Phototransistor common emitter amp

Discussion in 'The Projects Forum' started by tadgoff, Mar 16, 2011.

  1. tadgoff

    Thread Starter New Member

    Sep 27, 2010
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    I am working on a heart rate monitor project, and I need some help with the light sensing circuit. Actually I have two questions. Basically I want to use a simple common emitter amplifier like this one.[​IMG] But I'm not sure I am understanding it correctly.

    Question 1:
    Which direction does current flow? It would make sense to me that the current should flow from Vcc to Gnd via Rc and the phototransistor, but I thought current in a transistor goes in the direction opposite to the arrow in the symbol, right? Are the rules just different for phototransistors, or am I missing something?
    If the current does go from collector to emitter (Vcc to Gnd/same direction as arrow) I can see that Ic is the current through the resistor and therefore Vo=Vcc-(Ic*Rc). And Ic is directly proportional to the amount of light, so this is exactly what I want...

    Question 2:
    What if I want to expand the range of brightnesses this can detect? My professor warned that factors like skin color could change Ic by a factor of ten. He also mentioned a logarithmic version of the first amplifier where the resistor is replaced by a diode. I don't have much experience with diodes, and there's still the issue of current direction from before, so could someone please try to explain it to me? Do I just set Ic=Id and solve the diode equation for Vd? Is this circuit really all I need?[​IMG]

    Thanks in advance!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. wayneh

    Expert

    Sep 9, 2010
    12,115
    3,039
    You're suffering from an unfortunate fluke of history, Murphy's law I suppose. Current flow is conventionally thought of as being from high voltage to low, and thinking that way is generally OK. But electrons of course flow from highest negative potential (voltage) to lower negative potential, ie. from minus to plus in a circuit. So electrons flow through a transistor in the opposite direction of the schematic arrow.
     
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