Application note error? - Page 1
Forum section: electronics / beginners

author: bloodyshell
author_rank: Newbie
timestamp: March 12, 2017, 11:15:31 pm
content: I'm working on a very simple project where I need to connect an IR phototransistor to a digital input pin on a microcontroller. While I was researching the best practices for this job, I found this application note from Sharp:https://www.scribd.com/document/51082097/Photodiode-Phototransistor-Application-CircuitThe configuration I chose for my application is the one on page 5, figure 11(A). It is working as expected on my first prototype but I was thinking that the base current of the amplifier transistor is not limited in any way and could fall well above the maximum rating for the transistor.I've tried to simulate the configuration using LTSpice. It does not provide a model for the phototransistor so I've replaced it with a regular NPN transistor and a constant current generator connected to the base. The result seems to confirm my theory and I'm wondering if it is possible that an application note from a reputable source could contain such a glaring error.Any thought and/or suggestion?

author: rstofer
author_rank: Super Contributor
timestamp: March 12, 2017, 11:35:28 pm
content: Measure the base current. I'm betting it is limited by Ip of the device.

author: bloodyshell
author_rank: Newbie
timestamp: March 12, 2017, 11:41:41 pm
content: What do you mean with "Ip of the device"?

author: danadak
author_rank: Super Contributor
timestamp: March 13, 2017, 12:47:18 am
content: In the ap note, its the photo current of the diode.Regards, Dana.

author: bloodyshell
author_rank: Newbie
timestamp: March 13, 2017, 07:40:40 am
content: Wait a moment. There is no Ip in Fig. 11(A) where it shows the amplification of the phototransistor signal. Ip is only used for the photodiodes schematics in the ap note and I'm using a phototransistor, not a photodiode.BTW, the phototransistor I'm using is this one:http://www.farnell.com/datasheets/1683594.pdfI don't really understand. The current on the emitter of the phototransistor should be roughly proportional to the IR light intensity, right? In my application the phototransistor sits a couple of millimeters from the IR LED on the other side and if I move the phototransistor away from the LED it keeps working until it's about 5 centimeters apart, so I guess the IR intensity in normal conditions should be very strong.The IR signal I'm processing consists of short bursts so measuring the current flowing into the transistor base is not trivial because I don't have any fancy lab tools: I guess I'll have to modify my prototype to accommodate a shunt resistor and measure the voltage drop using my crappy USB DSO.

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2017, 03:29:27 am
content: I don't really know what you're asking, but as I've done a few photo transistor and photo diode circuits I'll try!"I was thinking that the base current of the amplifier transistor is not limited in any way and could fall well above the maximum rating for the transistor."That phototran's bad data sheet suggests the max. collector current is around 3mA, that won't be enough to destroy the base of most transistors."if I move the phototransistor away from the LED it keeps working until it's about 5 centimeters apart, so I guess the IR intensity in normal conditions should be very strong."Yes, but the change in intensity with distance might not be as large as you think if the photo TX and RX have narrow 'beams'/'viewing angles'."The IR signal I'm processing consists of short bursts"Photo transistors tend to be quite slow compared with photo diodes especially if the biasing/load resistor isn't right for the light level, if yoursignalis very fast the photo transistor might only pick up theaverageincrease of light during the burst.If it's possible to safely have the IF emitter permanently on, you could measure the levels of the received light signal with DC measurements.

author: bloodyshell
author_rank: Newbie
timestamp: March 14, 2017, 07:47:16 am
content: Fair enough. My question is: "Don't you think the schematic in the ap note is inherently wrong?" and, while you are there, "What would be the best fix?".I guess It is all about finding the correct interpretation for the datasheet and understanding how phototransistors work. The datasheet defines a minimum and a typical on state collector current, 0.7 and 3 mA respectively. It does not state anywhere that the current will be limited in any way. What's the meaning of this value? Because the datasheet also states the absolute maximum power dissipation is 100mW, I thougth the on state collector current was a suggestion of how strongly the device should be driven in normal conditions without exceeding the maximum power dissipation which, at the stated Vce(sat) of 0.8V means 125mA.I've also tried with the more expensive but better documented VISHAY BPW96C (http://www.farnell.com/datasheets/2047104.pdf): the prototype works just the same (but I didn't expect this since the two devices are designed for different wavelengths).The speed of the bursts is not an issue for the speed of the phototransistor but it is a problem if you want to measure the collector current with a regular multimeterThe shortest signal is over 50µs: it should be ok since the datasheet reports a typical rise of 15µs (the vishay seems to be much better in this regard, with a rise time of only 2µs).I have very limited control on how the IR source works and there is definitely no way to keep it on long enough to take a measurement with a multimeter. I could use an IR LED to stimulate the phototransistor, but I'm not sure if the IR intensity would be the same.Going back to the original questions, I think the resistor connected between the emitter and ground should be connected between emitter and the base of the transistor: in this case I think the resistor value should be (Vcc-Vcesat-Vbe)/Icesat. Does it sound reasonable?