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:

bc848b: http://www.nxp.com/documents/data_sheet/BC848_SER.pdf

bc858b: http://www.nxp.com/documents/data_sheet/BC856_BC857_BC858.pdf

phototransistor: http://www.megasan.com/service/pdfhandler.ashx?fileid=3565

^{simulate this circuit – Schematic created using CircuitLab}