I am working on classAB amplifier design which uses floating current source to bias the inverter(used as second stage)

I have read in couple of books saying that floating curr source will not present any impedance to the signal (proved mathematically also)  
in the fig shown below it can be proved mathematically that vtest/itest =Rncas (approximately when gmp=gmn)

can some one explain how to understand/interpret this result?

also what is the impedance of  floating curr. source at hi frequencies?


regards,
yvkrishna