gm/ID

1. 优点：直接和VGS-VTH 对应，查找表，短沟道，亚阈值。

2. 长沟道模型：

• 截止区：ID = 0。

• 线性区：Ron，ID = \frac{1}{2}uCoxW/L[2(VGS-Vth)VDS-VDS^2]

• 饱和区：ID = 1/2uCoxW/L(Vgs-Vth)^2

  
  

• gm/Id = 2/(Vgs-Vth)

• transit frequency fT =  1/2pi gm/Cgs , Cgs = 2/3 Cox WL   

• 传统设计方法

  
  

假设Vov = 300mV， ID = 1mA；Vout = -vin gm 1k, gm = 2ID/Vov = 2.1mA/300mV = 6.7ms

Av = -gmRL = -6.7ms.1k = -6.7, 带宽500Mhz, Av = 10

gm = 10/1k = 10ms，Cgs = 1/2pi/300/500M = 1.1pf

fT = gm/cgs/2pi = 9.4G。 

可以知道Vov, gm/Id, Id

 gm/Id 与fT

500Mhz， 10

gm = 10ms

Cgs = 1/2pi/300/500 = 1.1

ft = gm/2pi/cgs = 9.4G

gm/Id <17.5

Id = 570u

确定L。确定W

W不影响，gm/Id，ft。影响Cgs, Id

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