To help understand LM317 , I made a
simplified equivalent circuit. that you can change at will , (enable) JAVA in browser.
View attachment 95645
I did not add bypass current booster but that can easily be done using PNP or Pch FET so that current sense biases the bypass switch.
The 16V input is an ideal source with a switch add +/-2V of ripple or sag. I chose 0.1Hz but it could be 100Hz just to show effect of dropout when Vin-out<2V or so.
I chose fixed values of hFE, so they are shown as ideal transistors not real.
RED parts dissipate heat.
Any questions for the forum?
Tony