Voltage buffer in power supply
You are doing it wrong. If you want to add a power transistor to a linear regulator, it has to be a PNP transistor for a positive regulator, and its base goes to the input. Not the output.

The reason why this is different from a simple Zener circuit is the Zener "steals" the base current from the NPN power transistor in its circuit when the voltage on its terminals is too high. An integrated voltage regulator will not do that. Instead, it "steals" the current on its input when the voltage on the output is too low.

See the following schematic. Q1, R1, ZD1 are a simplistic linear regulator, using a Zener. Q2 is the external power transistor you added.







simulate this circuit &ndash; Schematic created using CircuitLab

R1 and R2 seem very similar in function at first, but in reality, their function is very different. While R1 is providing a maximum base current for Q1 that ZD1 could "steal away" if the output voltage (+0,7V BE voltage of Q1) is too high, effectively steer Q1 closed in that event, R2 is providing an "idle current" to the linear regulator circuit. It does nothing to Q2, as it's bypassing BE, not connecting BC.

The collector current of Q2 follows that of Q1 as the collector current through Q1 is the base current through Q2.