Here is my simple circuit
I need a 2 stage transimpedance amp with
Gain: 6M times (1nA will result in 6mV output)
Rise time: 400nsec worst case
Frequency response: 100KHz to 500KHz

So I have calculated the resistor values arbitarily(R1,R2,R3) for producing a Gain of 6M.
I am having difficulty finding the capacitor values in order to maintain a 6M gain and also the rise time must be < 400 nsec.

I am a newbie and need help with the formulas in understanding this. I am ot sure if Iam missing out on some basic points.
I need to have the value of C2 without compromising the gain.

Please help me with this.
Thanks in advance

 

I see a slew rate proble. The OPA355 can do 360 volts per microsecond. In 400 nsec, it can only move the output by 1.1111 millivolts.