Faulty schematic.

• Faulty schematic.
  Posted by bastouw on 28 Feb, 2014 16:30
• Hello, I am reading the The Art Of Electronics 2e Edition.
  
  The topic VOLTAGE REGULATORS AND POWER CIRCUITS have examples of bad cirquits.
  And 2 of them i have troubble reasoning whet the fault is.
  Can someone help me?  Here are the attached schematics:
  

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• #1 Reply
  Posted by grumpydoc on 28 Feb, 2014 16:40
• I'm wondering how the first one starts given that the op amp power supply is taken after the pass element.
  
  Not sure about the second for a moment....
  
  Ah, yes - once the current limit transistor has turned on it will feed lots of base current to the pass transistor, probably not what was intended.
  
  I think those are the faults - anyone agree (or disagree for that matter).
  
  
  
• #2 Reply
  Posted by Andy Watson on 28 Feb, 2014 17:14
• Agreed. The first pass transistor in the first circuit will require driving with a base voltage 0.6-0.7 above the supply rail to the op-amp. It's not going to happen, even with a rail-to-rail op-amp!