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Here is a similiary application circuits of LTC2054.
The voltage on Gate of FET is about 1.3 ~ 1.5V. How the FET to adjust the current Id?


http://www.linear.com/product/LTC2054

To understand this circuit, you have to understand that an op amp with negative feedback will always have the v(-in)≈v(+in), and that the input currents are negligible.
See the attached annotated schematic. Vgs will be about 2V.

 

Here is a similiary application circuits of LTC2054.
The voltage on Gate of FET is about 1.3 ~ 1.5V. How the FET to adjust the current Id?


http://www.linear.com/product/LTC2054