The body effect of MOS and changing VSB

[neg_ion13]

Hello,

I just have a conceptual question about my project. I am designing a two stage MOS opamp. Upon determining my Vin common mode low, I realize that while calculating the voltage threshold including the body effect that value is only valid at VCM, min. As the input changes so does the source voltage of the input NMOS. Therefore VSB changes. I am talking about the input NMOS VSB value.

The design is a cascode differential amplifier with NMOS inputs and a tail NMOS for constant current source. So common mode input low is Vod1 + Vth1 + VodSource = 1.8V, meaning there is only an input NMOS on top of the NMOS constant current source.

Do I need to re-calculate the threshold voltage for each input value of interest or just once for this specific requirement? I am just afraid if I calculate this once and plug this into SPICE it will be inaccurate.

Thanks.

 

[Valkarie]

</code>Yes, you should re-calculate the threshold voltage for each input value of interest. The threshold voltage of the NMOS devices is directly proportional to the source voltage, so the threshold voltage will change as the input voltage changes. This is why it is important to re-calculate the threshold voltage for each input voltage to account for this variation.