Problems with Darlington output stage

Discussion in 'General Electronics Chat' started by microelectronix, Mar 15, 2010.

  1. microelectronix

    Thread Starter Member

    Jan 21, 2008
    20
    0
    Hi all,

    I'm working on am amplifier and having a problem I absolutely can't explain. The last stage is a push-pull of Darlington pairs driven by the previous stage, with a transistor regulator set around 2.5V (4 base-emitters plus bias current through resistors).

    I'm using 3904/6's and MJ21194/3's for the Darlingtons. The rails are +/-15V. The emitter resistors are .1Ohm/5W.

    Even though the bias for the pairs is 2.5V, the voltage across the output resistors is 1.2V (drawing 6 amps through the total .2Ohm resistance). The second (power) transistors have the expected .7V base-emitter, however the first transistors have a REVERSE BIASED base emitter.

    If I disconnect the first collectors (so its like just having a diode), it functions just fine.

    So wheres the current to drive the second transistors coming from? I assume its collector/emitter current, but if the base-emitter is reverse biased how is this possible?

    All parts are functional and not defective. I tried having a professor explain this, and she said it could be some kind of a current loop and I have to put more bias current into the first transistors, however I upped the bias voltage with no success.

    I do need to have darlington pairs, as the previous stage cannot provide enough current to drive the output with any hopes for quality gain.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Please provide a circuit diagram showing indicated measurements at relevant points to enable members to better respond to your questions.
     
Loading...