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:

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

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%

  • \$\begingroup\$ 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. \$\endgroup\$ Dec 11, 2014 at 17:41
  • \$\begingroup\$ 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 \$\endgroup\$ Dec 11, 2014 at 22:38
  • \$\begingroup\$ 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.) \$\endgroup\$ Dec 12, 2014 at 4:18

1 Answer 1


The photo-transistor collector is connected via 330R to the local supply. Try shorting the 330R out and decouple the transistor's collector with 100nF to ground (bottom of R3) to reduce miller capacitance slowing the response of the transistor down.

  • \$\begingroup\$ It improved the rise time of phototransistor; however, there was no significant effects of propagation delay. Duty Cycle was about 33% at 8kHz, a bit far from a good result. Adjusting phototransistor gain has significant effect on this; but, I'm not sure what circuit respond if R2&R3 becomes less than 1k. Get stuck at this point. \$\endgroup\$ Dec 13, 2014 at 17:12
  • 1
    \$\begingroup\$ The question I answered was "What should I consider to increase bandwidth?". Propagation delay may not increase proportionally with bandwidth. There should be no problem lowering R2 and R3. You might also consider 100 ohms in the emitter of Q3 with maybe 100nF in parallel. Also try lowering R1 to 1k. \$\endgroup\$
    – Andy aka
    Dec 13, 2014 at 18:00

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