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Design >> Analog Design >> Zero in Common sourse amplifer
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Message started by hjzs18 on Dec 19^th, 2005, 11:06pm

Title: Zero in Common sourse amplifer
Post by hjzs18 on Dec 19^th, 2005, 11:06pm

Hi, all;

suppose i have a garden-variaty common source amplifer , and also suppose i put a Miller compensation cap,say Cc , between the drain and gate, now i add a zero to my system and the value is Gm/Cc, i wonder if anybody here can give me an intuitive explaination why it is this form.how to think of it.

in Razavi's book, he gives a explaination, it is something about current flowing back from the output. but i just can not get it

Thanks all

Title: Re: Zero in Common sourse amplifer
Post by vivkr on Dec 24^th, 2005, 7:03am

Hi,

At a zero, the output is zero for a finite input Vin. This only happens at
one frequency and so there is no physical short. This can only happen if
the current through the two parallel branches cancels out exactly, and no net
current flows into the load.

Setting Vout=0, path1 gives -Gm.Vin current into the load, path2 gives +sCc current into the load. Net zero current => s=Gm/Cc (RHP zero)

Regards
Vivek