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741 Op Amp Circuitry Question

Discussion in 'Electronic Components' started by [email protected], May 2, 2007.

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  1. Guest

    Take a look at the schematic of the 741 on Wikipedia:

    http://en.wikipedia.org/wiki/Operational_amplifier

    Here's my question. Let's say the rails are at +15 and -15 volts and
    the output of the differential amplifier (blue box in the figure) is
    +12 volts. That would mean that the base of Q15 would be at 12 volts,
    the base of Q19 would be at about 11.4 volts, and the emitter of Q19
    would be at about 10.8 volts. That would result in 25.8 volts across
    the 50 ohm resistor, requiring 516mA of current.

    However, using the assumed values at the rails, the Q12/Q13 current
    mirror would only be able to provide a maximum of 738uA, which is 700
    times less current than would be required by the emitter output of
    Q19.

    How does this circuit work? What am I missing?
     
  2. Bill S.

    Bill S. Guest

    Won't happen. The output of the diff amp is *current*, and the
    voltage at that node will be in the neighborhood of 1 Volt.
     
  3. Guest

    Ok, if you apply 1 Volt at that node, you'll have -0.2V at the emitter
    of Q19, which will try to draw 296mA (more than is available) through
    Q19, correct? That would be 14.8 volts across the 50 ohm resistor.
     
  4. Dick Ballard

    Dick Ballard Guest

    Q15/Q19 are Darlington connected to provide hi Z input load for Q4/Q6
    high impedance current output. This is where most of the single pole
    (with the 30pf cap) gain is achieved. Under op amp closed loop
    conditions, Q15+Q19 current exactly matches the Q12/Q13 mirror current
    to produce zero op amp output voltage. This would not turn Q22 on at
    all. The Q22/50ohm combo is there as a current limit for op amp output
    negative excursion short circuit current limiting. Note that positive
    excursion output current limiting is via Q17.

    Dick Ballard
     
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