Common Source (CS) FET amplifier at high frequency-page-1

 

High frequency small signal model

                        Vgs = vi

Rs reduces the gain of the amplifier although it improves bias stability.

Applying KCL at node D

(vi - VO) Ygd – gmvi – VO (Yd + Yds + YL) = 0   Ygd = jωCgd

                       

Vi (Ygd - gm) – VO (Ygd + Yd + Yds + YL) = 0

For low frequency small signal model

[RS = 0]

AV = -gmRd

Where Rd’ = Rd | | RL

nput Impedance

Yi = jωCgs + jωCgd (1- AV)

Ci = Cgs + Cgd (1+gmRd’)      [ AV = - gmRd’]

Miller’s Effect

Problem

Taking VDD + ve

ID = -2.25 mA

VDS = -10.5V


 

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