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IC Output stage question

  1. Nov 16, 2009 #1
    I'm having trouble understanding the output stage of the venerable 324 op-amp. The top part of the schematic is simple enough, but I don't understand the bottom half of the output stage - is it some kind of complimentary symmetry Darlington? Thanks for any advice.

  2. jcsd
  3. Nov 16, 2009 #2
    If the voltage drop across Rsc is >= about 0.6 volts, then Q1 will begin limiting the output source current.
    Bob S
  4. Nov 16, 2009 #3


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    output stage - is it some kind of complimentary symmetry Darlington?

    Yes it is. Q13 will only conduct if there is a path from its base to ground and Q12 provides this path.
    And Bob S has just explained the function of Q7
  5. Nov 16, 2009 #4
    The Q7 transistor current limiter is common in design of various power supplies. When the voltage drop across Rsc exceeds that of 0.6v, it will turn on Q7 that is rigged to a 50uA current sink. Once turned on, the current sink will suck in some of the current from the 100uA current source which facilitates in turning on Q5 and Q6. That way the current at Rsc will be limited.
  6. Nov 16, 2009 #5
    Thanks for your help, everyone!
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