some days ago,I asked this question here.the address of orignal topic is
+12V . | . R1 5k to 5M . | . +-------------------------, . | 470pF | . | ,---||---, | . | | | | . '---+-|-\\ | 100 ||--' S . | >-+-+--/\\/\\--||->
. +7V -----|+/ | ||--, VP0610L . 15k | BS250 . | | VP0106 . -12V +-----------o . | | . |< .-. . +5.5V----| Q2 | |RL . |\\ | | . | '-' . | | . o-----------o . | . === . GND
Thanks all.I have made great improve by the circuit above.But there is something confused. When the current is 100uA/10uA/1uA,this circuit works right.Buth when the output is 1mA,that means R1=5K,the output is not constant. When RL=0,the output is about 1.000324mA,Vs is about 7.002V,but Vg is more than 8.3V.When RL=240,the output is about 1.000219mA,Vs is about
7.003V,but Vg is more than 8.2V.When RL=5K,the output is only 0.87mA,Vs is about 7.003V,Vg is about 4.5V.
I don't understand the reason.I know when Vgs is positive when Rl is less than about 2.5K,but Vgs is negtive when RL is about 5K. Could someone help me?