A question about biasing phototransistors (the kind with 3 leads).
ElectricAye
Posts: 4,561
I'm baffled by this. There are some phototransistors that have leads to their bases. These base leads can be used to bias the phototransistors, but this biasing doesn't work the way I would guess it should. For example, the webpage that provides the diagram below, says this: "High values of base resistor Rb prevent low levels of light from raising the current levels in the collector emitter circuit and in this way ensuring a more reliable digital output. All other aspects of the circuit function remain the same."
To me (armed with my pathetically limited understanding of transistors), it should be the other way around. I would think that a high value for Rb would make the phototransistor more sensitive to light, not less sensitive to light. If Rb were infinite (an open circuit), I would think low levels of light would make it work. If Rb = 0, I would think that no amount of light would ever get the phototransistor to work since all the current at the base would be shorted to ground. What's wrong with my notion of this?
http://www.radio-electronics.com/info/data/semicond/phototransistor/photo-transistor-circuits-symbols.php
To me (armed with my pathetically limited understanding of transistors), it should be the other way around. I would think that a high value for Rb would make the phototransistor more sensitive to light, not less sensitive to light. If Rb were infinite (an open circuit), I would think low levels of light would make it work. If Rb = 0, I would think that no amount of light would ever get the phototransistor to work since all the current at the base would be shorted to ground. What's wrong with my notion of this?
http://www.radio-electronics.com/info/data/semicond/phototransistor/photo-transistor-circuits-symbols.php
Comments
I think you have it correct.
That writeup is a misprint.
Duane J
+1
I've built numerous ptx circuits where the base is left disconnected, light on the junction does all the biasing. Essentially infinite resistance, as you noted. Adding any resistance would desensitize the ptx IMO.
Low values prevent it from working at all, so the 'high value' is intended to alert designers "to approach this problem from above".
Thanks, Duane. That's a relief! I was worried about this all night.
And +10 to you (gain factor here) for helping relieve my anxieties about this.
Ah, that explains why I've read this sort of thing in more than one place. Thanks, jmg.
Poor choice of words in the description.
The photo-current in photo transistors is usually in the pA to low uA range.
So, the resisters are relatively high values of resistance, often in the 100KO to MO range.
Higher values increase sensitivity as they bled of less photo-current although high switching speed is slowed.
Lower values reduce sensitivity but tend to increase switching speed.
Duane J
That's perfect. That's exactly what I was hoping for. Thanks again!