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This is a 5V 100mA linear regulator circuit made up of discrete transistors. Short circuit current is around 105mA.
The open circuit voltage is 5.13V and the quiescent current draw is 4mA at 10V input. The output voltage close to full load (at 10V input) is 5.02V. That's a load regulation of 2.14%. The line regulation at close to full load is 0.2% (10V - 20V input change). Bad? Yes. With MOSFETs, the regulation is pretty good.
Starting from the left, the first 2 PNP transistors, along with the resistors near them, make a constant current source. It is needed to maintain the stability of the output voltage, because a constant current through the CE path of the lower NPN transistor is needed for output voltage stability. Why, I don't know. I don't see this problem with MOSFETs (coming soon). The NPN transistor is needed to "steal" some base current from the pass transistor to maintain a constant output voltage. The 5k and 1k resistors are feedback resistors. During startup, the output voltage starts increasing. When the output voltage is high enough to produce 0.7V across the 1k resistor, the output stops increasing and 5V is maintained. Now it's time to talk about the upside down transistor and the 7ohm resistor. Together, they take care of current limiting. When the drop across the 7ohm resistor exceeds 0.7V, the transistor starts "stealing" base current from the pass transistor, dropping the output voltage and preventing an overcurrent.
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