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OPA847: Some questions about the transimpedance Amplifier application

Mickey Zhang
Genius 12080 points
Part Number: OPA847
Other Parts Discussed in Thread: TINA-TI, TL052

Hi team,

I have some questions about the  transimpedance amplifiers. 

 How to calculate the CF value? Which formula can I use?

E.g. OPA847, I note the formula 1 of the datasheet which is on page 11 and 

the  formula 5 of the datasheet which is on page 12. 

I also note this document: 

Q1 So, Based on the stability of the op amp, I want to verify if the formula 1 of the OPA847 datasheet

is derived by the Figure 2 BodePlot Magnitude of the above document.

If this is true,  I can use  the formula 1 of the OPA847 datasheet to calculate the CF value,correct?

Q2. What is the formula 5 of the OPA847 datasheet which is on page 12? When can I use this

formula to calculate the CF value?  Could you explain the formula 5? How to determine

the Zo value and NG2 value to calculate the CF value?

Q3. For the  formula 1 and  formula 5, could you give me an example for each formula ? 

  • 0
    TI__Genius 15975 points
    Hi Mickey,

    To answer your questions:
    1. For transimpedance op-amp Cf calculation, the formula 1 of OPA847 datasheet should be correct, and should be similar to derivation of Figure 2 Bode plot magnitude in the attached app-note.
    2. The forumla 5 of the OPA847 datasheet is for low gain compensation design and not for transimpedance design.
    3. See answer 1 & 2.

    Best Regards,
    Rohit
  • 0
    Guru 140200 points
    Hi Mickey,

    I strongly suggest doing a phase stability analysis of your TIA with TINA-TI. Using these simple, partially idealized formulas is often not enough to get a stable design. In any case Zo of OPAmp, the detector capacitance, the phase lead capaitance and the feedback resistance must be taking into account when doing the phase stability analysis.

    Kai
  • 0 in reply to Rohit Bhat
    TI__Genius 12080 points
    Hi Rohit,

    Thanks for your reply. So, for each transimpedance op-amp , not only OPA847, I can use the formula 1 to calculate CF, correct?
  • 0 in reply to kai klaas69
    TI__Genius 12080 points
    Hi kai,

    Thanks for your help. Would you send me a TIAN circuit about this application?
  • 0 in reply to Mickey Zhang
    Guru 140200 points

    Hi Mickey,

    in the following you see the result of a simplified phase stability analysis for the TL052, as an example. Open loop output impedance is 250R, input capacitance is 12pF. A photo diode with a detector capacitance of 100pF was assumed:

    You see that the phase margin is always beyond 30°. So, the circuit is stable.

    No ringing in the transient response...

    ...and no peak in the frequency respsonse.

    mick1.TSC

    mick2.TSC

    Kai