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Design >> Analog Design >> body effect decreases output impedance in source followers?
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Message started by surreyian on May 18^th, 2011, 1:37am

Title: body effect decreases output impedance in source followers?
Post by surreyian on May 18^th, 2011, 1:37am

Hello all,

In razavi's design of analog CMOS integrated circuit, chapter 3, it states/shows that body effect decrease the output resistance.

But I find it puzzling. I bias the NMOS source with a certain voltage, eg 0.3V.
Setup 1: Base and Source are connected together to 0.3V bias. (No body effect)
Setup 2. Source is connected to 0.3V whilst base is connect to ground. (with Body effect)

Id=(Vgd-Vth)^2K.
Based on the equation, setup 2 Vth will be greater than setup 1, hence reducing Id, hence increase output resistance. Am I right in the analysis?
If that is the case how does body effect reduces output impedance?

Title: Re: body effect decreases output impedance in source followers?
Post by raja.cedt on May 19^th, 2011, 11:57am

hello..this is small signal o/p impedance and expression is 1/(gm+gmb), so it decrease. but you are arguing from large signal and that to some mistake is there in that equation.

Thanks.