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
Best Answer
First, you do not identify what phototransistor you're using, so there is no way anyone can answer your question about collector identification. "flat side" almost certainly identifies the photosensistive face, not collector or emitter.
It doesn't help that you haven't provided a number for the ouput voltage, either. "Low" could mean 1 volt, or .5 volt, or .1 volt.
That said, you need to do the following: put your circuit (without the LED) in a completely light-tight enclosure and see what the output is. Most likely you're picking up stray IR.
If you're still having problems, go back to the data sheet for your phototransistor and look at the dark current spec. If you're using a bc847b, your DC gain could be as high as 450. Assuming "low" means 0.5 volts, that's a collector current of about .5 mA, and a dark current of 1 uA will provide enough base current to do that.