BJT basics

Discussion in 'Homework Help' started by Manmeet Singh, May 21, 2008.

  1. Manmeet Singh

    Manmeet Singh Thread Starter Active Member

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    Im a bit confused about the BJT explanation provided by the site, Ive attached the part that is confusing me

    For the first diagram this is pretty bad but since 2 batteries are connected to that node lets say the one on the left is 5V and the right 12V then what would the voltage of the base be? Since both are connected which one sets the voltage of the base lead?

    For the second diagram in the reading it said that the base-collector junction is reversed biased therefore it would only be conducting leakage current but then it shows that the forward bias of the base-emitter is allowing a current to flow through this area. How does this make sense?

    Attached Files:

  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Bipolar transistors are current devices, the BE junction will forward drop .6 Volts, same as a diode and for similar reasons. This is why you always see resistors in the drawings, to limit the current.

    Beta is the parameter name given to define the DC gain a transistor has. It is defined by the Collector to Emitter Current created by the Base to Emitter Current. The Collector to Emitter Current is similar to a variable resistor. B=Ice/Ibe

    In both drawings the bias is identical. Care to point the specific paragraph in the book out?

    ***********************

    Found it.
    http://www.allaboutcircuits.com/vol_3/chpt_2/8.html

    You mismatched what was said to the wrong drawing, which has confused the issue quite a bit. What they are talking about is a transistor in the off state, and are trying to explain why it is off. The drawing is showing a transistor that is biased on, where the BE junction has been forward biased, and the collector conducts current.

    When the BE junction is reverse biased it will act like a diode, and start dropping voltage in exactly the same way a reverse biased diode will. Look below the discription for the correct drawing.
    Last edited: May 22, 2008
  3. Manmeet Singh

    Manmeet Singh Thread Starter Active Member

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    "It is customary to reverse bias the base-collector junction of a bipolar junction transistor"...."There is no current flow, except leakage current, in the collector circuit."....."Normally we forward bias the emitter-base junction, overcoming the 0.6 V potential barrier."

    This is before the diagrams Ive posted and then below those diagrams it states

    "1) forward bias the emitter-base junction, 2) reverse bias the base-collector junction. "

    well if this is the case then how would any current be getting through the base collector junction? is there another path the current is taking? the way im seeing it is that the emitter base junction is forward biased but the collector base junction is revsered biased so theres no way you would have any emitter collector current
    Last edited: May 22, 2008
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