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I read the Chris Basso APEC 2010 enter image description here I don't understand why the C1 can connect the Op amp output directly when using the TL431.

Because the TL431 output has a BJT.

enter image description here

Verbal Kint
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Jitter456
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  • Look very carefully at the + and - inputs of the error amplifier and which one connects to the $V_{ref}$. Note that the NPN inverts the output. So C1 connects between CATHODE and REF. – Bimpelrekkie Apr 07 '21 at 13:32
  • @Bimpelrekkie you mean the NPN is a common emitter so the output signal is inverted. so it looks like negative feedback? – Jitter456 Apr 07 '21 at 13:47
  • so it looks like negative feedback? Do the analysis yourself, bottom picture: if the voltage at the REF, goes up a tiny bit, then (since REF is + input) the output of the error amp will also go up, that makes the NPN conduct {more/less} which causes the voltage at CATHODE to go {up/down}... For circuits with feedback, you should master this skill, fortunately it is very easy to learn. – Bimpelrekkie Apr 07 '21 at 13:56
  • @Bimpelrekkie the V+ should be the same with V- , why the V7 can get higher than V-? – Jitter456 Apr 07 '21 at 16:46
  • What is V7? I do not see it. Have you read the datasheet of the TL431? Can you explain what it does and how it works? – Bimpelrekkie Apr 07 '21 at 17:29
  • @Bimpelrekkie Sorry, not V7. This is V+. I have read the datasheet. in the datasheet, if V+>2.5 the BJT will turn on if V+<2.5 the BJT like an open circuit. – Jitter456 Apr 07 '21 at 17:46
  • @Bimpelrekkie in the figure 23 this circuit, V+ is the same with the V- right? – Jitter456 Apr 07 '21 at 17:49

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