OPA656 vs OPA857

Hello Steven,

  For transimpedance applications decompensated amplifiers like the OPA657, OPA847 and OPA857 will have the best performance with regards to BW vs IQ, so I think we should stick to the decomp amplifiers. The OPA857 is our highest BW TIA and has many benefits including internal fixed resistors which are already compensated, but there are a few restrictions:

1, Transimpedance gain is fixed at 5K and 20K ohm.

2. Output only swings negative from the common-mode so you will have to ensure that the Photodiode is biased correctly to source current into the OPA857 non-inverting input.

3. The photodiode is compensated for 1.5pF diode capacitance so if your diode has much higher capacitance it will have to be recompensated appropriately.

The OPA857 does have an internal overload recovery circuit which makes it recover fairly quickly after an over-current spike. I have done some testing on the OPA857 and found that it will recover in < 5us to within 0.3mV with a 500uA input current. I am in the process of comparing this recovery to the OPA847 and OPA657 amplifiers.

With regards to noise, it comes down to how much gain you have. Please see attached application note which has some numbers which should help guide you.

http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=sboa122&docCategoryId=1&familyId=72&keyMatch=SBOA122&tisearch=Search-EN

Samir

Hi Samir,

I am working with you on my senior design project  in case you didn't know. I have some more questions, the OPA657 (decompensated)  seems superior to the OPA656 (compensated) in GBW and slew rate and noise, but for our project you wanted us to use the OPA656, is there a reason or should we go with the OPA657 instead. Also I've been looking at bootstrapping circuit shown below,  that could increase the bandwidth of the transimpedance amplifier because we would like to have a large gain around 1k and we might be using a pretty large photodiode capacitance around 150pF which would limit our bandwidth by alot, do you happen to have any advice about bootstrapping.

Hello Steven,

  Decompensated amplifiers are a little trickier to stabilize for transimpedance applications as they have a 2nd pole in their open loop response who's location is before the 0dB crossing. Do you know how much BW you will need? How much gain is required? With the 150pF capacitance if you want any decent BW you are going to have to look at amplifiers like the OPA846, OPA847 and LMH6629. The last 2 are our highest BW amplifiers.

 I dont really know much about bootstrapping. For High speed applications we tend to use Avalanche Photodiodes which usually have > 30V supplies on them in order to get low capacitance and better responsivity.

Thanks,
Samir

Hi Samir,

I don't know how much bandwidth we need yet, but I think we need a gain around 1000 or 60dB. Are the decompensated amplifiers much harder to use because I think I will need to used the OPA847 for more bandwidth. I was reading on stability of Amps and we just have to design the the Loop gain AB /= -1 or the feedback network beta so that there is a  phase margin greater than zero at the 0dB crossing, I read phase margin of 60 is best.