at high freq, gain of CS Amp

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020170

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in Razavi, this circuit's gain is Av = - gm RD

but why? in my opinion, this amp's gain is Av = - gm ( RD \parallel\ Rds)

why this circuit's gain is Av = -gm RD ??

thanks
 

Hi.
If you read it more carefully again, the author assumed λ=0. which means gds=0 or rds goes to infinity (chapter 6 -> 6.2 -> 2nd paragraph.)

Regards,
EZT
 

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    020170

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ezt said:
Hi.
If you read it more carefully again, the author assumed λ=0. which means rds=0 (chapter 6 -> 6.2 -> 2nd paragraph.)

Regards,
EZT
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hi EZT, if λ=0 , rds should be infinite, not 0
 

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    020170

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thanks everyone. i got it

Gds = ( lambda*Ids) / (1+ lambda Vds) ,

if lambda = 0 , gds = 0 , so then 1/rds = ∞

infinite means, open circuit. yes, rds can be ignored.
 

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