Voltage/Current Control & Impedance, BJT v. FET

Discussion in 'General Electronics Chat' started by ryancousins, May 31, 2014.

  1. ryancousins

    Thread Starter New Member

    Jul 15, 2012
    11
    0
    I am under the general understanding that a Bipolar Junction Transistor is a current controlled current source, while a Field-Effect Transistor is a voltage controlled current source. But I don't understand why. I have read in numerous places that the relatively lower input impedance of the BJT, and the higher impedance of the JFET and MOSFET are the reason this distinction is true, but I don't understand that. Both the base of a BJT, and the gate of a FET require some amount of voltage and current to operate.

    Anyone able to make shed some light on this?

    Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    In a steady state condition, the mosfet does not require current flow to or from the gate but the bjt does. Bjt current is proportional to the base-emitter current. Mosfet current is proportional to the gate-source voltage.

    When you change the voltage on a mosfet gate, you have to charge or discharge the capacitance of the gate, and that gets significant at high frequencies, but, once you arrive at the drain current you want, you don't have to keep providing current to the mosfet gate.
     
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