OPA858: Bandwidth of transimpedance amplifier

Vegard A

Part Number: OPA858
Other Parts Discussed in Thread: OPA188,

Tool/software:

Hi. What is the right equation to use for calculating the bandwidth of a transimpeance amplifier? I have seen this equation in some application notes:

Where Cin is differential, common mode, junction of pd and pcb capasitances. Why is not Cf included here?

I have also seen people using 1/(2*pi*Rf*Cf), which is cut off frequency of RC-filter.

Is it right that the first equation is the bandwidth of the TIA itself, but the last equation is the BW of the total system?

3 months ago

0 Vikas Jeevannavar 3 months ago

TI__Intellectual 2985 points

Hi Vegard A,

You can refer to the article mentioned in OPA188: how to calculate the signal bandwidth that can pass in a TIA circuit - Amplifiers forum - Amplifiers - TI E2E support forums to accurately calculate and simulate the TIA bandwidth.

Article for reference: Op Amp Bandwidth for Transimpedance Amplifiers (1).pdf

Thanks
Vikas J

0 Vegard A 3 months ago in reply to Vikas Jeevannavar

Prodigy 10 points

I read to the article you provided, also the datasheet for OPA188. I saw that the closed-loop bandwitdh was determined by 1/(2*pi*rf*cf) because it forms a pole in the frequency response of the amplifier. However, the application note "What You Need to Know about Transimpedance Amplifiers – Part 1" (https://www.ti.com/document-viewer/lit/html/SSZTBC4) uses the closed-loop bandwidth defined by the first equation iIgave in my original question. I'm very confused on which equation is right.

When simulating a circuit with OPA858, I observe that Cj, Cdiff, Ccm and Cf all affect the frequency at the -3db point on the output. Using this, both equations should then be wrong?

I want to know which equation gives the best result compared to simulations and real world measurments. Do they both have some sort of role?

+1 Vikas Jeevannavar 3 months ago in reply to Vegard A

TI__Intellectual 2985 points

Hi Vegard A,

Sorry for the confusion here. The pole frequency (fp) formed by rf and cf is not the bandwidth of the TIA.cf is introduced to insert a pole to recover the phase loss due to fz and stabilize the loop. TIA bandwidth is given by the equation that you have mentioned in the original question (f_BW). When cin >> cf, some assumptions are done which may not be captured in all articles.

However, as mentioned,

are the equations for all the frequencies of interest in a TIA circuit.

All the articles demonstrate the calculations considering the Cj, Cdiff, Ccm and Cf. For practical calculations, I suggest you use Op Amp Bandwidth for Transimpedance Amplifiers (1).pdf to calculate the f_GBW. We also have an online interactive tool to find the best amplifier for TIA application in Analog circuit design studio.

For more info, you can refer to more descriptive article with derivations: Transimpedance Considerations for High-Speed Operational Amplifiers

Thanks
Vikas J