Thursday, 1 August 2013


It is the device which works on the principle of absorption of of light is phototransistor. Which is more sensitive than a pn photodiode. The phototransistor is an npn transistor usually connected in  CE configuaration with the base kept open. The radiation is allowed to fall near the junction.
If light radiation is now to fall near th collector junction additional minority carriers are photo generated. These carriers contribute to the reverse current in exactly the same manner, as do the thermally generated minority carriers.

         The transistor action amplifies th light radiation current . Thus the sensitivity of a phototransistor is much higher than that of photodiode.
          The amount of light intensity is increased more and more minority carriers are generated.which increases the total current.
   Photodiodes, photo transiators are also used in photodetectors, in octocouplers.

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