What's the zener. Just a slightly modified diode with controlled reverse junction breakdown. The diode regulates the voltage while the current through it varies (in this case significantly). Well that variance of the current causes the current through the diode to constantly deviate from the correct operating point. So that's why instead of the passive resistor i placed a constant current source instead. That mixing of a constant voltage source and a constant current source proved to be quite stable. I mean 300uV output ripple from 4.8V input ripple is quite astonishing. You can see that the other zener (which constantly deviates from it's right operating point) doesn't keep the voltage well regulated. The ripple is 40mV. That's over a hundred times as larger ripples as the well regulated zener.
