Re: [請益] 小信號條件
※ 引述《creation (cc)》之銘言：
: https://imgur.com/a/YisHGLx
: (9.117 & 9.118式 置於 圖3)
: 請問一下，圖1的最後一行 & 圖2的第一行：
: 為什麼 "限制gm誤差在5%以內，明確的小信號條件是9.129式" ?!
what we said for gm is:
gm = dI/dVi
therefore, in a linear model, we have
Id = Id_0 + gm*Vid/2
Here, gm = 2Id/Vov = I_tail/Vov (or I/Vov in brief)
from 9.117, you can see that the real Id is expressed as:
Id = Id_0 + gm*Vid/2*sqrt(1-(Vid/2Vov)^2)
the error could be expressed as:
gm_e(%) = sqrt(1-(Vid/2Vov)^2)-1 ~ -(Vid/2Vov)^2/2
we want to have this no more than 5% (1/20)
so we have:
(Vid/2Vov)^2/2 < 1/20
Vid/2Vov< sqrt(1/10)
Vid < Vov*2*sqrt(1/10) = 0.632Vov#
the wording here is not precise enough, what should be written is:
"若我們希望gm誤差在5%以內，則...Vid應該限制在Vov的0.632倍以內"
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