Problems with Darlington output stage

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

  1. microelectronix

    Thread Starter Member

    Jan 21, 2008
    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
    Please provide a circuit diagram showing indicated measurements at relevant points to enable members to better respond to your questions.