15V Regulator Simple with Transistors

Not much to say.
The schematic explains it.
The darlington TIP120G is avialable at Mouser. It is TO-220.

Regulator Simple 15V TL431.jpg
 
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In these types of voltage regulation circuits it is traditional to include another resistor, not shown on your schematic. The new resistor is connected between the regulated output node (U4 emitter) and the reference voltage node (D1 cathode). The new resistor squirts some extra current into the Zener diode, which reduces the voltage fluctuations on VREF when the load current varies. So for example when the load current is a 1 kHz sinewave with peak current of 1 ampere and trough current of 0.5 amperes, VREF varies less and so the regulator's output voltage varies less.

The definition of OutputImpedance is (OutputVoltageChange)/(OutputCurrentChange), and this resistor reduces OutputVoltageChange. Thus we conclude that this new resistor reduces the regulator's output impedance. How? It reduces the numerator of the dV/dI ratio.

I recommend experimenting with resistor values which increase the cathode current of D1 by about 3x. 4.15mA from the emitter of U1, and 8.30 mA (another 2x more) from the new resistor.