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Is unity-gain stability in the op amp required for application as a transimpedance amplifier?

Eva Yang
Prodigy 160 points
Other Parts Discussed in Thread: OPA657

Hi TI Engineers,

      On the datasheet of OPA657, page 11, it said that  unity-gain stability in the op amp is not required for application as a transimpedance amplifier.But I was wandering why?

      If the I-V circuit can be seen as a UNITY GAIN voltage amplifier, the unity-gian stability must be required!

      OPA657 has a high gain bandwidth product 1.6GHz and very low noise performance, so can I use it in the following circuit if i got a high-ohm resistor with very low parasitic capacitor.

          

Thanks!

Eva

  • 0
    TI__Expert 5400 points

    Eva,

    It is true that decompensated amplifiers can be used for transimpedance applications.  In fact, they can offer some design advantages, such as lower noise and increased gain-bandwidth product.  This is the case because stability is determined only by the source capacitance and feedback capacitance.  A complete discussion of this concept can be found on pages 2-3 of the application note Transimpedance Considerations for High-Speed Amplifiers.

    In your circuit, 500Mohm is a very large transimpedance gain, and will dramatically limit the bandwidth of the amplifier, and necessitate an extremely small feedback capacitor.  If 500Mohm of transimpedance gain is indeed necessary, you may consider cascading multiple amplifier stages to increase the bandwidth.

  • 0 in reply to Bart Stiller
    Prodigy 160 points

         Thanks Bart!

          I will read that application note carefully.

          Actually ,we just need 4K bandwidth, but the 500Mohm is indeed necessary.Accroding to the equation(the following picture which is taken from Art Kay's paper) for selecting the feedback capacitangce that ensure stablity, if the fc is large enough ,we will get small feedback capacitor,even  though Cin is hundred pF or 10nF. That is why I choose 657. Of course, all above is on the condition that the 500Mohm resistor has a very small parasitic capacitor.