For a while I have been looking for a low cost reasonable performer Zener amplifier solution.
Conrad Hoffman's idea appealed to me how ever it was not quite the minimalist approach.
Here is something for Your consideration and critique:
A single opamp with a single resistor and a ( compensated )Zener diode.
The op amp is an LM 3900 a Norton op amp.
Norton op amp relies on a current differencing current mirror input stage. Connecting non inverting input to ground disables parts of the input stage and produces lowest noise outcomes.
The input pin will happily work with 200 uA max input current and when driven within the input range will develop typically 0,55V on the inverting input.
The current through the zener diode is set by the resistor such that I zener = 0.55/ R typically.
The other limit on zener current is the aximum drive capability of the op amp ... typically 10mA.
The implication is that the resistor should not be any less than 56 ohm and probably no more than 1Kohm.
I have built a prototype consisting of four sources using 6.2V zener and following resistor 56ohm, 100ohm, 220 ohm and 470ohm,
Unfortunately all i have is a Fluke 8050 4.5 digit instrument and my measurements are inconclusive to the extent that i see a variation of a couple of millivolts as the temperature in my workshop fluctuates from 18 C to 24 C over a 24 hour cycle. and the four references drift by unequal amounts.
The nice thing about the LM3900 package is there are four opamps.
I am yet t o build the temperture controller using one of them while the other three are used as zener amplifiers.
A very simple bang bang controller based on a comparator can be built with an LED status indicator. The LED should flash on and off at an increased frquency when set point is achieved and this would be a cue that the source has stabilised thermally.
As usual comments invited.