Emitter ballasting on bjt

Discussion in 'Homework Help' started by Cerkit, Oct 21, 2009.

  1. Cerkit

    Thread Starter Senior Member

    Jan 4, 2009
    Hi. I am currently looking at emitter ballasting on bjt's when paralleling them in order to increase the current output. Can someone explain to me how the negative feedback in this instance works? It says that if the emitter current rises then Vbe is reduced and hence the emnitter current is reduced.
    Can someone please explain this further?
  2. JDT

    Well-Known Member

    Feb 12, 2009
    This is used to enforce current sharing when transistors are in parallel by having equal value emitter resistors on each transistor. This is because Vbe (the base - emitter) voltage is fairly consistant between transistors of the same type, the current gain (beta) is not.

    Therefore, if one transistor has more gain than the other, it will try to conduct more current than the other. But the voltage drop through its emitter resistor will be higher than other one so its Vbe will be less. Less Vbe - less base current, so its current will be reduced. Negative feedback!