# selective photodiode transimpedance amplifier

i would like measure a tiny bit off reflected light from a modulated led in an ambient light environment. I'm afraid the large ambient light output wil saturate my amplifier with the high amplification i need for the modulated led output.

Is there such a thing as a selective photodiode amplifier ? So only the light at the led carrier frequency gets amplified ?

• A simple High-pass Filter can help here. – JimmyB Jun 21 '16 at 14:51
• You might also want an optical band-pass filter at your frequency (light color) of interest. – user2943160 Jun 21 '16 at 15:03

• I think it can't be directly AC coupled. First use an transimpedance amplifier like you have seen (no AC couling yet) and than AC couple ist output to another amplifier. It's important that the amplification ($V_{out}/I_{photo}$ ratio, i.e. feedback-resistor) of the transimpedance amplifier is not too large in order to avoid saturation by unwanted light. The more unwanted light you have the smaller the amplification in the transimpedance amplifier needs to be. That's why it is good to filter out as much unwanted light as possible. – Curd Jun 22 '16 at 9:20