The bottom transistor is the actual current source transistor, ordinarily the load ( here the 1K resistor ) is connected to the collector of this transistor. The top transistor is the cascode transistor and by holding the CCS transistor at a fixed voltage the CCS is much steadier with varying load or supply voltage. When I built this circuit the impedance was about 12Mohm at DC less at higher frequencies.
Note I used a jfet and resistor for the 200uA CCS in the real circuit.
