# Common source amplifier using mosfet: Why is my Id dependent on my Rd?

Since I'm (trying to work) in the saturation mode, the following formula should be valid:

Since Vto is a static parameter it shouldn't change, same for K. And Vgs is 2 as determined by the voltage divider circuit. My DC operating point simulation confirms this. This leads met to believe that Id should be stable if you change the resister R1. However, according to the simulations this isn't the case. (Even though Vgs is 2, and Vds is higher than 2. My Vto should be 0 so I should be working in the saturation region) If I make R1 higher, Vds becomes lower and Id also becomes lower. Can someone explain this to me?

• Channel length modulation? Saturation current also increases with Vds – sarthak Jan 1 '18 at 12:27

NMOS will be in saturation as long as: $$(V_{GS} - V_{TH}) < (V_{DD} - I_D R_D)$$ your formula for $I_{DS}$ will be valid only in this region. If you increase $R_D$, its obvious that at some point, the NMOS will come out of the saturation region and your formula for $I_{DS}$ is not valid anymore.