Gain of transimpedance op amp

Question

I am trying to calculate the gain-bandwidth product of a project where I am reading photodiode output current measurements. I aim to find an op-amp to buy to use as a transimpedance amplifier. The typical output current for the photodiode is 9 microamps (https://www.vishay.com/docs/81519/bpw21r.pdf), and this being the typical output current, I want this to correspond to 2.5V from a voltage swing of 0V to 5V. The bandwidth required in my application is 113 kHz.

I commonly see that the gain for transimpedance amplifiers are in the form of V/A (e.g. 400 kV/A). Am I right to use the 2.5V/0.9uA for the gain value in the gain-bandwidth product?

If not, how do I convert the gain value to be unitless in this situation?

Answer 1

how do I convert the gain value to be unitless in this situation?

You don't; it's a transimpedance amplifier and the circuit gain is defined in terms of ohms (V/I). A transimpedance amplifier takes a current as its input and the op-amp effectively operates in a gain of below 1 situation. This is because the signal source at the input is current and, in effect it is equivalent to a voltage source in series with a very high value of resistance. You have to think of TIAs a little bit differently than a conventional inverting voltage amplifier.

At the end of the day the feedback resistor equals the transimpedance gain at low frequencies and, above low frequencies, parasitic capacitances can radically alter the frequency response and noise density output but, for now just think in terms of the feedback resistance value defining the circuit gain (volts per amp).