OPA855: Input bias resistor in high speed TIA design?

Hi Martin,

Rb is actually not need, you can connect the noninverting input directly to ground, that should also work.

Regards

Anant

Hello Anant,

in my case Rf is small and Idiode is high compared to other TIA applications.

Shure it is not advantageous?

Regards
Martin 

The diode current does not matter.

For BJT opamps, the input bias current that flows into/out of both inputs is the same, and will affect the voltage drop over Rf. Adding Rb will compensate for that.

The OPA855 has an input bias current of 12 µA. How much would the error over your Rf be?

The dark current is 10pA.

I would like to amplify diode currents I_diode>=1uA.

Hi Martin,

The input bias current for OPA855 is typically 12uA so this will create a DC off set of 4.8mV (12u*400ohm) at the output.

If this value of DC offset is a concern for you then you add Rb of 400ohm, otherwise it is not needed.

But please also note that Rb will add resistor noise also, though in this case since the Value of R is not very big so it won't add much noise, but if you want to design low noise system then this should also be considered.

Regards

Anant