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OPT101 - changing responsivity, dynamic response and a wideband frequency compensation circuit (essentially a differentiator to restore the frequency response)

Alex Baikov
Prodigy 210 points
Other Parts Discussed in Thread: OPT101, TLE2071A, OPA132, OPA141, OPA140, TLE2071

Since working with low intensity light I need to increase the responsivity. Rext= 50 MOhm gives me desired voltage responsivity but it will lead to a 3 dB rolloff at 340 Hz,  well less than  the  desired 5 kHz bandwidth. Could it possible to use the second stage to wider bandwidth, i.e. a differentiating  amplifier whose input  RC combination is chosen to match the RC product of the feedback impedance in the OPT101 amplifier stage. The second stage’s feedback RC combination will be smaller, which serves as frequency compensation. The second stage will acts to restore the frequency response of the circuit. Will it help? A plan to use the TLE2071A.

  • 0 in reply to Bruce Trump
    Prodigy 210 points

    Since working with low intensity light I need to increase the responsivity. Rext= 50 MOhm gives me desired voltage responsivity but it will lead to a 3 dB rolloff at 340 Hz,  well less than  the  desired 5 kHz bandwidth. Could it possible to use the second stage to wider bandwidth, i.e. a differentiating  amplifier whose input  RC combination is chosen to match the RC product of the feedback impedance in the OPT101 amplifier stage. Will it help? A plan to use the TLE2071A.

     

  • 0
    Guru 21975 points

    Alex,

    I believe you would suffer a significant noise increase at high frequency with this approach.

    You might achieve better overall performance by using a flat frequency response amplifier after the OPT101. First, increase the gain of the OPT101 as much as possible while maintaining satisfactory 5kHz frequency response. You could probably add 2Mohm to the feedback resistor, preserving as much noise advantage as possible of the transimpedance amplifier. Then add a flat gain of 15 or so, to achieve the desired overall gain. This approach would, of course, amplify the pedestal offset voltage in the OPT101, a possible disadvantage.

    It may be that a combination of the two methods would be the best compromise.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 210 points

    Bruce!

    Which OpAmp will be better to add a flat gain of 15 or so... I think about TLE2071, OPA132, OPA140 and OPA141. What would you recommend?

  • 0 in reply to Alex Baikov
    Guru 21975 points

    Alex,

    Any one of these amplifiers has more than sufficient bandwidth for the additional gain stage.

    I'll assume that you set up the OPT101 for 5kHz bandwidth in the maximum possible gain.  You probably don't want an additional pole to break near 5kHz, so assume the pole from the second amplifier is at 20kHz. And assume that the second stage is approximately G=15. Then the second op amp requires a gain-bandwidth of 20kHz x 15 = 300KHz or greater. All of the op amps you propose have a GBW much greater than 300kHz.

    You can base the choice of the second op amp on other factors such as package type, quiescent current, etc.

    Regards, Bruce.