See Sedra&Smith 4th edition Chapter 6 Differential and Multistage Amplifiers
Channel length modulation in MESFET is greater than MOSFET because λ between 0,1 and 0,3 V^-1
We need another techniques different from current mirror to attain high output resistance, from the semiconductor device.
For example:
In GaAs - depletion MESFET we have a implicit channel Vt<0
**Simple current source: MESFET with VGS=0 , rout is the problem
**Cascode of another MESFET to the simple current souce, to increase output impedance
**Increasing the output resistance with a bootraping circuit that put voltage gain between drain and source approached to 1 (Miller Theorem).
And so on