I'm trying to sense IR wave with a phototransistor circuit. The circuit operates well under DC operations (I mean 3.3V through inputs). However, if a square wave is supplied from UART input, after 2kHz output voltage waveform become noisy DC about 200mV. What should I consider to increase bandwidth to 100kHz (maximum)? There is 1cm space between IR & phototransistor in vertical axis. LED draws 20mA current.
Here are the datasheets: