Transistor CCS resistor value calculation.

[ppppenguin]

I ask again, how much does the Zout matter in this application? if the single transistor isn't good enough then you need a better circuit such as this cascode. But that still raises the question: How good does the CCS need to be in this application? As you've said, precision isn't important in this valve circuit.

If temperature goes up and (for example) the zener voltage stays the same but the transistor Vbe goes down then the colelctor current will rise. The effect is reduced by the emitter resistor. With a 5V zener the effect will be fairly small as there will be around 4.5V across the resistor and the change in the Vbe will be small compared to the total voltage.

There are many different current source and current mirror topologies. Just look online and you'll find plenty. Some are really only for use on chips where you can match transistors accurately.