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thus r0 can be increased by increasing the length of the transistor or by decreasing the current... logically this means that the channel length modulation factor is less and you'll have a near ideal current source behavior from the transistor...
I think that regulated cascoding (as DZC said before) (unknown also by Gain Boosting technique ) will provide you will high output impedance with the same headroom consumed by single transistor but with more power consumption.