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discrete opamp current sources?

Discussion in 'Electronic Design' started by martin griffith, May 12, 2007.

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  1. Found this discrete audio type opamp from 1977

    The designer used two sets of diodes for setting the bias for Q3, Q4

    I would normally just use 2 diodes common to Q3, Q4

    What's the advantage in using 2 sets of diodes?

  2. Eeyore

    Eeyore Guest

    It looks like an afterthought to me.

  3. John Larkin

    John Larkin Guest

    There's probably a latchup mode. If they shared and the output rails
    high, Q4 could saturate and steal all the bias from Q3, turning off
    Q6-Q7, and it's dead forever.

    Old original LM324s suffer from shared bias supply problems, but at
    least they recover.

  4. Robert Baer

    Robert Baer Guest

    You forgot Q5...
    Better to use matched transistor pairs in same package (for better
    thermal coupling), and use a resistor in place of that "extra" diode
    where the resistor matches (in value) that used for Q3, Q4 and Q5.
  5. Thanks, I'd hadn't thought about fault conditions, maybe I'll
    LTspiceit, and see if that happens, but I still dont trust (LT)spice
    for wierd stuff, but thats probably just my ignorance

  6. Hello Martin,

    If the opamp is driven near saturation (Q6 off), a lot of
    current will flow into the base of Q4 and thus lowering the
    voltage across the two diodes. This would be a large
    unwanted change in the current for the differential input stage.
    The base current of Q4 will also change in the normal
    operation with every change of the output voltage.
    This will modulate the bias current source of the input
    pair too. Think on additional distortion here.

    Over all two good reasons for seperate bias voltages.

    Best regards,
  7. John Larkin

    John Larkin Guest

    The three 27K resistors could be reduced to two.

    The input diodes prevent another latch mode, zenering an input b-e
    junction. It may have some other hazards if connected as a follower.

  8. Probably to avoid a latchup if the -IN voltage ever
    gets high enough to bottom Q3. With joined bases the
    Q4 cc-source would then suddenly change current.
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