I'm trying to detect a small AC signal (~10kHz) generated by a phototransistor on a magnitude of 10nA~1uA using the following circuit by converting the signal to voltage signal first and then filter it:
However, environmental light generates a constant offset of approximately 50uA, which is much greater than my signal. This means that if need to convert the current signal to voltage signal first before filtering, assuming I power the amplifier with a 5V voltage source, R needs to be smaller than 100K Ohm in order to prevent saturation of the amplifier. As a result, the voltage signal would be merely 1~100mV, which is too small for accurate detection due to noise issues. If the 50uA background could be filtered out, I can use a larger R to convert current signal to a larger voltage signal.
In theory, some form of high-pass filter should solve the problem, but I have no idea how to apply a high-pass filter that operates directly on the current signal.
Any hint would be much appreciated! thanks!
Note: phototransistor's collector should be fixed at 0V.