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Concerning the CCS,' I fond this else where and does this pertain to the low current draw in this circuit? Figure 2.62 COSS output capacitance of the DN2540. As can be seen from the graph, COSS is alarmingly high at low voltages and it is not until Vds>15 V that it falls to its asymptotic value of ≈12 pF. Fortunately, we would always operate at Vds>15 V (preferably 40 V, or more) because a BJT cascode is a better choice at lower voltages. An EF184 pentode CCS could achieve an output capacitance of 3 pF, but once we add typical strays of 3 pF to both circuits, the valve advantage degrades to 6 pF against 15 pF. This somewhat poorer output capacitance is the price we pay for the undoubted convenience of a two-terminal CCS. The device has a maximum continuous current rating of 500 mA, so it should come as no surprise to learn that performance degrades at low currents. If you need a current <10 mA, then you owe it to yourself to see if there’s an alternative solution because the device is much better >10 mA, and at 25 mA it really comes alive ( rout of a cascode CCS at 25 mA was four times that at 10 mA, all other factors kept constant). References