作者deathcustom (Full House)
看板Electronics
标题Re: [请益] 小信号条件
时间Mon Jan 3 14:38:28 2022
※ 引述《creation (cc)》之铭言：
: https://imgur.com/a/YisHGLx
: (9.117 & 9.118式 置於 图3)
: 请问一下，图1的最後一行 & 图2的第一行：
: 为什麽 "限制gm误差在5%以内，明确的小信号条件是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:
"若我们希望gm误差在5%以内，则...Vid应该限制在Vov的0.632倍以内"
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1F：→ deathcustom: 明确来说，小信号本来就是近似，这个模型带来的误差 01/03 14:39
2F：→ deathcustom: 会造成(放大器)输出的高次谐波 01/03 14:40
3F：→ deathcustom: 当你有某个THD需求的时候，本来就需要检视相应的 01/03 14:40
4F：→ deathcustom: 输入限制，不是甚麽"应限制gm在5%以内"，而是(因为实 01/03 14:41
5F：→ deathcustom: 际电路需求，我的gm误差必须限制在5%以内" 01/03 14:42
6F：推 creation: 不好意思 请问为什麽gm_e(%) = sqrt(1-(Vid/2Vov)^2)-1 01/03 16:14
7F：→ creation: -1 1是"100%"的意思吗? 01/03 16:16
8F：→ creation: ~ -(Vid/2Vov)^2/2 是"约等於"-(Vid/2Vov)^2/2 吗?! 01/03 16:18
9F：→ creation: 为什麽呢?! ( ~ -(Vid/2Vov)^2/2 ) 01/03 16:20
10F：推 creation: THD是threshold的意思吗?! 01/03 16:23
11F：→ samm3320: total harmonic distortion 01/03 16:35