In Reply to: Re: Analog CMOS Design posted by siswoyo edo on 02/04/01 at 11:14 AM:
When the body(backgate) is tied to Vdd/Gnd(depending on whether substrate is P or N sub) instead of to the source, the Vth of the MOS will be increased meaning that it will be more difficult to turn on. the relationship is given
Vth=Vt0 + gamma(sqrt(phi-Vbs) -sqrt(phi))
Vt0 - threshold voltage for Vbs=0
gamma - bulk threshold parameter
phi - strong inversion surface potential
Vbs - backgate to source voltage
In normal applications, Vt0, gamma and phi are fixed as constants leaving only Vbs to change. As Vbs change the MOS will have different Vth.