# Phototransistor bandwidth issue

I'm trying to sense IR wave with a phototransistor circuit. The circuit operates well under DC operations (I mean 3V or 0V through inputs). However, if a square wave is supplied from UART input, propagation delay increases after some frequency due to bandwidth of phototransistor(I suppose). What should I consider to increase bandwidth? There is 1.5cm space between IR & phototransistor in vertical axis. LED draws about 15mA current when it conducts.

For example;

When 8kHz, %50 DC, square wave input is applied from UART pin, the DC of output decreases to 27% with about 20us propagation delay.

I've tried increasing the distance between IR & phototransistor to decrease photocurrent gain. Is it a good approach, or should I vary resistance values to decrease gain? Lowering the value of R2&R3 gave me better result; but, what other effects should I take into account?

Here are the datasheets:

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

simulate this circuit – Schematic created using CircuitLab

edit: Problem has solved according to values of updated schematic. For square wave input with 3Vpp, 50%DC up to 9.6kHz has been tested. @4.8kHz -> DC: 45%, propagation delay: 10us @9.6kHz -> DC: 40%

• I've hardly ever used a photo-transistor, But for more speed I'd want more light and less resistance... (reduce R2/ R3 maybe) A bigger supply voltage may help also. Dec 11, 2014 at 17:41
• Supply is not an option. Reducing resistors was my primary solution, and it worked; however, DC is relatively low, %33. Contradiction to what you say, increasing the distance, meaning less light I think also improves propagation delay. I finally make R2&R3 1.1k. How much are they decreased, and what may be the effects? @GeorgeHerold Dec 11, 2014 at 22:38
• Re light level, OK I perhaps shouldn't have commented. I don't know photo-transistors... (the only time I looked photo-diodes were better.. more area, photons.) Dec 12, 2014 at 4:18