Alternate voltage regulation for a linear power supply

Question

I'm building a linear power supply. The first stages are all working great. Coming out of the rectifier and filter caps, I have 28V DC. I also have a voltage reference circuit (LM317) that gives me a stable 14V.

Here's a schematic of what I have so far:

^{simulate this circuit – Schematic created using CircuitLab}

I now want to add the regulation circuit for the output transistors. Similar to a power supply I built before, I was just going to use an op amp to drive the output transistors (probably through a high current buffer) so the output voltage matches the reference voltage. I also had a transistor for current limiting, like this:

^{simulate this circuit}

However, that would require me powering the op amp off of 28V (under no load), and the most any of the op amps I have can handle is 22V supply. I'm figuring I can't run the op amp off of the 14V line since its output will likely need to be higher than 14V to drive the output transistor.

I could buy other op amps, but I'm curious if I can get by with what I have.

I know I could just make a step down circuit to power the op amp, but I'm thinking there has to be a more elegant solution. Is there a more elegant/better solution?

Answer 1

Reconsider a derived power supply for the opamp. If the Power Q is a darlington, the opamp needs only 5 to 10 mA of operating and output current. This can be done with 1 zener diode, 1 resistor, and 1 noise filter capacitor. 50 cents? You don't show a power supply decoupling capacitor for the opamp, but it already should be a part of the control schematic.