Question about positive real zero of CS stage in Sedra&S

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leohart

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The book said for a CS stage,it has a positive real zero at s=gm/Cgd,this is plausible to me,but it also said this means a zero for frequency gm/Cgd??
We all know s=jw,so how can this be?there is no physical frequency for s=gm/Cgd(cause w=-jgm/Cgd)!

I try Gray's book for this stuff,it did't say there's zero at frequency gm/Cgd but instead with there will be a zero at very high frequency(not infinity freq.),this doesnt make sense to me at all,higher freq. wont make s=gm/Cgd...

Any comments from you guys will be helpful,THX!
 

Re: Question about positive real zero of CS stage in Sedra&a

hi
although i have not good reason to prove your statement is un-correct.
i think, in most case, we just care about the amplitude-freq response, and phase-freq response, in these cases, the frequency is only w not jw.
s is a complex variable, just for stands for amplitude and phase vs. freq together.
good luck
jeff
 

hi,jfyan,thx for your comment
but the amplitude transfer function is defined with complex variable s=σ+jw,in any cases of w,we will get imagine part from jw.
As u said,for freq. itself,it is w,but when in s domain,we should use jw instead!
 

Poles and zeros refer to the s-domain anyway.
But try plotting the amplitude and the phase of (jw-gm/Cgd) and convince yourself that this corresponds to a zero at +gm/Cgd, with amplitude gm/Cgd*sqrt(2) and phase +45 degrees.
 

So,zeros do not necessary mean there exist transimmison nulls,but just implication for there is a s+w(z) numerator ?
If w(z) is purely imaginery,we got a zero at some physical freq.,if not we just got a trend of reduing amplitude when freq. moving to |w(z)|,but it won't be zero at |w(z)|

For example s+1/RC as numerator of a transfer function,in s domain,the real zero should be w=-j/RC
but in real world,the zero freq is 1/2piRC,also amplitude is not zero there
Am I right?
 

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