The TL431 is a very inexpensive programmable shunt regulator that can be used as the main active element in a series regulator. Here I have used a Sziklai pair instead of a Darlington pair to get low dropout voltage. This circuit can supply 5 V with a loss of about 1 V at as high a current as the transistors allow.

The problem with this configuration is that it requires a PNP pass transistor. Most of the power transistors in my junk box are NPN. Unfortunately the TL431 cannot be used inverted. The 2.5 V reference is with respect to the "zener" anode. One way would be to use a current mirror to reverse the direction of the drive current, The problem with that is it requires a matched pair, so is generally implemented on a monolithic chip. These are not jellybean parts.

But I thought, there is a feedback loop involving the TL431 so we don't need a precise linear mirror. We only need to reverse the direction of the current, perhaps with an amplification factor due to the CTR of the optocoupler, and over a limited operation range. Which is why I've called it a current reflector. So I came up with this circuit.

There is another advantage with this circuit. It allows a single ground rail for positive and negative supplies. With the standard positive side regulator, you need split windings on the transformer and rectifiers; or separate SMPSes.

Another possibility is a Sziklai triple version of the first circuit so that I can use a NPN pass transistor. This avoids the linearity issues with the optocoupler.

I have implemented the first cicruit, which works very well. I don't know if I'll implement the second and third circuits. It may be simpler to just buy some PNP power transistors, rather than trying too hard to use up what's in my junk box. 🤪

PS: Please ignore that fact that I've used different load resistor values, 100, 50 and 10 Ω in the three circuits. They do work up to the limits imposed by the gain of the transistors.