This is a figure I encountered in this article: LC_Oscillator, I calculated the impedance in this way and I got -2/gm, is there anyone can give me a hand? I think maybe I misunderstood something,some concepts about "impedance for port".
My calculation
For ideal NMOS FET, [LATEX]i_g=0[/LATEX], hence denote the voltage difference applied on [LATEX]d_1,d_2[/LATEX] is [LATEX]V[/LATEX], and the corresponding current is [LATEX]I[/LATEX], then
How do you get I = -id1 = id2 ? Where is I flowing?
If I is the current difference id1-id2 (tho' it's not apparent from the drawing why it should be), then I = gm*(vg1 - vg2) = gm*(vd2-vd1) = gm*(-V), so Z = V/I = -V/(gm*V) = -1/gm.
How do you get I = -id1 = id2 ? Where is I flowing?
If I is the current difference id1-id2 (tho' it's not apparent from the drawing why it should be), then I = gm*(vg1 - vg2) = gm*(vd2-vd1) = gm*(-V), so Z = V/I = -V/(gm*V) = -1/gm.
In my opinion,[LATEX] I [/LATEX] is assumed to be flowing into d2, and, out of d1, to comply with the direction of [LATEX]V[/LATEX], hence [LATEX]2I=i_{d_2}-i_{d_1}[/LATEX]