough information regarding the sensor's resistance and capacitance, the circuit setup, application, etc but is the 2.5 pF capacitance too much?

    TL081 referred to in the document is a very old part (1977) and only a 3 MHz device. Hence, input capacitance reduction techniques may be needed but a device like OPA659 has much lower input capacitance.

    Also, I could not find the document you are referring to here to get more background information on the circuit or some of the items referred to in the post (cannot find Rv resistor). However, the passive (first figure) and the active bootstrapping (fig 10.17) techniques referred to are complex and very susceptible to oscillations. Perhaps you may not even need to use the technique.

    Thanks,

    Keyur