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I am trying to learn about circuits from some slides I obtained and I am stuck on a slide.

Here is a screenshot of it (it is in German): http://i.imgur.com/HCtWF.png

It says: Example: Illustration of maximum achievable amplification of a MOSFET with the use of the parameters g

_{m}(transconductance) and g

_{DS}(output conductance).

On the left is the circuit configuration. On the right is the effect of an input voltage change with x-axis drain voltage and y-axis drain current. It also has the derivation for the amplification.

What confuses me is how they obtained:

g

_{m}* ΔU

_{GS}+ g

_{DS}* ΔU

_{DS}= 0

as a result of I

_{0}being constant. Perhaps I am not understanding the graph completely. Can someone point me in the correct direction?

For reference:

g

_{m}= dI

_{D}/ dU

_{G}with U

_{D}const.

g

_{DS}= dI

_{D}/ dU

_{D}with U

_{G}const.

Many thanks.