I've been asked to produce a photodiode transimpedance amplifier with a 10MHz bandwidth. Current range of **1 nA to 100 uA** (5 decades current) -> 100dB Dynamic Range.

Photodiode is a Silicon APD with junction capacitance **6pF**.

Logarithmic compression is not suitable since 1.) this is imaging, and 2.) full bandwidth required for all signal strengths.

I believe output referred noise requirement, Vrms is therefore < 1nA * Rf. I understand bias current should be in the 1-10pA range. Other parameters influencing noise/BW are GBW, Input Noise Voltage Density, Input Capacitance.

**The problem is, I have not found a single low-bias op-amp that seems capable. Is this performance even possible to achieve with commercial components?**

Calculation tool I'm using for bandwidth/total output noise (let me know if this is bad)

Full-scale output of 1 or 2 V is reasonable. For max input 100uA this implies Rf ~= 20Kohm. Therefore, the noise at output should be <= 1nA * 20K = 20uVrms.

For example, I've looked at the following op-amps: ADA4817, LTC-6268, OPA657 all have ~1000 uVrms order noise, but meet bandwidth requirement.

And these lower GBW products: AD8651, ADA4807, AD8655, OPA2301, OPA2365, just barely make the bandwidth, but still show output noise on the order of 100-200 uVrms.

Also, **could I possibly use an external input stage?** Like some sort of FET buffer to drive the input of a higher bias current op-amp? Discrete transistor circuits are not my forte.

Thanks for any help you can offer.

## Best Answer

Grin, Well your list includes all the good TIA opamps that I know of.

Are you allowed to change the gain resistor at the lower light levels?

If not then I would say maybe a bigger feedback R to get up to 10-20 V at the maximum light level. I would also direct you to Phil Hobbs. Here's his website.

http://electrooptical.net/ You can scroll down for his take on low noise PD front ends.

His book will also be useful for you.