**Part Number:**OPA847

**Other Parts Discussed in Thread:**OPA856

Hello,

We have a PCB circuit with the OPA847 amplifier in transimpedance configuration. We wish to work in a low gain regime with a resistance in the feedback loop of 220 Ohm. The circuit has been tested with feedback resistance values from 1kOhm to several kOhm without any problems (there were no capacitor in the feedback loop and the bandwidth was around 50 MHz). When we set a resistance lower than kOhm, we see oscillations with amplitudes in the hundreds of mV. With this resistance value of 220Ohm fixed (we need a low gain), we want to find the value of the capacitance to be shunted in the feedback loop. Taking into account the equations in the tutorial section, we found that the capacitance value should be less than a hundred nF. We've tested with a different values of capacitance between few pF to hundred of nF, but the problem persists. Our only constraint is to have a bandwidth of at least 20/30 MHz and a feedback resistor of 220Ohm. How can we determine the value of the capacitor?

We have a related question. Is it possible to buy an evaluation PCB designed for a transimpedance configuration with the OPA847 amp, as well as a place for a photodiode?

Thank you very much,