emitter follower query

Discussion in 'General Electronics Chat' started by harry99932, May 17, 2011.

  1. harry99932

    Thread Starter Member

    Dec 30, 2010
    38
    2
    Hi guys im trying to get my head round transistors at a bit more useable and in depth level and im struggling with the emitter resistor in the emitter follower format. Am i right in thinking the emitter resistor is simply there to set quiescent current values (ignoring its effect on input and output resistance?) but does have an effect on current gain as your effectively losing a percentage of your gain to ground through the emitter resistor?

    thanks
    harry
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    An emitter-follower has no voltage gain, just a small voltage loss.
    The range of current gain of a transistor is listed in its datasheet.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You're basically correct. The emitter bias resistance will be in parallel with your load resistance, so its current is basically wasted. Keep in mind, though, that the emitter can only source current (NPN), so if you have an AC-coupled load, the emitter resistor has to sink the load current when it goes negative. My point is, you can't make the bias resistor arbitrarily large.
     
  4. harry99932

    Thread Starter Member

    Dec 30, 2010
    38
    2
    cheers guys just wanted make sure i had that right, so for a steady dc signal (i.e using purely as a dc current amplifier) i can dispose of re so long as i can keep the transistor in the right operating are via other means? (the load resistance in this case will be permanent to the circuit)

    cheers
    harry
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Yes, you can drive a DC load directly. Just observe max power, current, and voltage ratings for the transistor.
     
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